Figure 19-01 The importance of electrical power on board boats has increased enormously in recent years. Every boater should understand the fundamentals of electricity, and a skipper who cruises away from his home waters should be capable of diagnosing common problems and making at least temporary repairs.
Fundamentals of Electricity & Electrical Circuits • Storage Batteries • Shorepower Alternative Energy Sources • Electrical Equipment on Board
Electricity has become an essential ingredient in boating. Originally used for engine starting and lighting only, electrical power is now used for many tasks—for running communications and navigation equipment, for cooking, for air conditioning and heating, and for such labor-intensive chores as raising the anchor and lifting the tender aboard.
Nowadays we take for granted the use of many electrical appliances that formerly would never have been afloat. More importantly, we place a greater reliance on electronic navigation and communication devices that must be reliably and constantly supplied with electrical power as a matter of safety rather than convenience.
Acquiring, storing, and delivering this power has become a complex matter, necessitating a knowledge of basic fundamentals, applicable standards, and an awareness that the boat’s batteries are under constant demand.
ELECTRICAL and ELECTRONIC are terms often used in an overlapping manner. In this book, “electrical equipment” includes such devices as motors, alternators, and lights. “Electronic equipment” refers to devices using transistors and integrated circuits—for example, radios, depth sounders, radar, and radionavigation systems. This chapter focuses on electrical systems, whereas Chapter 20 concentrates on radio, emergency, and satellite communications and Chapter 16 introduces the use of electronics in navigation.
GENERATING & STORING ELECTRICAL POWER
There is little need for a boater to delve deeply into the physics of electrical power—after all, most of us deal with electricity perfectly well at home without first undertaking a course in electromotive forces. Nevertheless, some fundamental background is helpful; with the convenience of electrical power aboard boats come certain potential hazards and maintenance problems.
CAUTION FOR BOATERS
Just as on shore, 120V or 240V AC electrical circuits on boats can produce a harmful, even deadly, electrical shock if improperly installed. This hazard exists not only to persons on board the craft, but also to people swimming in the water in proximity to an improperly wired or inadequately maintained vessel. If you do not possess a good degree of technical competence in this area, it is suggested that you always employ a fully qualified marine electrician to accomplish major installations and repairs to your boat’s electrical system, especially the 120V or 240V AC circuits.
Basic Electrical Terminology
Electrical systems can be described in terms of the type of electricity used and the voltage level used.
Direct & Alternating Current
The terms “DC” and “AC” are used by just about everyone, but usually with little recognition of the differences between them. DIRECT CURRENT flows in one direction only and is substantially constant in value; DC is used on boats at a relatively low voltage similar to how it is used on automobiles and trucks, typically described as “12 volts,” although actually between 12 and 14; see Figure 19-02. Larger craft may use 24- or even 32-volt DC systems. DC can be applied to a reversible electrochemical CELL, storing or accumulating chemical energy until the reaction is reversed, delivering electrical energy. Each cell in a boat’s lead-acid storage battery delivers 2.1 volts when fully charged. Six cells connected in series and housed in a common container create a “12”-volt battery. The amount of energy stored depends on the physical size of the cells. A Group 27 battery is shown in Figure 19-03.
Figure 19-02 The basic source of electric power on boats is an alternator, belt-driven from the propulsion engine. The alternator produces AC current, which is rectified by internal diodes to provide 12- or 24-volt DC current for the boat’s electrical system.
While each wire in a DC circuit is always either positive or negative (+ or –), the polarity of the wires in an AC current system alternate between plus and minus at a regular interval called the AC FREQUENCY. The standard AC frequency in North America and most of South America is 60 hertz (often called 60 cycles), with 50 hertz used elsewhere. The advantage of AC power is the ability to use a TRANSFORMER to change its voltage upward or downward as needed for each application. AC power systems on boats typically operate at either 120 or 240 volts, with the higher voltage able to deliver four times as much energy using the same size wires as a 120-volt system. Electric motors and some other devices are frequency sensitive. Operating 60-Hz devices from 50-Hz power may cause improper operation, overheating, and irreversible damage.
SHOREPOWER (electrical power brought on board a boat from any source on land) is always alternating current, which can also be produced by an engine-driven high-voltage alternator (GENSET) or a DC to AC INVERTER. Power supplied by storage batteries is always direct current. However, it should not be assumed that, although a vessel’s AC and DC systems are isolated from one another, there is never an interchange of power between them. Quite to the contrary, the AC power brought on board from a shorepower connection, and that produced by an onboard genset, can be changed by a battery charger into DC and used to recharge a vessel’s storage batteries. Conversely, DC drawn from the ship’s storage batteries can be changed into AC by an inverter and used to power the vessel’s AC distribution system, within the power limit of the inverter; see Figure 19-04.
