Figure 16–01 This helmsman can steer from the binnacle-mounted ship’s compass while her navigator takes bearings with a hand-bearing compass. These traditional piloting instruments remain vital in the age of GPS (Global Positioning System) navigation.
Basic Piloting and Navigation Tools & Procedures
The Dimensions & Tools of Piloting • Dead Reckoning • Precision & Accuracy, Rounding Numbers • Making a Speed Curve • GPS, Chartplotters, Radar, Depth Sounders, & Multi-function Displays
PILOTING is the use of landmarks, aids to navigation, and soundings to conduct a vessel safely through channels and harbors, and along coasts where depths of water and dangers to navigation require constant attention to the boat’s position and course; see Figure 16-01.
Piloting is one of the principal subdivisions of NAVIGATION—the science and art of directing the movements of a vessel from one position to another in a safe and efficient manner. It is a science because it uses principles and procedures based on centuries of observation, analysis, and study; it is an art because interpretations of observations and other information require individual judgment and skill.
An adjunct to piloting is DEAD RECKONING, a procedure by which a boat’s approximate location is determined at any time by its movements since the last accurate determination of position. The term is said to have evolved from deduced reckoning, often abbreviated “de’ed” reckoning in old ships’ logs.
Other types of navigation include CELESTIAL and ELECTRONIC NAVIGATION. The latter is covered here and in Chapters 17 and 18, while celestial navigation is covered in more advanced texts such as Dutton’s Nautical Navigation or Bowditch.
THE IMPORTANCE OF PILOTING
Piloting is used by boaters in rivers, bays, lakes, sounds, as well as close alongshore when on the open ocean or large lakes—all areas known as PILOT WATERS. The navigator of a boat of any size in pilot waters must have adequate training and knowledge; he or she must be constantly alert and give his task close attention. Frequent determinations of position are usually essential, and course or speed changes may be necessary at relatively short intervals.
Always keep in mind that the use of “hightech” electronic navigation instruments does not make basic piloting and chart work obsolete, as many GPS users seem to believe. Piloting and chartwork are integral to electronic navigation because, without a graphic representation of your pilot waters, you cannot know what lies ahead along your course. A GPS receiver will give you a very accurate and reliable readout of your boat’s position—i.e., its latitude and longitude coordinates—but without a chart, and without plotting your GPS position on it, you cannot know whether or not you are headed into danger.
Electronic chartplotters take GPS navigation a step further, using the GPS signal to show your boat’s position on a moving digital chart display. While this may relieve you of the routine necessity of chart-and-compass piloting, it does not relieve you of the responsibility for knowing how to do it. Traditional piloting techniques are not merely a hedge against the possible failure of your electronic navigation equipment, though this in itself is a sufficient reason to learn and practice piloting. The practice of piloting also places you in more intimate contact with your surroundings than electronic navigation can. Time spent studying paper charts, taking bearings on buoys and headlands, matching half-tide ledges you see across the water with their representations on the chart, and steering compass courses while searching the waters ahead makes you a more alert, more engaged, better, safer boater. Traditional piloting is also deeply satisfying, and you will retain a much more detailed and useful mental picture of waters you have piloted than of waters you transited with the aid of a chartplotter alone.
In this chapter we’ll look at piloting tools and methods first. Then we’ll explore how electronic navigation can make wayfinding more accurate and convenient.
The High Seas vs. Pilot Waters
On the high seas, or well offshore in such large bodies of water as the Great Lakes, your navigation can be more relaxed. An error or uncertainty of position of a few miles presents no immediate hazard to the safety of boat and crew. But when you approach pilot waters, you need greater precision and accuracy. An error of only a few yards, for example, can result in your running aground with possibly serious consequences. The presence of other vessels nearby underscores your constant need to know where the dangers lie and where your own boat can be steered safely if you need to maneuver to avoid a collision with another craft.
The Enjoyment of Piloting
You will enjoy piloting most when you can do it without anxiety, which means that you need a background of study and practice. Try “overnavigating” in times of fair weather so you acquire the skill needed to direct your boat safely through fog, rain, or night without fear or strain. Despite the availability and use of electronic systems, a good navigator will retain his or her basic skills, because piloting a boat with a chart and a pencil is a satisfying activity and a pleasure.
THE DIMENSIONS OF PILOTING
The basic dimensions of piloting are direction, distance, and time. Other quantities that must be measured, calculated, or used include speed, position, and depths and heights. You must have a ready understanding of how each of these dimensions is measured, expressed in units, used in calculations, and plotted on charts.
Direction
DIRECTION is the position of one point relative to another without reference to the distance between them. As discussed in Chapters 13 and 15 on compasses and charts, modern navigation uses a system of angular measurement in which a complete circle is divided into 360 units called degrees. In some piloting procedures, degrees may be subdivided into minutes (there are 60 minutes in one degree) or into common or decimal fractions of a degree.
True, Magnetic, or Compass
Also as discussed, directions are frequently referenced to a base line running from the origin point toward the geographic North Pole. Such directions can be measured on a chart with reference to the meridians of longitude and are called TRUE DIRECTIONS. Measurements made with respect to the direction of the earth’s magnetic field at that particular point are termed MAGNETIC DIRECTIONS. Those referred to local magnetic conditions as measured by the vessel’s compass are designated COMPASS DIRECTIONS; see Figure 16-02.
To ensure accuracy, it is essential that you designate the direction reference when magnetic (M) or compass (C) is used; true is assumed when there is no letter designation.
Figure 16-02 Direction, one of the basic dimensions of piloting, can be measured with respect to true, magnetic, or compass North. The reference used must always be designated when stating a direction.
Directions & Angles
The basic system of measurement uses the reference direction as 0° (North) and measures clockwise through 90° (East), 180° (South), and 270° (West) around to 360°, which is North again. Directions are expressed in three-digit form, such as 005°, 030°, 150°. Note that zeros are added before the direction figures to make a three-digit number, for example, 005° or 055°; see Figure 16-03, left. ANGLES that are not directions are expressed in one, two, or three digits as appropriate; 5°, 30°, 150°; see Figure 16-03, right. The use of three digits for directions helps distinguish them from angles.
Figure 16-03 Directions are always designated by a three-digit number. Add zeros before a single-digit or two-digit value (left). An angle between two directions (right), as distinguished from the directions themselves, is not expressed as a three-digit number.
Reciprocals For any given direction, there is its RECIPROCAL direction: its direct opposite, differing by 180°. Thus the reciprocal of 030° is 210°, and the reciprocal of 300° is 120°. To find the reciprocal of any direction, simply add 180° if the given direction is less than that amount, or subtract 180° if it is more; see Figure 16-04.
Figure 16-04 The reciprocal of a given direction can be found by adding 180° to its value. If the total exceeds 360°, subtract 360°. Alternatively, subtract 180° from the original direction rather than doing this two-step addition and subtraction.
Distance DISTANCE is defined as the spatial separation between two points without regard to the direction of one from the other. It is the length of the shortest line that can be drawn between the two points on the earth’s surface.
The basic unit of distance in piloting in the U.S. is the MILE, but as noted in Chapter 15, a boater may encounter two types of miles. The familiar STATUTE MILE is used on many inland bodies of water, such as the Mississippi River and its tributaries, the Great Lakes, and the Atlantic and Gulf Intracoastal Waterways. It is 5,280 feet in length; the same mile as commonly used on land. On the high seas and connecting tidal waters, the unit of measurement is the NAUTICAL MILE set at 6,076.1 feet. This “saltwater” mile was conceived as equal to one minute of latitude, and this relationship is often used in navigation on both the sea and in the air. The conversion between nautical and statute miles is a factor of 1.15; roughly, seven nautical miles equals eight statute miles.
You may cruise from an area using one kind of mile into waters where the other kind is used. Be sure to determine which type of mile is to be used in your calculations. For shorter distances, a few miles or less, the unit of measurement may be the YARD; feet are seldom used except for depths and heights.
Should the metric system be adopted in the United States, boaters must be prepared for greater use of METERS (m) and KILOMETERS (km); they must be familiar with these units and conversion to and from conventional units. Factors are: 1 foot = 0.3048 meter; 1 yard = 0.9144 meter; 1 statute mile = 1.609 kilometers, 1 nautical mile = 1.852 kilometers exactly. See Appendix D for additional conversion factors. Even though many charts will use mostly metric measurements, distances will probably continue to be stated in nautical miles because of its relationship to latitude.
Figure 16-05 A boat’s position may be described in “relative” terms, such as distance and direction from an identifiable object, for example, a landmark or aid to navigation. The position shown here might be stated as “1.8 miles, 130 true, from Point Judith Light.”
Time
Although a navigator in pilot waters does not need so accurate a knowledge of the exact time of day as does a celestial navigator, ability to determine the passage of time and to perform calculations with such ELAPSED TIME is essential.
Units of Time
The units of time used in boating are the everyday ones of hours and minutes. In piloting, measurements are seldom carried to the precision of seconds of time, although decimal fractions of minutes may occur occasionally in calculations. Seconds and fractions of minutes may be used in competitive events such as races and predicted log contests, but seldom otherwise.
The 24-Hour Clock System
In navigation, including piloting, the time of day is expressed in a 24-hour system that eliminates the designations of A.M. and P.M. Time is written in four-digit figures; the first two are the hour and the last two are the minutes. The day starts at 0000 or midnight; the next minute is 0001, 12:01 A.M. in the “shore” system; 0100 would be 1 A.M., and so on to 1200 noon. The second half of the day continues in the same pattern, with 1300 being 1 p.m., 1832 being 6:32 P.M., etc., to 2400 for midnight. (The time 2400 for one day is the same as 0000 for the next day.)
Time is spoken as “zero seven hundred” or “fifteen forty.” The word “hours” is not used. The times of 1000 and 2000 are correctly spoken as “ten hundred” and “twenty hundred,” not as “one thousand” or “two thousand.”
When performing arithmetic computations with time, remember that when “borrowing” or “carrying” there are 60 minutes in an hour, and not 100. As obvious and simple a matter as this may be, miscalculations occur all too often.
Times Zones
A navigator in pilot waters must also be alert to TIME ZONES. Even in coastal or inland waters, you can cruise from one zone to another, necessitating the resetting of clocks and watches.
Daylight Time
A further complication in time is the prevalence of DAYLIGHT TIME during the summer months. Government publications may use standard time; local sources of information such as newspapers and radio broadcasts use daylight time if it is in effect. Daylight time is one hour later than standard time. When converting from standard time to daylight, add one hour; from daylight to standard, subtract one hour.
Speed
No matter what type of boat you have, SPEED is an essential dimension of piloting. Speed is defined as the number of units of distance traveled in a specified unit of time. The basic unit of speed is MILES PER HOUR, whether these are nautical or statute miles, as determined by location. A nautical mile per hour is termed a KNOT. Since this term includes “per hour,” it is incorrect to say “knots per hour.” The unit “knot” may be abbreviated either as “kn” or “kt”; the former has a somewhat greater usage.
Conversion of Knots & mph
The conversion factors between statute miles per hour (mph) and knots are the same as for the corresponding units of distance—1 knot = 1.15 mph, or 7 knots roughly equals 8 mph. (1 knot = 1.852 km/h).
Position
The ability to describe the position of his vessel accurately is an essential requirement for a boater, and one that marks him or her as well qualified. To realize the importance and the difficulties of this seemingly simple task, listen to your radio on VHF Channel 16 or 22A on a weekend afternoon during the boating season. The hesitant, and inaccurate, attempts by skippers to simply say where they are surely must irritate the Coast Guard and embarrass competent boaters, and almost always delays the arrival of assistance. The availability of GPS receivers with their direct and highly accurate display of position has alleviated but not eliminated this problem.
Relative & Geographic Coordinates
POSITION may be described in two ways: in relative terms or by geographic coordinates. To state your boat’s RELATIVE POSITION, describe it as being a certain distance and direction from an identifiable point such as a landmark or aid to navigation. Precision depends on the accuracy of the data on which you base the position: you might say that you were “about two miles southeast of Point Judith Light”; or if you had the capability of being precise, you could say, “I’m 1.8 miles, 130° true from Point Judith Light”; see Figure 16-05. Be careful not to confuse the direction you measure toward the identifiable point with your report of position from the object. In relative positioning, the distance can, of course, be essentially zero, as would be the case with a position report such as “I am at Bell Buoy 4.”
These examples of position description have all used visible identifiable objects. You can also state your GEOGRAPHIC POSITION in terms of latitude and longitude using those straight, uniformly spaced lines on most charts. Using this procedure, measure the position from the markings on either side and on the top or bottom of your chart.
The units of measurement for geographic coordinates are degrees and minutes, but this degree is not the same unit as used in the measurement of direction, nor is this minute the same as the unit of time. Be careful not to confuse these units of position with other units with similar names. For more precise position definition, you can use either SECONDS or DECIMAL FRACTIONS OF MINUTES (usually tenths), as determined by your chart. The smallest unit on the marginal scales of NOS coast charts is tenths of minutes, but for harbor charts it is seconds; refer to Chapter 15, Figures 15-12 and 15-13.
To state your boat’s geographic position, give the latitude first, before longitude, and follow the figures by “north” or “south” as appropriate. Similarly, longitude must be designated “east” or “west” to be complete. All U.S. waters are in north latitude and west longitude, but the position labels should be stated nevertheless; see Figure 16-06.
Skippers of boats with GPS normally describe their position in terms of latitude and longitude.
Figure 16-06 A boat’s position may also be described in geographic coordinates. Using the subdivisions on the chart’s borders, latitude and longitude are measured as shown here and are recorded with time: 41°11.7'N, 071°27.8'W at 0700.
Depths
The DEPTH of the water is important both for safety of a boat in preventing grounding, and for navigational purposes. Thus, this vertical measurement from the surface of the water to the bottom is an essential dimension of piloting. Measurements may be made continuously or only occasionally as appropriate.
In pilot waters, depths are normally measured in FEET; refer to Figure 16-05. In open ocean waters the small-boat operator may use charts indicating depths in FATHOMS of six feet each. Some charts of other nations may use METERS and DECIMETERS (tenths of a meter). Check each chart when you buy it, and again when you use it, to note its unit of depth measurement. Check also the DATUM—the horizontal plane of reference, such as MEAN LOWER LOW WATER—from which depths on the chart are measured; refer to Chapter 15, Figure 15-09. Note that “mean lower low water” is the average of the lower low water of each tidal day, and there will be many days that will have a low water level below the mean; at those times, actual depths will be less than the figures printed on the chart. Charts of inland waters will have a datum at a specified level.