Figure 19-04 Electrical power for 120-volt AC devices can be obtained from the boat’s low-voltage DC system with the use of a DC/AC inverter. The maximum power is determined by the rating of the inverter and ability of the batteries to supply the current, and is usually limited to about 2,800 watts on a 12-volt system. Some inverters can function in reverse as battery chargers, operating from AC power.
Electrical Units & Calculations
Conceptually, the behavior of electricity flowing in wires can be likened to that of water flowing in pipes. The units of electrical measure are likewise analogous: VOLTAGE is analogous to pressure in a water pipe. An AMPERE (commonly called an amp) measures the amount of current that is flowing, which is comparable to the rate of flow of water moving through a pipe. And OHMS measure resistance, which can be visualized as corresponding to the friction that is generated when water flows through pipes.
Figure 19-03 For times when the engine is not running, and the alternator is not producing electricity for the boat’s needs, electrical power is stored in batteries. Groups of 2-volt cells are connected in series to form a battery providing the desired voltage. Shown here is a 12-volt battery consisting of six cells. In higher-voltage systems, batteries themselves may be connected in series, such as four 6-volt batteries in a 24-volt system.
There is a simple equation—Ohm’s Law—that explains the relationship between current (I), voltage (E), and resistance (R),
Current flow is directly proportional to voltage and inversely proportional to the resistance in the circuit. In other words, the voltage drop in a circuit (E, in volts) equals the current flow in the circuit (I, in amperes, abbreviated “A”) times the resistance of that circuit (R, in ohms).
E = I x R
WATTS and KILOWATTS (1,000 watts = 1kW) are measures of power—the rate at which energy is consumed or generated. Using the water analogy, power compares to the volume of water moving through a pipe, and energy to the total volume of water that flows in a stated period of time.
Another simple equation expresses the relationship between watts, volts, and amps:
Watts = Volts x Amps
This will be useful when we need to determine the electrical needs of a system we might want for a particular vessel.
In DC systems, power requirements are commonly stated in amperes, and energy consumption in AMPERE-HOURS (AH).
Onboard Electrical Systems
The onboard source of electrical power can be divided into three broad categories:
• Power stored in batteries.
• Power brought aboard a vessel from landlines via a shorepower connection.
• Power generated as needed and used.
Associated with each of these categories are one or more distinct, identifiable supply subsystems. For instance, electrical power that is created on board a boat as needed may be produced by alternators that are belt-driven by the vessel’s main propulsion engine or engines. Often it may also be produced by a 60-Hz alternator driven by a separate gasoline or diesel engine (a genset).
Supply subsystems feed power to the vessel’s electrical delivery systems, which consist of a network of distribution panels, bus bars, circuit breakers (or fuses), switches, wiring, outlets, and the like.
AC System Voltages
Not only does electrical current differ in terms of being direct or alternating; it differs in terms of voltage. Appliances designed for a specified voltage or voltage range must not be used on systems having other voltages. Most modern electrical equipment is designed to cope with a modest range of voltage, but it is always important to check and comply with the manufacturer’s specifications for that range. Otherwise, irreversible damage to the equipment involved may occur, and personnel may be put at risk.
A system using a higher voltage has the advantage that, for a given wattage demand, it does not require as heavy wiring as at a lower voltage. Moreover, higher voltages are better at overcoming localized resistances, for example, minor buildups of corrosion at connecting terminals.
When high wattages are involved, the smaller wires that are practical with higher voltages are advantageous in terms of cost, weight, and the physical difficulties of installation. (Smaller-diameter wires are lighter and more flexible, making them easier to snake through the con -stricted passages available on boats.) This is why high-demand electric cooking ranges and clothes dryers are most often designed to operate on 240-volt AC, rather than the 120-volt electricity used for light bulbs, stereos, TVs, and the like.
ELECTRICAL STANDARDS FOR BOATS
When it comes to understanding and following various standards of construction and recommended practices, there is no more important area for standards than that of the vessel’s electrical systems. If improperly installed or poorly maintained, these systems can pose not only fire and explosion as well as corrosion hazards, but also the potential for electrocution.
The United States Coast Guard (USCG) has certain basic electrical standards that are applicable to gasoline-powered vessels and that speak primarily of ignition protection (reduction of explosion hazard) and overcurrent protection (to avoid overheating and fires). A review of these standards will show that they are very basic and provide little “practical” information on vessel wiring other than for these two purposes.
The National Fire Protection Association (NFPA) publishes Standard No. 302 for Pleasure and Commercial Motor Craft. This covers a wide range of various topics with respect to fire protection, especially in the areas of electrical circuits and lightning protection.
However, by far the most comprehensive standards available today are those published by the American Boat and Yacht Council (ABYC—refer to Appendix B). In covering almost every major system on a vessel, the ABYC Standards go into great detail on the subjects of DC and AC electrical systems, battery chargers and inverters, lightning protection, cathodic protection, and others. The ABYC Standards also have been used for the development of various international standards being adopted in Europe.
At dockside in North America, you’ll find 120-volt AC power and often also 240-volt AC power. Actually, these voltage designations are nominal—the designed voltages of shorepower will vary from 110 to 120 AC, and 220 to 240 AC, respectively. Moreover, in practice, shorepower voltage may drop well below these lower range limits. In Europe and many other parts of the world, nominal shorepower is 220- to 240-volt AC.
AC System Frequencies
Many items of equipment used on boats, such as battery chargers, microwaves, televisions, stereos, and fluorescent lights, contain transformers for the purpose of changing the voltage within the equipment. When operating on “foreign” power (such as 50 Hz), it is imperative that this equipment be “compatible” with the 50-Hz shorepower source. This may be ascertained by looking at the nameplate on the equipment and verifying that it says “50/60 Hz.” If this type of equipment is not installed on your boat, you have two choices: Either outfit with special equipment to be compatible or, when in foreign ports, operate this equipment only from the vessel’s genset in order to obtain the requisite 60-Hz power.
There are transformers that can step shorepower AC voltage up or down to match the voltage requirements of a vessel’s AC equipment. However, none of these change the frequency of the shorepower alternating current, so care must be taken when hooking up to foreign shorepower sources, even when you have managed to adapt to noncompatible outlet configurations. If a shoreline isolation transformer is installed on board the boat, it must be compatible with 50-Hz AC power if operation in foreign ports is anticipated where this frequency is likely to be encountered.
DC System Voltages
Direct current for shipboard use is generally 12 or 24 volts, with 12 volts most common (32-volt systems may be encountered on older boats). The 12- or 24-volt potential is obtained by connecting individual 2-volt cells together to form a battery (3 cells = 6 volts, 6 cells = 12 volts). A fully charged “2-volt” lead-acid cell produces a potential of between 2.10 and 2.13 volts. A fully charged 6-cell, “12-volt” battery will deliver between 12.6 and 12.8 volts. Although the main DC electrical system in a boat may operate at a nominal 12 volts, the starting motors on large diesel engines and those that power anchor windlasses and bow thrusters may operate from 24 VDC to reduce the amount of current they draw through the connecting wiring. Doubling the voltage cuts the current in half and reduces the voltage drop in the wiring.
Modest size 12-volt systems are most often powered from a 12-volt battery of a size that will provide the required amount of stored energy. The five most common sizes are Groups 25, 27, 31, 4D, and 8D and range in weight from 45 to 160 pounds. Systems that require a large amount of stored energy often use two large 6-volt batteries, each weighing about 120 pounds, connected in series to yield 12 volts.
Determining Electrical Needs
Electrical power is the lifeblood of all electronic equipment. Although a few devices may operate from self-contained batteries, most electronic equipment is powered from the boat’s electrical system. With outboards as small as 4 hp available with alternators, virtually any powered boat can be equipped with navigation lights and electronics, provided the total load does not exceed the charging capacity of the alternator.
The electrical power demand in a 12-volt system is usually stated in amperes. Longand short-duration loads are considered separately when determining the required energystorage capacity of the battery and the charging capability of the alternator. The total of the short- and long-term load currents is used to determine the required wire size and protective device ratings for the electrical power system. Required battery capacity is determined by adding the ampere-hours of energy required by each consumer (amps x minutes of operation / 60) during the period when no charging system is operating; for example, when at anchor. (Daily or 24-hour demand is often chosen as the default for required-capacity calculations.) A typical small cruiser or sailboat anchored out overnight might require 64 amp-hours. Two Group 27 deep-cycle batteries would be sufficient for this electrical load.
For 120-volt AC systems, the load is normally computed in watts. The wattage rating on an onboard item of equipment serves as an approximate measure of the maximum amount of power that the electrical item will require when operated. By adding together the maximum wattage requirements of all equipment that will be operated simultaneously, a vessel’s maximum requirements for AC electrical power can be determined. Wattage ratings also provide important information about the required sizes for wiring and the required ratings for protective circuit breakers and/or fuses. Gensets are normally rated in terms of kilowatts; shorepower cords are rated in terms of ampere capacity.