In many areas, depths often fluctuate from the chart’s printed figures because of tidal changes; in other areas, depths change in seasonal patterns.
Heights
The HEIGHT or ELEVATION of objects is also of concern to a boater. The height of some landmarks and lighted aids to navigation may determine their range of visibility. Of more critical importance, however, are such vertical measurements as the clearance under a bridge; refer to Chapter 17, Figure 17-02. Heights, or vertical dimensions upward from the surface of the water, are measured in feet (or meters).
You will find that in tidal areas the plane of reference for heights is not the same as for depths.
The usual datum for height measurement is MEAN HIGH WATER. This will be an imaginary plane surface above the mean low-water datum for depths by an amount equal to the MEAN TIDE RANGE; charts usually show the height at mean high water; refer to Chapter 15, Figure 15-17.
Note that “mean high water” is the average of all highs, and there will be many instances of normal highs being above the datum, with correspondingly lesser vertical clearances.
THE INSTRUMENTS OF PILOTING
For piloting in coastal and inland waters, you will need a few simple tools or instruments. They are not particularly expensive items, but they should be of good quality, well cared for, and used with respect. Most navigators augment these time-honored tools with one or more electronic navigation instruments, beginning with a handheld or fixed-mount GPS (Global Positioning System) receiver, as covered later in this chapter.
Direction
Many instruments are used for measuring direction—some directly and others on a chart. Still other instruments measure direction both from a chart and from observation.
Determining Direction
The basic instrument for determining direction is a COMPASS. With rare exceptions it will be a MAGNETIC compass, as described in Chapter 13. Its directions are properly called “compass” directions—not “magnetic,” not “true.” Usually, before a direction such as 057° C can be used or plotted, it must be corrected for both deviation and variation.
The compass is used primarily to determine the direction in which the boat is headed. Depending upon its mounting and its location in the boat, you may also be able to use the steering compass to determine the direction of other objects from your boat. Readings may be taken by sighting across the compass itself and estimating the reading of the compass card. Steering compass sights are generally accurate enough, but physical obstructions usually limit them to objects forward of the beam.
Hand-bearing Compass
You can take compass sights more flexibly with a HAND-BEARING COMPASS, which is held in the hand and not in a stationary mounting. These come in many styles, but each is basically a small liquid-filled compass with a suitable grip for holding in front of your eye, plus a set of sights and/or a prismatic optical system for simultaneously observing a distant object through the sights and reading the compass card. A typical hand-bearing compass is shown in Figure 16-07; this model uses a prism in lieu of sights; see also Figure 16-01. A small steering compass can sometimes be removed from its mounting (but only if the construction is such that the compensating magnets are part of the mounting and do not come with the compass when it is removed) and taken to where there is an unobstructed line of sight to a distant object. Some binoculars have a built-in compass that allows the user to simultaneously see the distant object more clearly and read its direction.
A hand-bearing compass is normally used on deck, away from the usual causes of magnetic deviation, as discussed in Chapter 13. Take care, however, not to get too close to objects of iron or similar material, such as anchors or stays and shrouds. To find a sighting position on deck that you can be sure is free of magnetic influences, take bearings on a distant charted landmark with your boat in a known position. Compare your hand-bearing readings taken from various locations on deck with the magnetic bearing from the chart. Put the boat on several different headings as you do this, in case the orientation of its magnetic influences has an important effect on the hand-bearing compass. When taking the bearing make sure your eyeglasses or such other items as a spare battery in your shirt pocket are not causing deviation.
Typically, you would find a clear space to stand and brace yourself (note that you will be concentrating on the sighting and not keeping a weather eye for the next large wave), then raise the hand-bearing compass to your eye and take the reading. In most situations, the compass is well clear of the various magnetic influences that cause deviation, but you should be alert to the possibility that you are standing too close to a mass of magnetic material.
Hand-bearing compasses are also very useful in determining whether a nearby vessel is changing position with respect to your boat. Are you on a collision course? Are you winning or losing ground in a race? A hand-bearing compass can help you answer these important questions—though not always definitively—while there is still time to do something about it.
Figure 16-07 A hand-bearing compass can be used from almost any location on a boat, but be careful to keep away from large masses of magnetic material that could cause deviation errors.
Plotting Directions
Once you have determined a direction by compass, pelorus, or other device, you must plot it on the chart. There are several instruments for doing this, and they are the same tools used for determining direction from the chart itself.
Course Plotters These are pieces of clear plastic, usually rectangular, which have one or more semicircular angular scales marked on them; see Figure 16-08. The center of the scales is at or near the center of one of the longer sides of the plotter and is usually emphasized with a small circle or bull’s-eye. Plotters normally have two main scales, one from 000° to 180° and the other from 180° to 360°; each calibrated in degrees. (Some models may not follow the three-digit rule.) There may also be smaller AUXILIARY SCALES, which are offset 90° from the main scales. Lines are marked on the plotter parallel to the longer sides.
Figure 16-08 A course plotter is a single piece of transparent plastic ruled with a set of lines parallel to the longer edges, and semicircular main and auxiliary scales in degrees. There may be distance scales on the two longer sides.
Course plotters are used in the following manner:
To determine the direction of a course or bearing from a given point: Place the plotter on the chart so that one of its longer sides is along your course or bearing line, and slide the plotter until the bull’s-eye is over a MERIDIAN (longitude lines running North-South). Read the true direction on the scale where it is intersected by the meridian. Easterly courses are read on the scale that reads from 000° to 180° and westerly courses on the other main scale; see Figure 16-09. If it is more convenient, find your plotted course or bearing with one of the plotter’s marked parallel lines rather than its edge. It is not essential that you actually draw in the line connecting the two points (the plotter can be aligned using only the two points concerned), but you will usually find it easier and safer to get in the habit of drawing in the connecting line.
Figure 16-09 The course plotter is lined up with the charted line along one of the longer sides and is then moved until a meridian cuts through the bull’s-eye. Direction can then be read from the appropriate main scale. If more convenient, the plotted line can be lined up with one of the parallel lines on the course plotter.
When the direction to be measured is within 20° or so of due North or South, it may be difficult to reach a meridian by sliding the course plotter across the chart. The small inner auxiliary scales have been included on the plotter for just such cases. Slide the plotter until the bull’s-eye intersects a PARALLEL OF LATITUDE (east-west line). The intersection of this line with the appropriate auxiliary scale indicates the direction of the course or bearings; see Figure 16-10.
Figure 16-10 In the case of plotted lines within 10° to 20° of North or South, the inner auxiliary scale and a parallel of latitude can be used.
To plot a specified direction (course or bearing) from a given point: Put a pencil point on the origin location, keep one of the longer edges of the course plotter snug against the pencil, and slide the plotter around until the center bull’seye and the desired mark on the appropriate main scale both lie along the same meridian. With the plotter thus positioned, draw in the specified direction from the given point.
Alternatively, you can first position the plotter using the bull’s-eye and scale markings without regard to the specified origin point, slide the plotter up or down the meridian until one of the longer edges is over the origin point and then draw in the direction line.
For directions nearly north or south, use one of the small auxiliary scales on a parallel of latitude.
To extend a line that must be longer than the length of the course plotter: Place a pair of dividers, opened to three or four inches (7–10 cm), tightly against the edge of the plotter and then slide the plotter along using the divider points as guides. Draw in the extension of the course or bearing line after the plotter has been advanced.
To draw a new line parallel to an existing course or bearing line: Use the parallel lines marked on the course plotter as guides.
Some course plotter models do more than just measure directions. One, called a nautical ruler, has distance scales along its longer sides for both nautical and statute miles at common chart scales.
There are other plotting instruments, such as the Quik-Course; see Figure 16-11. This can be used to determine the direction of a course line by placing its center at any point along the line, orienting the horizontal and vertical lines with similar lines on the chart, and reading the true direction from the printed scales. After correction from compass to true, bearings can be plotted by placing the center of the Quik-Course on the object sighted upon, aligning the plotter, and placing a pencil mark on the chart for the reciprocal of the bearing as read on the scale around the outer edge of the plotter. After you remove the Quik-Course from the chart, you can draw a line from this pencil mark back to the location of the sighted object.
Figure 16-11 A specialized plotting tool, such as a Quik-Course plotter, is handy to use in plotting course lines and bearing. It is particularly useful in smaller, open boats.
Course Protractors Many skippers use one of these as their primary plotting tool; see Figure 16-12. This instrument, with its moving parts, is slightly more complex and takes a little more practice to use comfortably and accurately than course plotters. Course protractors typically consist of a small clear plastic square marked with a 360° compass scale and a grid of fine lines. Riveted on the center of the circular scale is a clear plastic arm that revolves like the hands of a clock—but it is a very long arm, extending about a foot (30.5 cm) beyond the circle of the compass scale. The principle is to duplicate the orientation of the compass rose of the chart (particularly the magnetic circle) and transfer it to the point of interest on the chart.
Figure 16-12 Some navigators prefer to use a course protractor for plotting. The long arm is movable with respect to the square grid.
Course protractors are used in the following manner:
To measure the direction of a course or bearing: Place the center of the course protractor on the chart exactly over the specified origin point, such as your boat’s position or an aid to navigation. Then swing the protractor’s arm around to the nearest compass rose on the chart, making the upper edge of the arm (which is in line with the center of the compass part of the course protractor) pass directly over the center of the compass rose.
Holding the course protractor arm firmly in this position, turn the compass part of the protractor around until the arm’s upper edge cuts across the same degree marking of the protractor compass as it does at the compass rose (true or magnetic). The compass of the protractor and the rose on the chart are now parallel.
Holding the protractor compass firmly against the chart, move the protractor arm around until its edge cuts across the second point involved in the course or bearing. You can now read the direction in degrees (true or magnetic) directly from the protractor compass scale.
To lay off a line in a specified direction from the given point: Line up the protractor rose with the chart’s compass rose as in the preceding instructions. Then rotate the arm until the desired direction is indicated on the compass scale, and draw in the line; extend the line back to the origin point after you have labeled the course and the protractor has been lifted off the chart.
Parallel Rulers These are traditional instruments for measurement and plotting directions on charts. Parallel rulers may be made of black or clear transparent plastic; see Figure 16- 13. The two rulers are connected by linkages that keep their edges parallel and, like ordinary drawing rulers, have beveled edges that are convenient for drawing lines. To measure the direction of a line, line up one ruler with the desired objects on the chart and then “walk” the pair across the chart to the nearest compass rose by alternately holding one ruler and moving the other toward the compass rose or other objective using the two small knobs that are on each ruler. One ruler is held firmly in place on the chart while the other is advanced. Then this second ruler is held in place while the first is closed up on it; this process can be repeated as necessary. To plot a line of stated direction, reverse the process; start at the compass rose and walk to the desired origin point and draw a line to the length desired. In both procedures, take care that the rulers do not slip.
Normally the outer circle of a compass rose will be used, giving true directions. With experience, however, the inner circle can be used for measurement of magnetic directions; remember that deviation must be applied to get compass directions.
Figure 16-13 Parallel rulers are a traditional plotting instrument. Two straightedges are kept in parallel alignment by the connecting links. Transfer directions from one location to another by “walking” the rulers from plotted line to a compass rose, or vice versa.
Drawing Triangles An ordinary plastic pair of these can also be used for transferring a direction from one part of a chart to another, although not for very great distances. The two triangles need not be similar in size or shape; it is helpful if each triangle has a handle. Place the two hypotenuses (longest sides) together and line up one of the other sides of one triangle with the course or bearing line, or with the desired direction at the compass rose; see Figure 16-14. Hold the other triangle firmly in place as a base, and slide the first one along its edge carrying the specified line to a new position while maintaining its direction. If necessary, alternately slide and hold the triangles for moving greater distances. Although it takes some practice to use this method, it does work.
Figure 16-14 A pair of ordinary drawing triangles can be used for easily transferring a direction from one part of a chart to another, but not for long distances.
Other Instruments There are a number of other instruments available for use in plotting directions, including patented variations on those mentioned above. You may use any that you feel comfortable with.
There are also several more comprehensive plotting systems available. These may consist of a means for mounting and protecting a chart and may provide a set of several specialized plotting devices that can make chartwork much easier to execute and much more accurate in practice. While the principles are the same (and still must be thoroughly understood), such chartwork systems are especially desirable for small-boat skippers. An electronic chart table, such as the Yeoman shown in Figure 16-15, provides a very convenient way to precisely plot position information from a connected GPS receiver and to measure bearings and distances on both printed and hand drawn charts.
Figure 16-15 Various electronic plotting systems are available that can simplify and speed up many procedures. Some, such as the Yeoman shown here, compute positions electronically once matched to the chart and can accept input from other instruments.
Distance
Distance to an object can be measured by RADAR, and distance traveled can be measured directly by a SUMMING LOG or on a GPS receiver.
Alternatively, a handheld OPTICAL RANGE-FINDER can be used to measure distance to an object up to a thousand yards or so. Most often in piloting, however, distances are measured by taking them from a chart.
Chart Measurements for Distance
Dividers Distance is measured on a chart with a pair of these; see Figure 16-16. Open the two arms of the dividers, and the friction at the pivot is sufficient to hold the separation between the points. Most dividers have some means for adjusting this friction; it should be enough to hold the arms in place, but not so much as to make opening or closing difficult. A special type of dividers has a center crosspiece (like the horizontal part of the capital letter “A”) which can be rotated by a knurled knob to set and maintain the opening between the arms; thus the distance between the points cannot accidentally change; see Figure 16-16, center. This type of dividers is particularly useful if kept set to some standard distance, such as one mile at the scale of the chart being used.
Figure 16-16 Three styles of dividers are shown here. When using the more common kind (left), the friction at the pivot holds the arms for the desired opening between the points. Another type (center) uses an adjustable center cross arm to maintain the separation between the points. When using traditional “one-hand” dividers (right), squeeze the lower part of the arms to close the gap, the upper part to open it.
To measure distance with dividers: First open them to the distance between the two points on the chart, then transfer them without change to the chart’s graphic scale. Although your first thought might be to place one divider pin on the zero position of the scale and then see where the other pin falls to read a distance, there is a more accurate procedure. For one pin of the dividers, choose a mark on the scale for a whole number of units so that the other pin falls on the subdivided unit to the left of zero; refer to Chapter 15, Figure 15-07. The distance measured is the sum of the whole units to the right of the zero mark and the fraction of a unit measured to the left of zero. Be sure to note the type of fraction used, such as seconds of latitude/longitude, tenths of a mile, or hundreds of yards/meters.
If the distance on the chart cannot be spanned with the dividers opened widely (about 60 degrees is the maximum practical opening), follow this procedure: set the points for a convenient opening for a whole number of units on the graphic scale or latitude subdivisions; then step this off the necessary number of times by swinging one leg past the other as you set the points alternately along the line. Although you have probably drawn in a line on the chart, it is possible to step the dividers along the edge of a ruler; remember to count the number of steps. Then adjust the dividers to measure the odd remainder. The total distance is then the simple sum of the parts stepped off and measured separately; see Figure 16-17.
To mark off a desired distance on the chart: Set the right pin of the dividers on the nearest lower whole number of units, and the left pin on the remaining fractional part of a unit measured leftward from zero on the scale. The dividers are now properly set for the specified distance at the scale of the chart being used and can be applied to the chart. If the distance is too great for one setting of dividers, step it off in increments.
Charts at a scale smaller than 1:80,000 do not have a graphic scale; distances are measured on the latitude scales at either side of the chart. Take care to measure on these scales near the same latitude as the portion of the chart being used; in other words, move directly horizontally across the chart to either of its sides to use the scales; refer to Figure 16-17.
With a little experience you can set and use conventional dividers with one hand. Making sure that the friction at the pivot is properly adjusted, practice this technique until it is easy for you; it will add convenience and speed to your piloting work. Traditional “one-hand” dividers have a pair of curved arms; squeezing them above or below the crossover opens or closes the gap between the points; refer to Figure 16-16, right.
An instrument that looks much like a pair of dividers, except that a pencil lead or pen is substituted for one point, is called a COMPASS (or DRAWING COMPASS to distinguish it from a magnetic compass). This is used primarily for drawing circles or arcs of circles.
Figure 16-17 When a chart distance is too great to be measured with a single setting of the dividers, open the points to a whole number of units. Step these along the charted line for the required number of times, and then measure any smaller distance left over in the usual manner.
Chart Measurer This device can also be used to measure distance across a chart, althought with reduced, but generally acceptable, accuracy. A chart measurer has a small wheel that rolls along the chart and is internally geared to an indicating dial. Chart measurers read distances directly in miles at various typical chart scales; there are separate models for charts using statute and nautical miles. They are particularly useful in measuring distances up and down rivers with many bends and changes of direction.
Time
Every skipper, no matter how small his or her boat, should have a dependable and reasonably accurate timepiece, whether a clock, wristwatch, or pocket watch. Long-term accuracy is of secondary importance in piloting, but short-term errors, those accumulated over the length of a day or half-day, should be small. A knowledge of the time of day within a few minutes is usually all you will need. If you use a clock, mount it where it is clearly visible from the helm-seat or plotting table.
ELAPSED TIME is often of greater interest than ABSOLUTE TIME; a stopwatch is handy for this; see Figure 16-18. Most stopwatches have a second hand that makes one revolution per minute, and this is the simplest kind to use, but there are some that sweep completely around in 30 seconds and even a few that have a 10-second period; some have a digital readout. Know for sure the type of stopwatch you are using.
Also quite useful is a COUNTDOWN TIMER with alarm. This can be either a simple kitchen timer from the galley, or a wristwatch with the countdown feature. Countdown timers save you the trouble of keeping an eye on the clock to be ready for a course change or an expected sighting of an aid to navigation.
Digital wristwatches are excellent for use in piloting. Some models have many features in addition to ordinary timekeeping, such as settable alarm, stopwatch, and countdown timer with alarm.
Figure 16-18 A stopwatch, as shown here, or a wristwatch that includes a timing function, is useful for measuring elapsed times. It eliminates the possibility of mistakes that might be made in subtracting clock times.
Speed
Speed is a dimension that can either be measured directly or calculated from knowledge of distance and time. Direct-reading instruments are convenient, but they give only the SPEED THROUGH THE WATER, not the speed made good over the bottom.
In “olden days,” the speed of a vessel through the water was measured by a “chip log.” A weighted wooden float was attached to the end of a line that was allowed to run out off a reel. Knots were tied in this line at regular, specific intervals (47 feet, 3 inches) and counted as the line ran out for a specified time, measured by turning an hourglass (actually a 28-second glass). The number of knots that had passed gave the vessel’s speed in nautical miles per hour. Hence the speed was measured in—what else—“knots,” and it still is today!
More modern marine speedometers use the pressure built up in a small tube by motion of the boat, or the rotation of an impeller. There are many different models with varying speed ranges, from those for sailboats to models reading high enough for fast powerboats; see Figure 16-19.
Figure 16-19 This cockpit- or wheelhouse-mounted digital display unit receives an electronic signal from a small impeller mounted through the bottom of the boat, and can display distance run as well as speed through the water.
SPEED MADE GOOD (SMG), also termed SPEED OVER GROUND (SOG), can be manually calculated from distance covered and elapsed time, as discussed later in this chapter, or with special calculating devices. These calculators are adaptations of general mathematical slide rules; they may be either linear or circular in form; see Figure 16-20.
Small electronic calculators are also very useful for speed-time-distance computations. Today’s GPS receivers provide quite accurate SOG data. This is especially true when differential corrections are being used.
Figure 16-20 A circular calculator or slide rule, such as the one seen here, enables a navigator to solve speed-distance-time problems as well as other conversions or measurements. It provides easily read, accurate results.
Depth-Measuring Tools
Depth of the water can be measured manually or electronically. A hand LEAD LINE is simple, accurate, and not subject to breakdowns, but it is awkward to use, inconvenient in bad weather, can give only one or two readings per minute, and can be used only at quite slow speeds or to check depths around an anchored boat or one that has gone aground. When it is useful, however, it is very useful. If you run aground, its readings can tell you the best direction to go to get off; it’s not always dead astern. The line need not be long, 10 feet or so with markings every foot, plus a special mark at your boat’s draft. A handheld lead line can also be helpful when used from a dinghy ahead of your boat if you are outside a channel in unknown depths. A marked boathook or other pole can be used in shallower waters.
Many small boats today have an ELECTRONIC DEPTH SOUNDER—a convenient device that gives clear and accurate measurements of the depth of water beneath the boat. A depth sounder gives readings many times each second, so frequently that they appear to be a smooth, continuous depth measurement. There is more information on these relatively inexpensive but most useful piloting aids later in this chapter.
Miscellaneous Piloting Tools
Among the most important of all piloting tools are ordinary PENCILS and ERASERS; see Figure 16-21. Pencils should be neither too hard nor too soft. If too hard they tend to score into the chart paper, and if too soft they smudge. A medium (HB or No. 2) pencil works well; experiment to see what hardness is best for you. Keep several pencils available, well sharpened, and handy to the plotting table. Fine lead (0.5mm) mechanical pencils are excellent. A soft eraser of the “Pink Pearl” type is good for most erasures; an art gum eraser works well for general chart cleaning.
Figure 16-21 Seen here are some of the basic tools for piloting: a high-quality, waterproof binocular, hand bearing compass, powerful searchlight, flashlight with magnifying lens, and, most important, pencil and eraser.
Binocular A good BINOCULAR is essential for most piloting situations. In choosing a binocular, remember that higher powers give greater magnification, bringing distant objects closer, but only at the cost of a more limited field of view. An adequate field of view makes it easier to find an object on small boats with their rapid and sometimes violent motion—just the time when you are most anxious to find that object.
A binocular is designated by two numbers, such as 6 x 30 or 10 x 50. The first figure indicates the power of magnification, the second is the diameter of the front lens in millimeters—a 6 x 30 binocular enlarges images 6 times and has a 30 mm front lens. The size of the front lens is an important consideration, as a larger lens gathers more light and is the main factor in determining how well the optics will assist your night vision in night use. Most authorities recommend a 7 x 50 binocular as best for marine use. A binocular may be individually focused (IF) for each eye or centrally focused (CF) for both eyes, with a minor adjustment on one eyepiece to balance any difference between a person’s two eyes. Both types are fine for marine use; choice is based on personal preference. There are also fixed-focus, nonadjusting binoculars.
Your hands cannot hold a binocular steady enough for boating use under many conditions. Two types of IMAGE-STABILIZED BINOCULARS are available that can deliver steady images despite boat motion. Two stabilization techniques are used:
Image Stabilization (IS), which deals most effectively with vibration, and gyrostabilization, a technique that stabilizes the optical path against the long-term motions common in rough sea conditions. Both types allow use of higher magnification—as high as 18 for IS types and 14 for gyro units—provided the batteries are supplying power. They complement but should not be considered a replacement for a 7 x 50 marine binocular.
They are somewhat larger and heavier than a regular binocular, but for most boaters, their advantages will outweigh these negative factors.
Keep your binocular in its case when you are not using it, and make sure the strap is around your neck when you are using it. Be careful when you put it down so that it cannot slide off and be damaged; even a rubber housing may not protect it from harm.
Flashlights Keep several FLASHLIGHTS or ELECTRIC LANTERNS on your boat for emergency use, and extra batteries for them. At least one should be a red LED flashlight. Unlike the conventional red light obtained from an incandescent lamp and a red filter, the pure, monochromatic light from the red LED will not diminish your night vision. A red LED-equipped hand-held magnifier will be invaluable for reading chart details.
Your Own Piloting Tools
Every navigator eventually has his own favorite set of piloting tools. Don’t be concerned with choosing the “right” or the “best” instrument—choose tools with which you feel confident and learn to use them intuitively.
MEASUREMENTS
If you are considering the measurement of various quantities for use in piloting, and the calculations in which they are used, you must first consider the appropriate standards of accuracy and precision.
Although often used interchangeably in everyday language, “accuracy” and “precision” are not synonymous. PRECISION relates to the degree of fineness of measurement of the value under consideration. ACCURACY relates to how close the stated value is to the true or correct value.
Statements of distance as 32 miles or 32.0 miles are not quite the same things. The first merely says that to the best of observation and measurement the distance is not 31 nor 33 miles; the second says that it is not 31.9 nor 32.1 miles. Note the difference is the degree of preciseness of these two statements of the same distance. Never write 32.0 for 32 unless your measurements are sufficiently precise to warrant it.
The accuracy of the value recorded, regardless of how precisely it may or may not be stated, is determined by the tools and/or techniques used to measure it. Accuracy is sometimes stated as a value, such as “accurate to a half mile.” It can be stated as a probability, such as “100 meters CPE (circular probable error),” meaning that 50 percent of measurements will be within the stated distance.
Accuracy and precision are independent of each other. A measurement or calculation may be stated in very precise terms, but at the same time it may be inaccurate.
Standard Limits of Precision
The navigation of vessels of various sizes naturally involves different standards of precision and accuracy, as befitting the conditions encountered. The piloting of small boats does not permit so high a degree of accuracy as on large ships that offer a more stable platform.
Direction
Direction is measured in small-craft navigation to the nearest whole degree. It is not reasonable to measure or calculate directions to a finer degree of precision when a boat is seldom steered closer than 2 or 3 degrees to the desired course.
Distance
Distances are normally expressed to the nearest tenth of a mile. This degree of precision, which works out to roughly 200 yards for a nautical mile, is reasonable in consideration of the size of the vessel and other measurement standards. On a chart at a typical scale of 1:40,000 this is less than 3/16 of an inch. Use of GPS affords greater degrees of precision and accuracy—commonly to within a few yards. But a malfunctioning GPS receiver may give you dangerously inaccurate positions and distances that are presented with a high (and highly deceptive) degree of precision. If you’re backstopping your electronic instruments with traditional piloting techniques, you won’t be lured into danger.
Time
Time is measured and calculated to the nearest minute. Fractions of a minute are rarely of any significance in routine piloting. In contests, however, time is calculated to decimal fractions and used in terms of seconds.
Speed
Measured electronically, speed through the water is usually indicated to the nearest tenth of a knot (miles per hour), especially at speeds less than 10 knots (mph). The accuracy of such readings does not support this level of precision unless the device has been carefully calibrated. Speed over the bottom (speed made good) may be accurately shown by a GPS receiver, and will be shown accurately when using Differential GPS. Speed is calculated to the nearest tenth of a knot or mile per hour. It is seldom accurately measurable to such fine units, but a calculation to the nearest tenth is not inconsistent with the expressed standards of precision of distance and time. This same degree of precision is used in calculations of current velocity.
Position
Geographic coordinates are expressed to the nearest tenth of a minute, or whole minute, of latitude and longitude, or to the nearest second, as determined by the scale of the chart. As explained in Chapter 15, latitude and longitude markings are subdivided into minutes and seconds on the larger-scale charts (1:50,000 and larger) and in fractions of minutes on smaller-scale charts (1:50,001 and smaller). The smallest scale charts may show whole minutes without fractions.
Don’t be misled by the displays of GPS receivers. Most models show position in latitude and longitude to a finer degree of precision than is warranted by the radionavigation system being used—one more decimal place than is justified.
Depths & Heights of Tide
Tidal variations in the depth of water are normally tabulated to the nearest tenth of a foot (or meter); calculations are carried out to the same degree of precision. Remember, however, that the effects on tidal action of winds and atmospheric pressure make this degree of precision hardly warranted.
ROUNDING OF NUMBERS
In this chapter, the phrase “to the nearest …” is used frequently. ROUNDING is often employed to reduce various quantities to such limitations. For example, if you were to make a distance calculation for a speed of 13 knots for a time of 5 minutes, you would get 1.08333 on most electronic calculators (perhaps even more trailing “3s”). However, since distance is normally expressed to the nearest tenth of a mile, you would use it as 1.1 miles, the value arrived at by “rounding off,” as described below.
Any mathematical expression of a quantity has a certain number of “significant figures.” The quantity 4 has one significant figure, for instance; 4.2 or 14 each have two significant figures; 5.12, 43.8, and 609 each have three significant figures.
The process of reducing the number of significant figures is called “rounding.” To have uniform results, rules have been established for the rounding of numbers.
1. If the digit to be rounded off is 4 or less, it is dropped or changed to a zero.
8.23 is rounded to 8.2
432 is rounded to 430
2. If the digit to be rounded off is 6 or larger, the preceding digit is raised to the next higher value and the rounded digit is dropped or changed to a zero.