The amount of power required to operate all of the AC consumers can be expressed in watts (volts x amps = watts). This simple relationship works well for cook stoves, hair dryers, water heaters, and other RESISTIVE loads. For example: cook stove, 2,500 watts; + hair dryer, 1,500 watts; + water heater, 1,250 watts; total load = 5,250 watts (5.25 kW). However, motor drivendevices—including refrigeration and air-conditioning equipment and some battery chargers—demand more from their electrical supply than simply the watt rating on the nameplate. Motors are INDUCTIVE loads and can demand 3 percent to as much as 30 percent more current from the supply than the wattage rating indicates and up to 3 times the current when starting. This extra drain is called REACTIVE power and can significantly limit a genset’s ability to cope with the boat’s total electrical load.
Typical Boat Systems
Most small craft have a basic 12-volt DC system consisting of storage batteries, an alternator on each propulsion engine, and perhaps a battery charger for use with shorepower. Typically, the boat will have both a STARTING BATTERY for each engine plus one or more HOUSE BATTERIES that provide power for lights, pumps, radios, and other accessory equipment. The readily apparent advantage of such a system is that you can be assured of having adequate engine starting power no matter how much you use lights, fans, etc. These batteries have design differences. The primary characteristic of a starting battery is the ability to deliver large amounts of current for a limited time; a house battery should have the ability to deliver energy at a moderate rate over many discharge/recharge cycles. A battery undergoes a “deep cycle” when 20 percent or more of its stored energy is consumed before it is recharged; for maximum battery life, discharge should be limited to no more than 50 percent (voltage dropping to about 12.2 volts). DEEP-CYCLE BATTERIES can provide 200 to more than 3,000 use-cycles. Those that provide the longest cycle life are more expensive to buy but cost less per cycle. A starting battery used in cycling service will be worn out after about 50 cycles. Deep-cycle batteries can be used for starting gasoline engines and diesel engines up to about 400 hp.
A separate AC system connected to shore power or to an engine-driven AC alternator—i.e., a genset like the one in Figure 19-05—will be required to power equipment such as air conditioners and home-style appliances. Modest amounts of AC power, up to about 2 to 3 kW, can be obtained from a 12-VDC to 120-VAC inverter; refer to Figure 19-04. However, an inverter powering an electric toaster (1,000 watts) will demand a current of 90+ amperes from the batteries. The battery’s limited ability to efficiently deliver high currents makes it necessary to limit the length of time inverters are used to power even relatively modest AC loads.
Figure 19-05 When the demands for 120-volt AC electrical power exceed what can be reasonably drawn from the boat’s batteries using an inverter, such as for electric stoves and air conditioners, an auxiliary generating plant (genset) must be installed. It should use the same fuel as the craft’s main engines.
Storage Batteries
LEAD-ACID batteries store chemical energy. They release it when needed by converting the energy into electrical form. This is accomplished with use of an electrochemical reaction between two molecularly dissimilar metals, LEAD PEROXIDE (at the positive plate) and SPONGY LEAD (at the negative plate), that are immersed in an ELECTROLYTE (a weak sulfuric-acid solution).
Connecting an electrical load to the battery enables a flow of electrons from a chemical reaction in which the lead peroxide and the spongy lead are converted into lead sulfate. The process will continue, with constantly decreasing voltage and, therefore, current, until the battery is considered to be exhausted (although a voltmeter connected to an exhausted 12-volt battery will still indicate about 10.5 volts).
A discharged battery is recharged by supplying DC voltage to its terminals at a strength sufficient to reverse the internal chemical reaction that delivered the energy consumed during the discharge process. Lead sulfate at the positive plate reverts to lead peroxide, while at the negative plate it converts back to spongy lead; the sulfur that was removed from the electrolyte to form the lead sulfate is restored to the electrolyte. While the creation of lead sulfate is a normal and necessary part of the operation of the battery, its presence will shorten and eventually end the life of the battery if it is not converted back into lead peroxide and spongy lead.
To obtain maximum energy storage performance and longest possible life from a lead-acid battery, the average level of discharge should be limited to 50 percent of its capacity (12.2 volts with no load connected), and the battery should be promptly recharged after each use cycle.
CARING FOR FLOODED WET-CELL BATTERIES
There are five essential actions in the proper maintenance of wet-cell lead-acid batteries:
• Minimize or eliminate deep-discharge cycles.