8.27 is rounded to 8.3
439 is rounded to 440
3. If the digit to be rounded off is a 5, it is desirable to round to the nearest even value, up or down.
8.25 is rounded to 8.2
435 is rounded to 440
This rule may seem arbitrary, but it is followed for consistency in results; it has an advantage that when two such rounded figures are added together and divided by two for an average, the result will not present a new need for rounding.
Note, however, that slightly different figures will result from using an electronic calculator. When a calculator or computer is set to display a fixed number of decimal places, a 5 to be rounded off nearly always results in the preceding digit being changed to the next higher—even or odd—number. (Internal calculations use the full range of numbers without rounding.) Computers, with their greater capabilities, can be programmed to round up only half of the time, when the preceding digit is odd; they round down for other numbers—this ensures less distortion of the final results.
4. Rounding can be applied to more than one final digit, but all such rounding must be done in one step. For example: 6148 is rounded to 6100 in a single action; do not round 6148 to 6150, and then round 6150 to 6200.
DEAD RECKONING
When operating your boat in large bodies of water, you should always have at least a rough knowledge of your position on the chart. Basic to such knowledge is a technique of navigation known as DEAD RECKONING (DR). This is the advancement of the boat’s position on the chart from its last accurately determined location, using the courses steered and speeds though the water. Note that no allowance is made for the effects of wind, waves, current, or steering errors. This may seem strange, but the reasoning will become clear in Chapter 18, the chapter covering position determination.
Much navigation is now done with electronics, but equipment can fail—often at the worst possible time. And when it does, it is dead reckoning that will get you safely to your destination. Dead reckoning is not some relic left over from the days of sailing ships; it is an essential part of the navigation of any vessel.
Terms Used in Dead Reckoning
Dead reckoning follows some important conventions for recording information on the chart. The DR TRACK (or DR TRACK LINE) is the path that a boat would be expected to follow, or is believed to be following, without any allowance for the offsetting influences of wind, current, waves, and steering error. It is represented on the chart by a line drawn from the last known position using courses and distances through the water. The path the boat travels through the real world may be different from its DR track across the chart due to one or more offsetting influences, to be considered in Chapter 18.
COURSE, abbreviated “C,” is the direction in which a boat is to be steered or is being steered—in other words, the direction of travel through the water. Courses are normally plotted as true directions labeled with three-digit figures, with leading zeros added as necessary: 8° becomes 008; 42° becomes 042. The degree symbol is not used. Some skippers may plot and label courses as magnetic or compass directions. There is no need to add a “T” following the numbers if the course is a true direction, but do add a space following with “M” or “C” to indicate a magnetic or compass direction, if applicable.
HEADING is different from course; it is the direction in which a boat is pointed at any given moment. For example, a sailboat proceeding upwind will typically have a heading that varies from its course by several degrees. Heading is often given in terms of magnetic or compass directions; these values are not part of a plot.
SPEED, abbreviated “S,” is the rate of travel through the water. This is the DR track speed; it is used, together with elapsed time, to determine DR POSITIONS along the track line. (Speed is shown on a plot only for a vessel underway.)
DISTANCE, abbreviated “D,” may be used with a DR plot of a future intended track.
The Basic Principles of Dead Reckoning
You should follow these basic principles of dead reckoning:
1. A DR track is always started from a known position.
2. For a DR track, use either true or magnetic courses consistently, and label appropriately.
3. Only the speed through the water is used for determining distance traveled and a DR position along the track. (The reason why speed over the bottom is not used is discussed in Chapter 18.)
Rules for when to plot a DR position are given later in this chapter.
The Importance of Dead Reckoning
You should always plot a DR track when navigating in large, open bodies of water, especially when aids to navigation or landmarks are not available, or when visibility is poor. You should also plot a DR track whenever there is the possibility of an emergency arising. In such a situation it might suddenly be necessary to report your position to the Coast Guard or other source of assistance. In other words, keeping a DR track is a part of safe boating and is almost always important.
A DR track is the primary representation of your boat’s path, the base to which other factors, such as the effect of current, are applied. Dead reckoning is the basic method of navigation to which you will apply corrections and adjustments from other sources of information.
At the same time, remember that this DR track rarely represents your boat’s actual progress. If there were no steering errors, speed errors, or external influences, the DR track could be used as a means of determining your boat’s position at any time, as well as the ETA (ESTIMATED TIME OF ARRIVAL) at a destination. But even with errors and external influences, a DR plot is always an important safety measure in the event of unexpected variations in current or an encounter with fog or other loss of visibility.
Plotting
Fundamental to the use of dead reckoning is the use of charts and plots of a boat’s intended and actual positions. You should always use standard SYMBOLS and LABELS so your chartwork will be clear to you and understandable to another person (and to yourself at a later date!).
Basic Requirements
The basic requirements of plotting are accuracy, neatness, and completeness. All measurements taken from the chart must be made carefully, all direct observations must be made as accurately as conditions on a small boat permit, and all calculations should be made in full and in writing. If time permits, each of these actions should be repeated as a check; errors can be costly!
When plotting a course, neatness is essential to avoid a confusion of information on the chart. Drawing extra lines or overly long lines on charts or scribbling extraneous notes on them may obscure vital information.
Information on a chart must also be complete. You will often need to refer back to information you placed on the chart hours or days ago, and it can be dangerous to rely on memory to supply details.
Labeling
Draw lines on your charts lightly and no longer then necessary. Keep your straightedge slightly off the desired position of the line you are drawing, to allow for the thickness of the pencil point, no matter how fine it may be. Failure to do this may result in small errors that can accumulate and make your DR positions inaccurate
The requirements of neatness and completeness combine to establish a need for labeling. Immediately after drawing any line on a chart, or plotting any point, label it. The basic rules for labeling are:
1. The label for any line is placed along that line.
2. The label for any point should not be along any line—it should make an angle with any line so that its nature as the label of a point will be unmistakably clear.
The above basic rules are applied in the labeling of DR plots in the following manner:
• Use plain, block upper-case letters and numbers. Labels should be placed so that they can be read with the top or left side of the chart up. If space does not allow a label to be placed as described below, it is acceptable to place it in an adjacent clear area with an S-shaped arrow pointing to where it would normally appear.
• The direction label is placed above the track line as a three-digit number, preceded by “C” for course and followed, if applicable, by “M” or “C” to indicate magnetic or compass. Use a space between the figures and any letters. Note that “T” (for true direction), periods, and the degree symbol “°” are omitted; see Figure 16-22.
• The speed along the track is indicated by numerals placed under the track line, usually directly beneath the direction and preceded by a space and the letter “S”; see Figure 16-22. Units, such as knots or mph, are omitted.
Figure 16-22 Course is labeled above the line with C followed by a space and the direction as a three-digit number (add leading zeros as necessary) followed by a space and then M or C if the direction is magnetic or compass; no letter is used if it is true. Speed is labeled beneath the line with the letter S in front of it. Or distance can be shown below the course line with the designator D. Note the space between the letters and the numbers here also.
• A known position at the start of a DR track, a FIX (see Chapter 18), is shown as a circle across the line; a small dot may be placed on the line for emphasis. It is labeled with time written horizontally; the word “fix” is understood and is not shown; see Figure 16-23.
Figure 16-23 A known position of the boat is plotted as a dot with a small circle around it. If it is at the intersection of two lines, the dot need not be used. Label the position with the time as a four-digit number in the 24-hour system, written horizontally.
• A DR position, calculated as a distance along the track at the set speed through the water, is shown as a half circle (with a dot) along the track line; it is labeled with the time placed at an angle to the course line but not horizontally. “DR” is understood and is not written in; see Figure 16-24.
Figure 16-24 A DR position along a track, without a change in course or speed, is plotted as a half-circle around a dot on the line. Add the time as for a known position, but not written horizontally.
• When planning and preplotting a run, speed, which is often affected by sea conditions, may not be known in advance. In this case, distance, “D,” may be labeled below the course line in lieu of speed; units (nautical or statute miles) are not shown.
Further applications of the basic rules of labeling will be given as additional piloting procedures and situations are introduced in Chapters 17 and 18.
All lines on a chart should be erased when no longer needed, to keep the chart clear. Erasures should be made as lightly as possible to avoid damaging the chart and obscuring its printed information.
D, T & S Calculations
As mentioned earlier, calculations involving distance (D), time (T), and speed (S) are often made with a small mechanical (slide rule or circular) calculator. Use of a calculator is acceptable, but you should also be able to make your calculations accurately and quickly without one, using only a simple set of equations and ordinary arithmetic. The three basic equations are:
Where D is distance in miles, T is time in hours, and S is speed in knots or miles per hour as determined by the type of mile being used. Note carefully that T is in hours in these basic equations. To use time in minutes, as is more normally the case, the equations are modified to read:
Examples of the use of these practical equations may serve to make them clearer:
1.You are cruising at 14 knots; how far will you travel in 40 minutes?
Note that the calculated answer of 9.33 is rounded to the nearest tenth according to the rule for the degree of precision to be used in stating distance.
2. On one of the Great Lakes, it took you 40 minutes to travel 11 miles; what is your speed?
3. You have 9.5 miles to go to reach your destination; on a broad reach you are sailing at 6.5 knots; how long will it take you to get there?
Note again the rounding of results; the calculated answer of 87.6 minutes is used as 88 minutes. In powerboat piloting contests, however, it would probably be used as 87.6 minutes, or as 87 minutes 36 seconds.
The equations are also usable with kilometers and km/h. Memorize the three equations for distance, speed, and time. Practice using them until you are thoroughly familiar with them, and can get correct answers quickly. An easy way to remember is “60 D Street,” for the basic equation 60D = S x T.
The Three-Minute Rule
For short distances, the “Three-Minute Rule” is very handy. Add two zeros to the speed you are making in knots, and you will have the distance, in yards, that you will travel in three minutes, to an approximation close enough for practical navigation.
Use of Logarithmic Scale on Charts
Charts of the National Ocean Service (NOS) at scales of 1:40,000 and larger have printed on them a logarithmic speed scale.
To find speed: Place one point of your dividers on the mark on the scale indicating the distance in nautical miles, and the other point on the number corresponding to the time in minutes; see Figure 16-25. Without changing the spread between the divider arms, place the right point on the “60” at the right end of the scale; the left point will then indicate on the scale the speed in knots.
Figure 16-25 To use a logarithmic speed scale, set one point of the dividers on the scale division indicating the miles traveled and the other point on the number corresponding to the time, in minutes. With the dividers maintaining the same spread, transfer them so that the right point is on the “60” of the scale; the left point then indicates the speed in knots or mph.
To determine time: Use the same logarithmic scale to determine the time required to cover a given distance at a specified speed (for situations not exceeding one hour). Set the two divider points on the scale marks representing speed in knots and distance in miles. Move the dividers, without changing the spread, until the right point is at “60” on the scale; the other point will indicate the time in minutes.
To determine distance: Using knowledge of time and speed, distance likewise can be determined from this logarithmic scale. Set the right point of the dividers on “60” and the left point at the mark on the scale corresponding to the speed in knots. Then, without changing the spread, move the right point to the mark on the scale representing the time in minutes; the left point indicates the distance in nautical miles.
The logarithmic scale is used in the same manner with distances in statute miles and speeds in mph.
NOS charts have instructions for determining speed printed beneath the logarithmic scale, but not the procedures for determining distance or time. In all cases, you must know two of the three quantities in order to determine the other.
Use of S-D-T Calculators
It is not practicable here to give detailed instructions for operating all models of speed-distance-time slide rule or circular calculators. In general, they will have two or more scales, each logarithmically subdivided. The calculator will be set using two of the factors and the answer, the third factor, will be read off at an index mark; refer to Figure 16-20.
If you have a calculator for S-D-T problems, read the instructions carefully and practice with it sufficiently, using simple, self-evident problems, to be sure you can get reliable results later even in emergency.
Speed Curves
Although many boats may have marine speedometers, the traditional method for determining speed of powerboats is through the use of engine speed as measured by a TACHOMETER in revolutions per minute (rpm). A SPEED CURVE is prepared as a plot on cross-section (graph) paper of the boat’s speed in knots or mph for various engine speeds in rpm.
Factors Affecting Speed Curves
The boat’s speed at a specified engine setting may be affected by several factors. The extent of each effect will vary with the size of the boat, type of hull, and other characteristics.
LOAD is a primary factor influencing a boat’s speed. The number of people aboard, the amount of fuel and water in the tanks, and the amount and location of other weights on board will affect the depth to which the hull sinks into the water (the displacement) and the angular trim. Both displacement and trim may be expected to have an effect on speed.
Another major factor affecting speed is the UNDERWATER HULL CONDITION. Fouling growth like barnacles or moss increases the drag (the resistance to movement through the water), and slows the boat at any speed. Fouling on the propeller itself will drastically affect performance.
Whenever preparing speed data on a boat, note the loading and underwater hull conditions as well as the figures for rpm and speed. If you make a speed curve at the start of the season, when the bottom is clean, check it later in the season if your boat is used in waters where fouling is a problem. You may need a new speed curve, or you may be able to determine a small correction that can give you a more accurate determination of speed. You should also know what speed differences to expect from full tanks to half or nearly empty; the differences can be surprising.
Obtaining Speed Curves
Speed curves are obtained by making repeated runs over a known distance using different throttle settings and timing each run accurately. You can use any reasonable distance, but it should not be less than a half mile so small timing errors will not excessively influence the results; yet, it need not be more than a mile, to avoid excessive time and fuel requirements for the trials.
The run need not be an even half mile or mile if the distance is accurately known. Do not depend upon floating aids to navigation—they may be slightly off station, and, in any event, they have some scope on their anchor chains and will swing about under the effects of wind and current. Some areas will have a MEASURED MILE (or half mile); see Figure 16-26. These are accurately surveyed distances with each end marked by a range. Use these courses whenever possible; they are accurate, and calculations are easier with the even-mile distance. But do not let the absence of a measured mile keep you from making a speed curve. Wharves, fixed aids to navigation, or points of land will also give you the accurate distance you need.
In most speed trials you will need to run the known distance twice, one in each direction, in order to allow for the effects of current. Even in waters not affected by currents, you should make round-trip runs for each throttle setting to allow for wind effects.
Figure 16-26 Measured miles are often established and indicated on charts for measuring vessel speed. Even if your boat has a speedometer, make timed runs over this distance to check its accuracy.
For each one-way run, measure the time and steer your boat carefully to make the most direct run. Compute the speed for each run by the equations earlier in this chapter. Tables are available that give speeds for various elapsed times over a measured mile. If the measured distance is an exact half mile, just divide the tabulated speeds by two. Then average the speeds of each pair of runs at a given rpm, for the true speed of the boat through the water. The strength of the current is one-half the difference between the speeds in the two directions of any pair of runs. Caution: do not average the times of a pair of runs to get a single time for use in the calculations; this will not give you the correct value for speed through the water.
If time is measured with a regular clock or watch, be careful in making the subtractions to get “elapsed time.” Remember that there are 60 seconds in each minute, not 100, and likewise 60 minutes in one hour. Most people are so used to decimal calculations that they make errors when “borrowing” in the subtraction of clock times.
If one is willing to use a slightly more complex equation, the boat’s speed through the water (or the strength of the current) can be found from a single calculation using the times of the two runs of each pair.
Where S is speed through the water in knots or mph
Cur is current in knots or mph
Tu is time upstream, in minutes
Td is time downstream, in minutes
D is distance, in nautical or statute miles
In preparing a speed curve for a boat, make enough pairs of runs to provide points for a plot of speed versus rpm; six or eight points will usually be enough for a satisfactory curve. With some types of hulls, there will be a break in the curve at a critical speed when the hull changes from displacement action to semi-planing action. At this portion of the curve you may need additional, more closely spaced measurements, so it is a good idea to calculate speed during runs and make a rough plot as you go along—obvious errors in timing will be readily seen.
You may also want to calculate the current’s strength for each pair of runs. The current values will probably vary during the speed trials, but the variations should be small and in a consistent direction, either steadily increasing or decreasing, or going through a slack period. You will get the best results by running your trials at a time of minimum current.
Example of a Speed Curve
A set of speed trials was run for the motor yacht Trident over the measured mile off Kent Island in Chesapeake Bay. This is an excellent course because it is marked by buoys offshore as well as by ranges on land. The presence of the buoys aids in steering a straight run from one end of the course to the other; the ranges are used for accuracy in timing.
On this particular day, it was not convenient to wait for slack water, but a time was selected that would result in something less than maximum ebbing current. A table was set up in the log, and runs were made in each direction at speeds of normal interest from 900 rpm to 2150 rpm, which was maximum for the Detroit 6-71 diesels.
The results of the runs are shown in Figure 16-27, upper. An entry was also made in the log that these trials were made with fuel tanks 0.4 full, the water tanks approximately one-third full, and with a clean bottom. The column of the table marked “Current” is not necessary, but it serves as a flag to quickly expose any inconsistent data. Note that on these trials the current is decreasing at a reasonably consistent rate.
After the runs had been completed, a plot was made on cross-section paper of the boat’s speed as a function of engine rpm. This resulted in the speed curve shown as Figure 16-27, lower.
Figure 16-27 During the speed trials for Trident, the results were tabulated as shown at top. Runs were made in each direction to account for the effect of current. Entries in the craft’s log were made for the hull bottom condition and the state of fuel and water tanks. The speed curve at bottom, plotted from these data, is accurate only for load and hull conditions similar to those during the speed trials.
Dead Reckoning Plots
With knowledge of dead reckoning terms and principles, the rules for labeling points and lines, and the procedures for making calculations involving distance, time, and speed, you can now consider the use of DR plots.
There are several specific rules for making and using DR plots.
• A DR plot should be started when leaving a known position; see Figure 16-28.
Figure 16-28 A dead reckoning (DR) plot is started when leaving a known position. This is plotted as a fix, with the time labeled; course and speed are labeled along the course line.
• A DR position should be shown whenever a change is made in course (see Figure 16-29, plot A) or in speed (see Figure 16-29, plot B) or both course and speed.
Figure 16-29 Whenever a change in course is made a DR position is plotted for that time; the new course and speed are labeled along the new DR track. If a variation in speed is made without a change in direction, B, a DR position is plotted for that time, and new course and speed labels are entered following it.
• A DR position should be plotted each hour on the hour (more frequently under conditions of reduced visibility); see Figure 16-30.
Figure 16-30 A DR position should basically be plotted every hour on the hour. In conditions of reduced visibility, the interval between DR positions should be shortened. The course shown here is referenced to true north.
• A new DR track should be started each time the boat’s position is fixed. The old DR position for the same time as the fix should also be shown at the end of the old DR track; see Figure 16-31.
Figure 16-31 Start a new DR track whenever the boat’s position is fixed. In this example, a fix is obtained when the craft passes alongside an identified aid to navigation. Label the old DR with the same time as the fix.
FUNDAMENTALS OF GPS NAVIGATION
After the ship’s compass, the most ubiquitous instrument of modern navigation is, without doubt, the GPS receiver. Created by the U.S. Department of Defense in the 1970s, GPS technology can now be found in everything from smartphones to airplanes, automobiles, rail and bus systems, a wilderness hiker’s gear, and boats of all kinds and descriptions. GPS is easy to use (automatic in many cases), world-wide in coverage, usually accurate within meters, and only marginally affected by weather conditions. It works in daylight, snow, hail, sleet, fog, and the dark of night.
Yet despite the advantages of GPS—which is rendered even more useful and convenient when a chartplotter or laptop software uses the GPS signal to place your boat on a digital chart—you are well advised to master the traditional piloting skills and to maintain an hourly DR plot as described above. The reasons for this are touched on in the beginning of this chapter and can be summed up by the phrase “situational awareness.” There is no surer way than plotting your intended DR track with a pencil on a paper chart to prevent you from shaping a course into danger. There is no more reliable way to ensure that you discover a malfunctioning GPS in timely fashion than by maintaining an hourly DR plot. (GPS receivers rarely malfunction, and GPS satellite signals are rarely degraded, but all it takes is once.) Should a GPS receiver or chartplotter cease to function altogether, there is no more sure-footed way to continue a safe voyage than by having a DR plot to fall back on. And there is no more concrete and intuitive way of assessing the impacts of current and leeway on your course and speed over ground than by comparing your DR position with a position fix for the same time; see Chapters 17 and 18.
A navigator needs to maintain situational awareness of the surrounding world just as an automobile driver does. The GPS receiver on an automobile dashboard might call for a left turn in 0.5 mile, but the driver considers what he or she knows of alternative routes, the traffic ahead, road construction, and other factors before following that advice. Blind adherence is never wise. A navigator should keep track of his or her position by at least two techniques, verifying one against the other. Electronic navigation is easy and accurate, but traditional piloting techniques never quit. Know and use both.
Figure 16-33 This handheld GPS displays charts and navigation data in an easy-to-use format, either as a primary source or as a backup on a larger vessel.
GPS Accuracy, DGPS, and WAAS
Modern GPS receivers are extremely accurate almost all the time, subject only to their ability to “see” satellites through potential blockages such as thick foliage or high buildings. Such hindrances are rarely an issue for marine use, and when functioning normally, most civilian GPS receivers are accurate to within a 15-meter range.
DIFFERENTIAL GPS (DGPS) corrects GPS signals to an accuracy of one to three meters. Run by the Coast Guard, the DGPS system consists of a network of towers that receive GPS signals and then transmit a corrected signal out to their broadcast range, from 75 to several hundred miles, depending on the station; see Figure 16-32. DGPS coverage is available for most of the coastal waters of the continental United States, Hawaii, and parts of Alaska. For coverage areas, see www.navcen.uscg.gov/?pageName=dgpsMain. Canada and many other nations operate similar systems. Most GPS receivers now have built-in DGPS reception.
The WAAS (WIDE AREA AUGMENTATION SYSTEM) corrects GPS signals to provide an accurate position within three meters 95 percent of the time. Developed by the Federal Aviation Administration and the U.S. Department of Transportation to provide differential corrections for aeronautical services, the WAAS system consists of ground stations that monitor GPS data. Master stations collect information from the other ground stations and broadcast corrections to geostationary satellites. These corrected WAAS signals are then retransmitted on the same frequency as the basic GPS signals down to your boat, so no separate receiver or antenna is needed. To date, WAAS coverage is available only in North America, although other governments are developing similar systems around the world. Many marine GPS receivers are now available that use WAAS corrections in lieu of the basic GPS service. Some allow the user to switch between DGPS and WAAS.
Figure 16-32 The U.S. Coast Guard operates a number of Differential GPS radiobeacons. The differences between the accurately known locations and the GPS-measured positions of these beacons are determined, and corrections are transmitted that can be received by boats and ships to improve the accuracy of their positions.
GPS Receivers
A wide variety of handheld and installed GPS receivers are available from many manufacturers. Some portable units are designed only to be handheld, and these operate off an integral antenna. Others can be either handheld or mounted at a vessel’s navigation station or helm using an externally mounted antenna and drawing power from the boat’s electrical system. For offshore cruisers, a handheld GPS receiver is a valuable component of the “abandon-ship bag.”
Some fixed-mount GPS receivers include their signal acquisition and navigation computing equipment along with their display in a single unit mounted at a vessel’s navigation station or helm. Other installed receivers have their signal acquisition and computing equipment in an antenna, called a GPS sensor, mounted with a clear “view” of the sky, and a separate display mounted at the navigation station or helm. The two components must be connected by a cable. For vessels with more than one helm, these units can operate multiple displays from a single antenna; position information can also be transmitted over a data network for display on a fishfinder, radar, or chartplotter, as discussed in the final section of this chapter.
GPS Receiver Features
A full-function GPS receiver can do much more than simply display a vessel’s geographical position with predictable accuracy. The GPS data display can include the vessel’s course and speed over the ground (COG and SOG), these being continuously computed from successive position data. Courses and bearings can be shown as true or magnetic, with magnetic courses derived either from stored values for variation or user-entered variation. The obvious utility of this information will be explored in Chapter 17. Since courses and headings are derived from successive positions, heading information is not available when the craft is stationary. There are, however, GPS compasses that use two or three antennas spaced apart from each other and a special receiver to derive highly accurate heading data.
Internal memory can be used to store the coordinates of WAYPOINTS for future use; ROUTES can be preplanned using a series of waypoints. Most sets can store multiple routes and can, with a single key press, reverse the direction of a route. Along a route, transfers from one leg to the next can be automatic or manual, with an alarm being sounded as each waypoint is approached; some models can be programmed to switch from one route to another.
Figure 16-34 This GPS unit displays fundamental navigation information on this screen: Latitude/longitude, speed over the ground, and course over the ground. Other screens will have more information to guide you to waypoints and routes. Note the MOB (man-overboard) button. A sophisticated unit that can be used on any vessel, either as a primary GPS or a secondary one on a bridge, this DGPS is accurate within a few yards.
Internal computing capabilities can determine the distance and direction (true or magnetic) between pairs of waypoints. Using computed track and present position data, a GPS receiver can show cross-track error and how to steer to get back on track. This information can be fed into an autopilot for continuous course correction without action by the helmsman. (But always remember to keep a sharp lookout for other vessels and navigational hazards.) Combining its knowledge of speed, distance, and time, a GPS receiver can display how long it will take to reach a destination and give an ETA.
GPS receivers can save a vessel’s position at any time with a push of a button. This feature, routinely used to establish new waypoints, is used as a “man overboard” function that will create a memory-protected MOB position, automatically presenting the heading to and distance from the MOB position.
GPS signals include very precise time information that can be displayed as local time or time at the prime meridian (0° longitude), formerly known as GMT (Greenwich Mean Time) but now referred to as Universal Coordinated Time (UTC) or “Zulu.” However, you must be aware that “GPS Time” is not the same as actual, correct UTC. Every year or so, UTC is adjusted to compensate for the very gradual slowing of the earth’s rotation; a “leap second” is added to UTC. The clocks of GPS satellites, out in space, are not adjusted. Some GPS receivers have a correction built in as of the date of their design and manufacture, while others do not; some receivers may have a slight lag between time calculation and display. Many units will also give sunrise and sunset times for the current position of the vessel and tidal information for a tabulated station located nearby.
Many GPS receivers can also be programmed to stand an anchor watch by describing a small circle around a vessel’s anchored position and having the receiver sound an alarm if the vessel drifts outside that circle. An alarm can be programmed for an off-track error greater than a preset amount or for an approach to a place that is to be avoided, or the receiver can be programmed to function as an alarm clock or countdown timer. Another valuable function is a “watch alert” that can be set to sound at preset intervals and must be manually turned off by the helmsman, who might have dozed off.
GPS Antennas
External GPS antennas are small. Some are 1 to 2 inches in diameter and 12 to 15 inches high; others are 3 inches in diameter but only 2 inches high. All must be mounted on a vessel’s exterior with a 360-degree view of the horizon. Mounting should only be high enough to provide a clear view all around the horizon—too great a height results in excessive motion of the antenna due to the pitch and roll of the vessel, which can degrade the accuracy of the position. A GPS antenna should not be mounted where it will be in the beam of radar pulses, but a separation from communications antennas of only a few feet is adequate.
HOW A GPS RECEIVER DETERMINES POSITION
In essence, a marine GPS receiver measures the distance between itself and three satellites in space, and uses those distances as the radii of three spheres, each having one of the satellites as its center. Then, using spherical geometry, it determines its position as the intersection of those three spheres.
The system is based on a network of 24 satellites launched in the late 1970s and early 1980s for military use; the U.S. Government made the system available for civilian use starting in the 1980s. The satellites circle the earth twice a day in six planes at an altitude of 10,900 nautical miles (20,200 km), an inclination angle to the equator of 55 degrees, and a speed of 7,000 miles an hour. Because of the satellites’ altitude and orbital patterns, at least four satellites are observable by a ground-based GPS receiver at all times; sometimes as many as eight or even ten can be received simultaneously. This means fixes can be obtained from the system continuously.
The signal from each satellite has two parts: One is a digital code, unique to that satellite, which identifies it. Also transmitted is a navigation message that contains updated information about the satellite’s orbit (technically referred to as “ephemeris data”); what time it is as far as the satellite is concerned (GPS time); almanac data for all the satellites in the constellation; and coefficients a groundbased receiver can plug into a computer model stored in its memory to calculate how the atmosphere is affecting the signal’s transmission through the ionosphere. (This is known as SIGNAL PROPAGATION.)
The satellite tells the receiver the instant in GPS time in which the signal was transmitted, and the receiver synchronizes itself (approximately) to GPS time. Since time, speed, and distance are interrelated—by multiplying the difference between the time that the satellite transmitted the signal and the time that it reached the receiver by the signal’s speed (186,000 statute miles per second)—the receiver calculates its approximate distance (called a PSEUDORANGE) from the satellite. The receiver’s computation of this distance could be absolutely accurate if it contained an atomic clock, but that would make the receiver prohibitively expensive. Instead, the receiver contains a crystal oscillator that is synchronized with the satellite’s clock to the precision required for accurate position fixing.
As shown in this illustration, a GPS receiver determines vessel position by taking virtually instantaneous readings from at least three satellites, the distance from each becoming the radius of a sphere. The receiver calculates the vessel’s position as the point at which the three spheres intersect.
If the receiver could determine distance with absolute accuracy, signals from two satellites would suffice to fix latitude and longitude. Since it cannot determine distance with absolute accuracy, it acquires the pseudorange from a third satellite and adjusts the three pseudoranges in equal amounts until the three LOPs converge to determine the clock error in all three signals and then eliminate it.
The receiver uses the pseudoranges it has computed from the satellites it is observing to solve three simultaneous equations (one for each satellite) with three unknowns (latitude, longitude, and clock error), and produces an estimate of its position. It next must account for its own velocity during the process of acquiring and processing the satellite signals. It does this by comparing the frequencies of the satellite signals against that of a reference signal that the receiver generates internally. From the Doppler effect (this is the effect you note in the sound-wave frequency of a train’s whistle as it approaches you, passes you, then recedes into the distance), the receiver computes its velocity relative to each of the satellites it is observing. The receiver then recalculates the earlier three equations using velocity rather than pseudoranges. After using the solution to those three equations to allow for its own velocity in the earlier estimated position, the receiver produces a fix. (The signal from a fourth satellite is required to determine altitude, a requirement for aeronautical but not for recreational marine applications.) From then on, the receiver can calculate other information, such as boat speed, track, range to a destination, and much more.
Figure 16-35 A chartplotter lets you choose from a series of navigation screens to help you find your way. A digital highway is displayed on the screen at left, illustrating your course to steer. The screen at right shows how you can put the curser on an object, in this case the Thorn Knoll green buoy, to pull down specific information. (The Thorn Knoll nun buoy is green rather than red because this example is from IALA-A waters.)
CHARTPLOTTERS
Electronic charts were covered in Chapter 15—here we will see how they are used, and their most frequent use as of this writing is in a chartplotter, which is also referred to by some as a “chart plotter” or an “electronic charting system”; see Figure 16-35. These are the small-craft parallels to the electronic chart display and information systems (ECDISs) used on large ships. It must be remembered, however, that a chartplotter is not a substitute for having the proper paper charts on board.
Most chartplotters now come preloaded with charts. Some show the entire U.S. and coastal waters, but many manufacturers offer one of four basic areas of the U.S., and if you want more than whatever was preloaded, you have to buy the charts separately. Formerly the charts would come on a CD-ROM, but now they usually come on a memory card or chip.
An electronic chart reader can be a standalone device merely displaying the information that is stored internally or inputted from a data card, but more often they are combined with a GPS or DGPS receiver so that the vessel’s position is shown on the chart display. (Disks and cards are available from several commercial sources.) In this manner, the tedious step of transferring a location in latitude and longitude from a GPS display to a paper chart is avoided, and the possibility of plotting error is eliminated. (Some GPS receivers have rudimentary chart information internally stored in memory, but without the capability of receiving external chart information; these are not true “chartplotters.”)
Chartplotter displays are LCD panels, either monochrome or color; the number of pixels (resolution) will vary with the size and cost of the unit. Screens are normally backlit at variable intensity for nighttime use; some models will have a transflective display for usable visibility in conditions of bright light, such as on a flybridge; these displays get brighter as the light falling on them gets stronger. Many models store track information that can be reviewed later or reversed to lead you back to a starting point. Displays can be selected as “north up” or “heading up,” similar to radar displays. Newer models have touchscreen displays for convenient, weather-resistant helm-station use. One problem with these touchscreens, however, is that they can quickly become so smudged by sunblock-coated fingers as to be unreadable. Note too that entering a waypoint on a touchscreen becomes a real challenge when you’re underway in a seaway. With the boat bouncing from wave to wave, it’s hard to touch the screen where you intend. You may have to stop the boat in order to make an entry.
Figure 16-36 A vivid split-screen 7-inch stand-alone chartplotter with stylized vector-type views of the course ahead. This is a touchscreen unit.
The full range of internally calculated information from a GPS receiver is usually available for display at the margins of a chartplotter screen. A split screen may allow the simultaneous display of data and a chart, or a chart and a fishfinder view of the water beneath the boat; some can show a chart and an aerial photograph of the same area, usually a harbor or harbor entrance. Radar displays can be superimposed on a chart at the same scale or shown on a split screen. On some models, the overlaid radar display can be made semitransparent so that chart details can be read through it.
Data from other electronic devices that can be shown on some models include depth information, speed through the water, water temperature, wind speed and direction, etc. Chartplotters interconnected with a VHF-DSC radio can show the position of a vessel in distress if the Mayday message contains that information; if set up to do so, the location of any boat calling can be shown on the plotter, and the DSC radio can be used to request another boat to transmit its position, which will be shown. Some models, on craft fitted with appropriate sensors, can even display fuel usage and remaining availability—the possibilities are almost endless!
A recently developed chartplotter, when used with an appropriate electronic chart, can sound an alarm when depths shoal beyond a preset minimum in the path ahead of the boat; this is based on charted depths and must be adjusted for tidal conditions; no measurements are taken. It can also warn of land, man-made objects, and other charted features as selected. The unit takes an input of position from its internal GPS receiver and scans the chart in an arc 15° either side of dead ahead out to a distance of ¼ to 1 mile, as selected; the distance should be chosen with reference to the speed of the craft. Caution: This system relies upon the information included on the chart; it cannot warn against an uncharted depth or obstruction.
Figure 16-37 This large multi-function 15-inch touchscreen plotter with radar overlay shows both land and sea details. You can create and store 5,000 waypoints and 100 routes with this unit.
WARNING
It must always be remembered that on small craft, a chartplotter is not a full replacement for paper charts. Even a 10-inch color display shows much less information than you can quickly scan from a conventional chart. Knowing how to zoom, pan, and scroll the display on your chartplotter will help, but you must still have a full-size chart at hand for when you need the full picture.
COMPUTERS ON BOARD
Figure 16-38 In this split-screen chartplotter, the colorful vector screen on the left with depth and channel information also has icons representing much more available information. The traditional raster screen on the right has the familiar look of NOAA paper charts, from which it has been digitized.
With the advance of solid-state technology, which packages more and more power and versatility into smaller and smaller components, using a sophisticated personal computer with a rechargeable power supply on board a recreational vessel is not difficult or complicated.
Typical desktop personal computers require 120-volt AC power input, but that’s because they are designed primarily for use on land, where that is the power normally available. In fact, the computer circuits themselves run on less than 6 volts of DC electrical current, which is provided by the computer’s power supply (a stepdown transformer plus a rectifier that converts 120-volt AC power to multiple DC voltages such as 12, 5, or 3.3).
The easiest way to use a personal computer on board a boat with 12-volt battery power is with an inverter that changes the battery’s 12-volt DC power to 120-volt AC power, which the computer’s power supply will then step down and rectify to the required voltages. The computer’s power supply should be from a dedicated circuit with its own breaker on the main distribution panel to protect it from sudden voltage drops as other equipment comes on and off line. The computer should then be connected to the inverter through a surge protector to guard against sudden voltage spikes.
At 120-volt input, the current draw of a desktop personal computer is about 1.5 amps. If the AC power is being produced by an inverter, that equates to approximately a 16-amp DC draw on a 12-volt system after allowing for the inverter’s 15 percent efficiency loss. While this is an easily manageable power level on board a boat equipped with a genset, it could be too great for a vessel that relies strictly on an engine alternator to recharge its batteries. A more practical approach on medium-size and smaller craft, especially those without a genset, is a portable laptop or notebook computer that can be operated for several hours on an internal battery; see Figure 16-39. Laptop computers are necessarily energy efficient and are therefore ideal for onboard use, especially when operated from a small DC/AC inverter. The power from the inverter will keep the computer’s battery at full charge, while the battery acts as an excellent electrical filter against electrical noise on the boat’s DC power bus and functions as an uninterruptible power supply, keeping the computer on line during engine starts or when switching batteries. The inverter’s power drain will typically be less than what would be required to power a desktop computer. (Also available are DC/DC power cords that eliminate the need for an inverter.)
Figure 16-39 A notebook computer can serve many purposes on a boat, including navigation, log keeping, and administration of supplies and spare parts. It can also serve the same uses as a computer at home or in an office—word processing, data recording, and entertainment. Alternatively, many boaters now are using chartplotters and multi-function displays at the helm and smartphones, iPads, or other wireless-enabled tablets elsewhere on board, leaving their laptops ashore.
More computers designed for marine use (rugged and weatherproof) are entering the market. Available software includes programs for tide and current predictions, current sailing, Mercator and great circle sailing, celestial navigation, and other boating activities. And, of course, the computer can be used for the same word processing, database, and spreadsheet work as at home or the office, and younger members of the crew can play their games. Internet connections are widely available both in port and underway.
A primary application is with navigation software to serve as a chartplotter. Starting in the 1990s, software companies developed navigation programs that enable boat owners to plan future trips from the comfort of their homes or offices. You could input waypoints and routes on your Mac or PC and then download them to a chartplotter at the helm or simply take the laptop down to the boat. There are still many such programs, and now they also operate on electronic pads and tablets, even on smartphones. You can download a full complement of U.S. coastal charts and some foreign charts from Navionics to your tablet. The program is GPS enabled, so you can see where you are. The screen is small, of course, and should by no means be your sole source of navigation information, but it’s great for cruise planning and for use as a backup underway.
The electronic charts used with a chartplotter, and the navigational software used with a personal computer or even a smartphone, may also include a database with additional information such as available marine facilities, tide predictions, and other information useful to a cruising boater. Because of the possibility of equipment failure or loss of electrical power, the availability of such supplemental information should be considered a convenience only—a full backup of texts and tables should always be on board.
Further Precautions
• Computer equipment must be carefully protected from rain and spray. This equipment should be run periodically to ensure that internal parts are dry.
• Disk drives and printers have moving parts that can be damaged by vibration and shock. They must be mounted securely and cushioned as much as possible. Laptop, notebook, and tablet computers, being designed to travel, are more resistant to shock and vibration, but even they should be cushioned when underway.
• Computer circuits may generate radio interference and should be mounted well away from sensitive navigation and communications electronics.
RADAR
Radar (Radio Detection and Ranging) is the only navigation tool that can measure both direction and distance to a moving or still target, and thus can be used both for position fixing (see Chapter 18) and for collision avoidance. What’s more, it can do so under almost any condition of visibility. In fact, radar can almost always make a measurement despite fog, light rain, or darkness. (Heavy rain, however, can decrease and may even prevent radar from detecting targets.) Radar measurements of distance are quite precise and accurate—much more so than vertical sextant measurements or optical range-finder readings discussed in Chapter 18.
Radar offers an excellent means of extending the coverage provided by a visual lookout, especially at night and in reduced visibility. This greater range of detection affords more time for a vessel to maneuver in order to avoid another craft or a detected obstacle. With a radar plot, an early determination can be made of another vessel’s course and speed. Radar also helps when making a landfall from offshore; running a coast and picking up landmarks for fixes; or traveling in confined waters, entering an inlet, and the like. Radar has real advantages, even in the daytime, and of course becomes particularly helpful at night or in conditions of reduced visibility.
Although the Navigation Rules require that radar, if on board, be used in conditions of reduced visibility, it is important to understand that radar is not a substitute for a human lookout required by Rule 5; refer to Chapter 5 and Figure 5-06.
The basic technology that radar employs has been available since before WW II, but the advent in recent years of digital signal processing and integrated circuitry has allowed a number of improvements in its ability to define (clearly outline) targets, as well as in how its information is displayed. A good radar set is an extremely valuable piece of equipment because of its ability to detect the presence, range, and bearing of distant objects in any weather, and to track some forms of weather, such as rainstorms.
HOW RADAR DETECTS OBJECTS
Radar works by transmitting a tightly focused beam of super-high-frequency radio waves from a rotating antenna mounted as high as practical on a vessel, then measuring the time it takes for the radio wave pulses that are reflected from distant objects to travel from and back to the antenna; refer to Figure 16-41. By multiplying that time interval by the speed of radio waves—161,875 nautical miles per second (299,793 km/s)—the system can determine the distance between the antenna and the objects that reflected radio waves back to it.
The bearing to an object is determined by the direction in which the antenna is pointing when it transmits its radio wave beam and receives a reflection (often called an echo). The antenna’s rotation during the time interval between the pulse being sent out and its return is not a factor in determining the bearing of an object, because it makes only one revolution every 0.8 to 4 seconds but transmits pulses of radio waves at the rate of 400 to 6,000 per second. Since it takes the radio waves only a fraction over 12 microseconds (millionths of a second) per nautical mile of range to make their round trip, the antenna can send and receive up to 25 pulses in the time it takes to rotate just one degree, which means that, electronically speaking, it is standing still.
Because the radio waves that a radar transmits are bent by the atmosphere and follow the earth’s curvature, its horizon at any particular mounting height is about 7 percent farther away than the horizon of the human eye at that same height. The maximum range at which a particular radar can detect an object and the clarity with which it can define targets depend on four factors—the strength of the radio wave signal it transmits, the sensitivity with which it can detect and interpret reflected radio waves, the length of its antenna and the height at which the antenna is mounted on the vessel, and the height of the object itself.
As the reflected radio waves are received, they are displayed on a screen normally mounted adjacent to a vessel’s helm. The pattern of all the reflected radio waves that are received on each successive sweep of the antenna clearly reveals the existence of the objects they detect.
This does not necessarily mean that the image displayed on a radar screen will match exactly what is seen with the human eye or on a navigation chart. Low-lying targets such as a beach along a coast will not show up as well as tall buildings several hundred yards inland. Radar cannot “see” through a mountain to reveal a harbor entrance on the other side, and the reflection from a large object such as a commercial ship can mask the reflection of a smaller vessel behind it or nearby.
A radar’s range depends on the mounting height of its antenna and on the height of the objects that reflect the radio waves it transmits. (Not to scale; curvature of the earth exaggerated for emphasis.)
Figure 16-40 A standard radar display presents the vessel’s position at the center of the screen and the vessel’s heading at the 12 o’clock position. With each rotation of the antenna, reflected radar pulses reveal the objects they detect. Shown here is an older-model radar with phosphor screen.
Radar Equipment
A standard radar’s components consist of a transmitter that generates radio waves and includes a modulator that causes the radio waves to be generated in brief pulses; an antenna that radiates the radio waves and collects the returning echoes (see Figure 16-41); a receiver that detects the returned reflections and amplifies them to usable strength; and a display that presents the pattern of received echoes on a cathode-ray tube (CRT) or liquid crystal display (LCD) and includes rotary knobs, buttons, key pads, and sometimes a joystick or trackball for certain operator-controlled functions. The radars used on small craft normally group these components in two units. The antenna, the transmitter, and a portion of the receiver are housed in one unit, mounted as high on the vessel as practical. The remainder of the receiver and the display are housed in a second unit, normally installed near the vessel’s helm. Many manufacturers also offer a remote display that allows a second presentation of the picture on the master display at a second steering station such as on a powerboat’s flybridge or a navigator’s station below.
Transmitters for use on recreational vessels are rated by their peak pulse power output, from 1 kW to about 25 kW. The power taken from the boat’s electrical system is much less, from 30 watts to about 600 watts.
Figure 16-41 A marine radar operates by transmitting very brief pulses of super high-frequency radio waves. These are reflected back by other vessels, aids to navigation, landmasses, and other “targets.” The speed of the pulses is so great that an echo is received before the next pulse is sent out.
Radar Antennas
Small-craft radar antennas range in length from about 17 inches to 6.5 feet (45 cm to 2.0 m). Antennas under about 2 feet (61 cm) in length are usually enclosed in a RADOME, a fiberglass housing most often used to prevent a sailboat’s sails from becoming entangled in the rotating antenna. A radome does not affect the quality of an antenna’s radio wave transmission but does limit the size of an antenna.
The longer an antenna, the narrower the horizontal width of the beam of radio waves it transmits. A 17-inch (43-cm) radome antenna, for example, would typically transmit a 5.7° horizontal beam; a 3.5-foot (1.1-m) openarray antenna might transmit a 2.4° beam; and a 6.5-foot (2.0 m) antenna might transmit a 1.23° beam. The narrower a radar’s beam width is, the greater will be its ability to separate targets; this is called BEARING DISCRIMINATION. The vertical beam width of radar antennas is typically 25° regardless of the length of the antenna and is necessary in order to assure the detection of close-in targets when the vessel is heeled or rolling from port to starboard.
Radar sets are designed with a number of different range scales—longer ranges for coverage of a larger area and the earlier detection of “targets,” and shorter ranges for better detail of nearby waters. The maximum range scales for radars found on board recreational boats generally vary from about 16 to 72 nautical miles, but these figures can be misleading. The maximum range at which an object can be detected is most limited by the curvature of the earth—with transmitter power, receiver sensitivity, and antenna performance playing lesser roles. Although a longer range may be used for tracking weather, such as thunderstorms or large rain squalls, a boat radar is usually operated on a range scale of 3 to 8 miles. Range scales of less than one mile are used when navigating a channel or in close quarters with other vessels.
Antenna Location
A radar antenna should be mounted high enough to ensure that the beam will not illuminate GPS antennas or anyone on board. While range increases somewhat with elevation, the gain is offset by the increased radius of the radar blind area created by the antenna’s 25° vertical beam angle. A height of about 20–22 feet is appropriate, though not possible on many small boats.
Figure 16-43 The larger a radar antenna is, the better will be its bearing discrimination, the ability to separate two targets close together. In some instances, however, such as on a sailboat, it is desirable to use a smaller antenna enclosed in a radome.
Radar Displays
Older analog radars presented target echo information on cathode-ray tubes equipped with long-persistence phosphor screens. These screens kept the image acquired during a single rotation of the antenna visible until refreshed by the next rotation. Today’s “raster scan” radars digitize and store the information they acquire in a memory and then repeatedly deliver the stored information to the screen of a TV-like fast-response CRT or LCD, presenting a non-fading, easy-to-see image.
LCD screens are used in almost all small-craft radars and, to an ever-increasing extent, in ship radars as well. Most displays use color to enhance the user’s ability to interpret the on-screen information. The information gathered by the radar is differentiated into four levels according to the strength of the detected echo, with color used to indicate the strength of the target signature to the operator. A substantial amount of alphanumeric information may also be presented on the screen. Most models now have connections so that networked data from other electronic devices, such as GPS and depth information, can be displayed on the screen.
Figure 16-42 An LCD radar display on a busy boating day clearly identifies other vessels. You can see where they’ve been by their digitized “tails” as they have moved through the water, and you can adjust course as necessary to avoid them. In this instance, the captain would want to keep a watch on the vessel approaching off his starboard quarter.
Radar Capabilities
As mentioned above, radar displays allow an operator to select from a variety of scales ranging from ⅛ mile up to the unit’s maximum range. Most displays overlay concentric circles (“rings”) on the display to divide the selected range into a number of equal units, thus allowing rapid estimates of the distance to any object that is reflecting the antenna’s transmitted radio waves. Most also offer at least one VARIABLE RANGE MARKER (VRM), a movable concentric circle that the operator can place directly over the image of a detected object with a knob, pushbutton, joystick, or trackball. The display will then read out digitally the range to the object.
Another common feature is an ELECTRONIC BEARING LINE (EBL), a movable straight line that pivots around the center point of the screen, which the operator can place over the image of a detected object. The display will then digitally read out the relative bearing from the vessel to the object. More expensive radars offer multiple VRMs and EBLs.
Some radars allow the operator to zoom in on a portion of the display to present that segment in greater detail. While most radars display the vessel’s position at the center of the screen, some radars allow the operator to “off-center” the display to provide greater coverage to one particular portion of the display.
The standard orientation of a radar display is “head-up”—the vessel’s heading is always toward the 12 o’clock position on the radar screen. The display of radars that are interfaced with an electronic compass can present a “north-up” display in which compass north is at the 12 o’clock position on the screen. Some electronic compasses can be programmed to automatically compute variation and present true north at the 12 o’clock position on the screen of an interfaced radar.
More sophisticated radars also offer such features as echo tracking, which graphically presents the track of other vessels on the display screen. Some radar manufacturers also offer optional target plotting aids, which allow selected models of their radars to track up to 10 targets simultaneously; display their range, bearing, and CLOSEST POINT OF APPROACH (CPA); and sound an alarm if a target violates preset CPA limits. If these radars are also interfaced with an appropriate navigation sensor such as a GPS receiver, they can also display target courses in relative or true degrees and present the TIME TO CLOSEST POINT OF APPROACH (TCPA).
Most radars can be interfaced with navigation sensors such as GPS receivers to digitally display one’s own vessel course and speed over the ground and destination waypoint on the radar screens. When interfaced with a GPS chartplotter, they can graphically plot the vessel’s track on either their radar picture or the chart representation.
Most radars also allow a guard zone—all-round or only for a defined sector—to be drawn around the vessel on which they are installed, and will sound an alarm if that zone is violated. This feature can be valuable as an anti-collision device when underway, and also as an anchor watch.
Developing Radar Skills
Radar, even more than other piloting methods, requires considerable practice and accumulation of experience. Develop your radar procedures during fair weather and good conditions so that they are readily available in difficult circumstances and you can use them with confidence. It is not possible to simply install radar on your boat and immediately use it effectively. For both navigation and collision avoidance, the scene shown on the display will require interpretation, which requires study and experience. The full value of having radar on your boat will not be realized until you can use it skillfully and with confidence.
The use of radar for obtaining bearings and ranges is discussed in Chapter 18. Most manuals that come with radar sets provide rudimentary information regarding the use of the many controls on the equipment. There are a number of books devoted to the use of radar, some focused on small craft. There are also videotapes and programs for personal computers that can be run interactively to simulate radar operation. Some larger electronic stores offer instructional courses that are well worthwhile for a skipper not experienced in radar use.
Solid-State Radar
Solid-state, continuous-wave radar (as opposed to the traditional pulse radar) lets you see targets very close to the boat—as close as six feet from the antenna. This is a tremendous advantage in navigating where other boats may be present, in seeing smaller navigation aids, or even in a man-overboard situation. Solid-state radar uses a different technology than is used by conventional radar, employing a magnetron to generate a microwave signal transmitted from the rotating antenna; the signal changes tone or frequency when it reflects off a target and returns to the radar. Traditional radar often runs into problems with interference patterns that tend to block near-in targets. This is not the case with solid-state, which significantly reduces sea clutter.
The scale on a solid-state radar display is set differently. You can set the range of a solid-state radar down to one-thirty-second of a mile and display a range on the screen of only 100 feet. Solid-state technology reduces power consumption and generates lower emissions than conventional radar. Sometimes described as "broadband" radar, solid-state radar models are now offered by most or all of the marine electronics manufacturers.
Figure 16-44 This split-screen radar shows two images on very different ranges. The screen on the left shows close-in broadband radar, with a 200-foot range, displaying a boat only 100 feet away. (A solid-state radar is capable of showing an object only 6 feet from the radome.) The screen on the right, with a distant 36-mile range, still displays the nearby vessel at 0.16 nautical mile at a bearing of 348 degrees magnetic.
FLIR
Figure 16-45 A FLIR handheld thermal imaging camera, which uses infrared technology to detect the heat signature of objects so they can be seen at night or in periods of reduced visibility.
Originally developed for military uses, forward-looking infrared cameras are now used on recreational vessels to help identify objects at night or in periods of reduced visibility. They can help with navigation—letting the user identify buoys and channel markers—as well as in collision-avoidance, since other boats can be seen clearly whether they’re moving or stationary. New FLIR cameras, available in both a small handheld monocular device or through larger radar-like fixed displays, do not rely on light from an external source to develop their images. Instead, they rely on thermal energy. Everything—a boat, a person, a rock—emits heat and develops a heat signature. FLIR cameras use a microbolometer sensor that detects infrared radiation far outside the visible light spectrum. The shape of an approaching boat, for example, is displayed clearly on the screen, even in the dark of night, since its heat signature is different from the water or background surrounding it. FLIR is easy to use and has obvious safety implications, particularly in MOB applications.
Figure 16-46 Two persons overboard as seen through a FLIR thermal imaging camera, either the handheld or fixed-mount version. These cameras detect the heat radiation of each object in view, making it possible to see objects in the water—a person overboard, other boats, navigation aids—as well as bridges and other structures.
Radar Detectors
Marine radars operate at “S” and “X” band frequencies. The C.A.R.D. (Collision Avoidance Radar Detector) can be used to provide an audio and visual warning using a four-bar orthogonal display when its antenna is being illuminated by the beam from an S-band or X-band marine radar. While this is very useful, particularly when operating shorthanded, it must be remembered that no signal is returned to the vessel whose radar has “illuminated” your craft, and that skipper may be entirely unaware of your presence and location.
Passive Radar Reflectors
The strength of radar echoes is based on the reflectivity of objects that return a minute portion of the outgoing radio energy from the radar set. Unfortunately, fiberglass has less reflectivity than metal and thus is a relatively poor radio wave reflector. Sails are extremely poor reflectors.
A boat owner who does not have radar equipment on board can still use radar to increase safety by installing a passive radar reflector as a desirable “defensive” device to alert radar-equipped vessels in his vicinity to his presence and position. Even radar-equipped craft should have a passive reflector mounted as high as reasonably practical. Having an operating radar on board a boat does not increase the likelihood of its being detected by other vessels unless those vessels themselves carry operative equipment that detects the presence and source of radar emissions.
Most passive radar reflectors consist of thin metal sheets arranged in mutually perpendicular planes. These may fold for storage, but must be flat and rigid with respect to each other when open for use. A reflector with each surface only about 2 feet (60 cm) square can provide a strong return signal if properly used. For maximum effectiveness, a reflector should be hoisted from three corners so that one of the eight “pockets” is straight up; this is sometimes termed the “rain-catching” position; see Figure 16-47. In certain cases, the reflector planes are held in their optimum configuration within a fiberglass shell.
Figure 16-47 Whether or not your boat is equipped with radar, it is highly advisable to use a passive radar reflector to make sure your craft shows up on the radar screen of any vessel in your vicinity that is using radar. Without such a reflector, there is a good chance your craft will not be detected by a ship’s radar.
Racons
Racons, also referred to as radar beacons, radar transponders, and radar transponder beacons, are receiver/transmitter devices used as an aid to navigation to assist in identifying landmarks or buoys on a marine radar display. As also described in Chapter 14, a racon responds to a received radar pulse, after an extremely short delay, by transmitting back an active signal, not just a reflected echo. The returned signal becomes an identifiable mark on the radar display with a length of several miles, encoded for identification as a Morse code character. Racons are assigned various identifications consisting of single letters that are all dashes—T, M, and O. (Racons in some foreign waters may signal “D” [– · ·] on new and uncharted hazards.) The delay at the racon causes the displayed response to appear at a greater distance than the echo from the structure on which the racon is mounted. Racons and their identifying marks are normally indicated on charts.
In the United States, racons are used:
• To identify aids to navigation, both on the water (buoys) and on land (lighthouses);
• To identify landfall or positions on inconspicuous coastlines;
• To indicate navigable spans under bridges;
• To identify offshore oil platforms and similar structures.
Outside the United States, racons are also used:
• To mark new and uncharted hazards (using the letter “D”);
• To identify center and turning points.
The U.S. Coast Guard operates approximately 105 racons on the Atlantic, Pacific, and Gulf of Mexico coasts and on the North Slope of Alaska; most of these are dual-band units responding to both X-band and S-band radars. Racons are relatively low-powered and have a range of 6 to 8 miles if located on a buoy, farther if installed at a greater height. To conserve power, those on buoys are not operated continuously, but typically with a duty cycle of 20 seconds on, 20 seconds off.
No additional equipment is required on a vessel, but some precautions are necessary in the operation of the radar. A rain clutter feature must be turned off (or, if necessary, set very low) to ensure that the racon signal is visible. U.S. Coast Guard racons are the “frequency-agile” type, responding on the exact frequency of the triggering radar; any interference rejection (IR) feature must be switched off and sea clutter suppression minimized.
DEPTH SOUNDERS & FISHFINDERS
Depth of the water can be measured manually or electronically. A hand lead line—as described earlier in this chapter—is simple, accurate, and not subject to breakdowns, but it is awkward to use, inconvenient in bad weather, and can give only one or two readings per minute. Many boats today have an electronic depth sounder—a convenient device that gives clear and accurate measurements of the depth of water beneath the boat. A depth sounder gives readings many times each second, so frequently that they appear to be a smooth, continuous depth measurement. The depth sounder may be an independent unit mounted at the helm station or displayed as part of a multi-function data or chartplotter screen.
A depth sounder has a wide range of uses on lakes, rivers, bays, and offshore, providing both safety and convenience, so it is doubly advantageous to have on board. Now depth sounders have been carried one step further—they have evolved into fishfinders. These units do indicate the depth of the water, but go beyond that capability to show a video display of the water between the boat and the bottom, with an emphasis on indicating any fish occupying that area.
How Depth Is Measured and Displayed
Depth sounders determine depth by measuring the roundtrip time required for pulses of ultrasonic energy to travel from the boat to the seabed and then to be reflected back to the vessel—the velocity of sound in water is about 4,800 feet/second. These are sound waves, but of a frequency above that which the human ear can hear; the ultrasonic frequencies used range from 25 kHz to 400 kHz, with 50 kHz and 200 kHz being typical. Lower frequencies will penetrate to greater depths, but higher frequencies will allow narrower beam widths and better definition of echoes in shallower water. The beam width can be as wide as 45° or as sharp as 15°; some depth sounders and fishfinders provide for the selection of a lower or a higher operating frequency. The pulses are sent out from a TRANSDUCER, and the same device picks up the returning echoes and sends them to a receiver, where they are processed to eliminate undesired “noise” and are then sent to some type of display; see Figure 16-48.
Figure 16-48 An electronic depth sounder measures the distance to the bottom by the time required for pulses of ultrasonic sound to travel to the bottom and back.
Transducers are normally mounted as a through-hull fitting, but transom mounts are available for smaller craft. Careful placement of the transducer is necessary to avoid turbulence that would prevent proper operation. With all depth sounders, the center of the beam should not be greater than 15° from the vertical; if the hull angle is greater, special models are available that have the transducer fixed at an angle within the throughhull fitting. Do not shorten the cable from the transducer to the main unit; coil up any excess.
Liquid crystal displays (LCD) are commonly used in depth sounders. An LCD is a digital display that is easily read in daylight and does not require a light shield in bright-light conditions, such as in an open boat or on a flying bridge. LCDs can be backlighted for night use; red backlighting is preferable. Depth information generally is available in feet, fathoms, or meters; see Figure 16-49. There are models of “black box” depth sounders that have no display of their own and connect via cable to other electronic navigation displays or a personal computer.
A video display sounder presents a visual “elevation” picture, displaying the acoustic images of underwater objects such as fish and the seabed beneath the transducer, thus creating a fishfinder. A time history of the information captured is created as each successive set of information is added to the display. The on-screen images are based on the character of the reflected acoustic energy.
Figure 16-49 Modern depth sounders display the depth digitally on an LCD screen. You can usually select feet, fathoms, or meters. Values less than 10 are usually shown to one decimal place.
Correction to “Zero” Depth Indication
A depth sounder’s transducer generally is mounted on the hull several feet below the water surface but (for its own protection) at some distance up from the lowest point of the keel. As depths are measured from the transducer, its readings will be neither the actual depth of the water nor the clearance between the keel and the bottom. On many models, the zero reading can be offset plus or minus a few feet so that the depth indications will be either the true water depth or the clearance under the boat, without having to mentally add or subtract a fixed amount. Although actual waterdepth readings are advantageous for navigation, some skippers prefer a direct reading of clearance under the keel.
Interpreting Displays
A depth sounder shows the depth over a small area as defined by the beam width of the transducer; it does not identify the bottom by type. Digital displays will typically show depths of 10 feet or less to a tenth of a foot, rounding to whole numbers of feet for greater depths. Some models also incorporate a seawater temperature sensor, and this value can be displayed simultaneously or alternately with depth. Other models incorporate speed data from a separate sensor. Many depth sounders have a digital output that can be sent over a data network to other navigation instruments such as a GPS receiver, radar, or a chartplotter. Portable depth sounders and even fishfinders are available that can be handheld over the side of a small boat, but their capabilities are less than those of installed units.
Depth Alarms
Most depth sounders have a shallow-water alarm, and some have multiple alarms. The latter are useful when the boat is anchored. With one alarm set just below the present depth and the other set just above it, the unit sounds if the boat drags anchor toward either shallower or deeper water. Dual alarms are also useful for depth-contour piloting; set the alarms for one or two feet shallower and deeper than the depth curve you are following. An alarm should be audible from the salon or the skipper’s bunk. Some units include remote repeaters.
Figure 16-50 This depth sounder is set to display the depth below the transducer (which probably is not located at the very bottom of your boat). Note that it also has an alarm setting to help you navigate shallow water or to act as a wake-up call if you’re dragging your anchor in the middle of the night.
Fishfinders
With a fishfinder, the depth of the water is indicated, but the primary focus is on the water column between the boat and the bottom—are there any fish there? This is much like an underwater radar, either looking downward or scanning around. LCD video displays for fishfinders may be either monochrome or color, with the latter being more expensive. A digital display of depth and sometimes water temperature is shown in addition to the “picture” of the water column beneath the boat. Monochrome displays can have multiple shades of gray or green to show various features, and a white line for the bottom. Color displays use various colors for this purpose. Care must be taken when selecting a fishfinder that the screen is readable in all levels of ambient light that will be encountered.
Figure 16-51 Fishfinders show the bottom and schools of fish or larger individual fish. Most models will show the depth digitally, and some can also show other data such as water temperature.
The ability to zoom in at different ratios on a specific portion of the water column provides greater information than a simple depth sounder with only a single “elevation” display of the entire column of water beneath the boat can provide. Some fishfinders feature multiple selectable beams to view the area around the boat, and at least one such model will project a beam ahead of or behind the craft. Split screens are available on many models. There are other special features on the more expensive units, varying with the manufacturer and model. A distinct advantage of fishfinders over simple digital depth sounders is their ability to give a general indication of the nature of the bottom, such as hard rock, firm sand, or soft mud.
Fishfinders are available in a range of sizes, and one can be fitted on a fishing boat of almost any size. Display screen resolution, the ability to show fine detail, is determined by the number of picture elements (pixels) available to create the image and is more important than the overall size of the LCD.
If a fishfinder is mounted on a boat that also has a depth sounder and/or a VHF radio, care should be taken that they operate on different frequencies to avoid interference problems, especially on VHF Channel 16.
Scanning Fishfinders
“Scanning sonar” units are available for larger vessels and sportfishing boats. Typically, these can direct a beam of ultrasonic sound pulses 360° around a vessel or to any desired sector 6° or wider; the center of the sector can be set on dead ahead or astern, or any direction to the side. The beam can be directed from 5° upward to 90° downward to greatly expand the volume of water that is being checked. Angles of 0° down from the horizontal will show fish swimming near the surface but nothing of the bottom. Down angles of 30° to 40° ill show the bottom; intermediate angles will show some of the bottom and the water area above the bottom. These are large and expensive units; the transducer dome retracts within the hull of the vessel when not in use, and speed is limited to about 15 knots when it is lowered. For smaller craft, units with a phased array of transducers (no moving parts) can scan either vertically from the surface to the bottom looking forward, or horizontally through a 90° or 180 arc centered on dead ahead, depending on the model installed; beam width is 12°.
Some fishfinders now come with two- or even three-dimensional cartography, displaying the sea bottom down to 3,000 feet with high resolution while showing underwater contours and even wrecks. Digital forward-looking sounders can be an all-weather help in navigation; they let you follow an unmarked channel, for example, to lead you to an inshore destination or keep you from running aground.
Figure 16-52 Fishfinders are available with color LCD displays that provide greater and more easily interpreted information, but these are more expensive than monochrome models. Some models can have a GPS module added so as to also show position. A “blind” fishfinder module can be connected to a chartplotter display.
ONBOARD DATA NETWORKS & MULTIFUNCTION DISPLAYS
One of the more interesting developments in marine electronics has been the ability to interconnect various devices for the exchange of data—and continuing advances in data and video networking can be expected in the years ahead; see Figure 16-53. A VHF radio receives position information from a GPS receiver and adds that data to an outgoing distress message. A chartplotter receives depth information from an electronic sounder and includes that information in its display. A radar unit receives information on the vessel’s course and speed from a GPS receiver and adds that to its screen. And many instruments can be used with a second display so that the same information is available in two locations. There are also LCD display-only units that can receive inputs from various navigation devices, switching from one to another as desired. The concentration of all data in a single monitor is a great space saver on crowded helm consoles. Such devices can even be used to display the output from a TV set, VCR, VDR, or DVD player.
Figure 16-53 Many items of electronic equipment can be connected by a network so that information can be shared. It is advantageous, for example, for a fishfinder to also display position data and course and speed information. A radar display may share space on a chartplotter, and many other combinations are possible.
A MULTI-FUNCTION DISPLAY (MFD) is useful and convenient even on a large boat with ample room for navigation equipment at the helm, but it’s even more valuable on a smaller boat where there might not be enough room for an array of separate instruments. The multi-function display is a result of total system integration, offering high-speed, high-resolution images and information on large screens controlled by a combination of easy-to-use touchscreens, keypads, and rotary dials. Many new MFDs come with a chartplotter that can be split into as many as four screens simultaneously, showing, for example, a GPS navigation screen, a radar screen, a satellite weather screen, and a sonar screen.
Exchange of information among devices in a network requires the use of a standard communication protocol, a common language. The National Marine Electronics Association (NMEA) established an initial standard, NMEA 0183, when the first marine data network systems became available. A “single-talker/singlelistener” system, it was adequate for most installations; however, the limitations it imposed on the number of users and on data transfer speeds became unacceptable as increasing numbers of devices appeared in even small systems. A lack of strict standardization of data formats and the relatively uncontrolled development of new versions of NMEA 0183 created the need for a new, higher-speed, disciplined communication system. The NMEA met this need with NMEA 2000, a multi-talker/multi-listener system that imposes strict requirements on all users and assigns unique addresses to each device in a network. NMEA 2000 makes it possible to display instantaneous information about everything from exhaust temperature to engine oil life.
Figure 16-54 A multi-function display with a vector graphic representation of the course ahead on the left screen and radar and fishfinder displays splitting the right screen.
Split screens even make it possible to view both raster and vector charts next to each other; for a description of electronic charts, see Chapter 15. Raster charts display information from NOAA and British Admiralty charts that has been digitally processed; they look like the paper charts you are used to seeing, with the same scale and depth datum as the paper chart. A problem is that when you zoom in or out on a raster chart, the typography stays the same size as it was originally, often making it hard to read, although on some sets you can switch to a different chart of a harbor or smaller body of water so that you do see a clear display. A vector chart, on the other hand, has information arranged in layers that you can turn on and off. It may have aerial photographs of the harbor, for example, information about local marinas, repair services and even waterfront pizza joints. Some have three-dimensional views of the waterway ahead, or even suggest the best route to follow. Vector charts, with all their information, can be more versatile than the more traditional raster version.
Manufacturers have used NMEA standards to send data to and from electronic units for some time, but more recently they have also developed proprietary video networks based on Ethernet protocols as used for computers in offices and homes; improved connectors are used to meet marine conditions. The various devices are connected to a hub as in small businesses and homes with several computers. Most Ethernet systems use standard RJ 45 connectors; however, waterproof connector housings may be used in place of the hardware commonly used in an office environment. Proprietary networks usually have a trademark name. They are quite satisfactory in use, but may require all devices to come from one manufacturer. Networks can start small, with only a few devices, and grow as new ones are added.
Figure 16-55 A multi-function unit mounted on a smaller boat, with a vector chart display on the left screen and course information and a radar display on the upper and lower right.
AUTOPILOTS
Autopilots are becoming more and more common on small craft because they offer two considerable operational advantages and at least one safety advantage. But first, what is an autopilot? It is an electronic device using mechanical or hydraulic power to steer a boat (or ship). The name is somewhat unfortunate as the equipment does not perform any true “piloting” functions, such as determining position or deciding what course to steer to a destination. A better name might be an “automatic helmsman.” An autopilot is basically a device that will ensure that a boat is maintained on a preset heading. More advanced models can receive positional information and adjust that heading to make good a desired track. An auto-pilot can be an important adjunct to the navigator’s tools—especially on a shorthanded boat—and thus autopilots are included in this chapter.
The operational advantages are: (1) the person at the helm is relieved of the job of constantly steering the boat, which can be tiresome and tedious (and cold, wet, and uncomfortable at times on some craft); and (2) the autopilot will in many, if not most, cases do a better job than a human for maintaining a desired heading! The safety advantage is that the person at the helm will have much more time to act as a lookout for other vessels and hazards to navigation.
The heading sensor is usually an electronic compass of the flux-gate type, which should be installed in a stable location (usually low in the hull) and well away from varying magnetic influences (steady influences can be compensated out by the autopilot’s computer). Data from this unit is sent to the central processor, which can be located anywhere that is convenient. From this electronic “brain,” signals are sent to a mechanical or hydraulic drive unit to turn the rudder(s) in a direction that will correct a heading error. To make control more effective and smoother, a rudder angle sensor will provide “feedback” to the control unit.
Sailboats may use a masthead wind vane to maintain a steady heading with respect to the apparent wind, and may be able to automatically take up a new heading that is 100° (or other preset value) greater or lesser following a tacking maneuver. Advanced models can “learn” the handling characteristics of the boat and its reaction to wind, currents, and waves.
The input power requirements are moderate and usually no problem. The convenience of having an autopilot will be appreciated by all who stand a watch at the helm. Although sometimes it will be necessary for a human to steer—in a significant following or quartering sea, for example—you will be amazed and satisfied as to the large percentage of the time that an autopilot can steer your craft.
As noted above, many autopilots will accept positional information in the form of CROSS-TRACK ERROR from a GPS receiver. This will alter the desired heading to compensate for offsetting influences such as currents or crosswinds. A DODGE feature will allow the preset heading to be overridden temporarily to avoid an obstruction in the waters ahead; the prior heading will be resumed when the dodging action has ended. A handheld remote, wired or wireless, makes it possible to control the autopilot when away from the helm.
Various alarms are included in many autopilots. An OFF-COURSE ALARM will sound if the vessel gets more than a preselected distance from the established track. A WATCH ALARM can be set to sound an audible signal at regular intervals until manually turned off—no more sleeping undisturbed at the helm! Some autopilots can be used in a POWER-STEERING MODE without the heading sensor being active.
Figure 16-56 Despite the suite of navigation electronics integrated on the helm console of this power cruiser, the ship’s compass (partially obscured by helm) retains its central location.