Each time a storage battery is cycled (dis-charged/recharged), some of its useful life is lost. The electrochemical reaction that produces the electrical current causes the plates to gradually deteriorate because of active material lost due to mechanical stress and the inevitable accumulation of lead sulfate that has not been completely reconverted into lead peroxide and spongy lead. Eventually the loss of active material reaches a point where the battery’s energy storage capacity decreases to 50% of what it was when new. The battery is considered dead at this point.
• Utilize properly controlled charging procedures.
Battery charging must be precisely managed. Most battery chargers and some engine-driven alternators use a 3-step charging process: a moderately high-current bulk charge; a lower-current absorption cycle; and, finally, a float charge at very low current. Some shore-powered chargers can apply an “equalizing” charge, a higher than usual charging voltage, regulated at a low current, that restores all of the battery’s cells to an equal and maximum state of charge. Delayed or improper charging is the most common cause of premature battery failure.
• Maintain the correct electrolyte level.
You must check electrolyte levels regularly and replace any lost electrolyte by refilling with distilled water according to the manufacturer’s specifications. (By using distilled water you will avoid the introduction of foreign chemicals and materials into the electrolyte.)
• Keep the battery clean and its terminal connections corrosion-free and tight.
Regularly clean the tops of your batteries with a cloth dampened in a baking soda solution (the baking soda neutralizes any acid that has escaped via the cell vent caps). Be very careful not to let any baking soda solution get into the battery’s interior, as the solution will degrade the electrolyte.
Using a wire brush, keep the terminals and cable connectors clean and shiny; make certain the connections are tight. After cleaning and reinstalling the cable connections, coat the terminals with an appropriate battery terminal compound to retard corrosion. Do not use petroleum jelly, which tends to liquefy and seep into the connection, causing more harm than good.
• Provide good ventilation.
The wet-cell type of lead-acid batteries produces flammable hydrogen gas during recharging. And although hydrogen gas is lighter than air and quickly dissipates, it does represent a danger that should not be ignored. Batteries can explode, so the prudent skipper is especially careful when working around them. When dealing with terminal connections, take care to vent the battery space well, make certain that the batteries are not being charged at the time, and take care not to create sparks. Moreover, it is always a good idea to wear splash-proof safety goggles when you are dealing with batteries. For further safety, remove all jewelry.
Caution: If you accidentally come in contact with battery acid, wash it off immediately to avoid severe burns to your skin. If battery acid contacts your eyes, flush them immediately with cool water, and seek medical attention. Also remember that battery acid will eat holes in most fabrics.
Replacing the Energy
Energy that has been removed from a battery must be restored with the least possible delay to assure maximum reconversion of the lead sulfate formed during discharge into active energyproducing materials. Energy for recharging is supplied from a shorepowered battery charger or from an engine-driven alternator similar to the type used in vehicles; refer to Figure 19-02. Although alternators can deliver a modest amount of energy when the engine is idling, significant amounts of power usually require an engine speed of at least 1500 to 1800 rpm. The voltage supplied to the battery as it is recharged will eventually reach approximately 13.6 volts for a 12-volt battery.
A VOLTAGE REGULATOR manages the alternator’s output voltage to assure safe and efficient recharging of the battery by automatically adjusting the current flowing through the alternator’s field windings in response to measurements of battery voltage and temperature. Most stock alternators use internal regulators that are better suited for vehicles than for boats. Substituting an external, marine-grade voltage regulator can significantly improve battery system performance. When shorepower is available, a marine-grade battery charger can automatically maintain the battery at a full state of charge while supplying power to the boat’s DC power system; see Figure 19-06, for example. Battery charger installation should be in accordance with ABYC Safety Standard A-31.
Figure 19-06 This advanced-design battery charger, or power converter, will charge batteries fast and fully, but most importantly, it will protect the batteries from being overcharged by continuously sensing their state of charge.
Supplying electrical energy to a discharged lead-acid battery reverses the electrochemical reaction that produced the lead sulfate, changing most of it to lead peroxide and spongy lead, thereby restoring the amount of chemical energy stored in the battery to a level close to its initial state.
Gel-cell & AGM Batteries
The maintenance-free aspect and low selfdischarge characteristics of gel-cell and AGM batteries make them particularly attractive for use on boats. Both types rely on the same electro-chemical process as the flooded-cell variety; however, they use a somewhat altered electrolyte and different alloys in the plates. Cells are sealed; the pressure of hydrogen and oxygen gas emitted during charging increases to the level needed to force them to recombine into water, eliminating the loss of water from the electrolyte. The gelled electrolyte is immobilized, permitting the battery to be operated in positions other than upright (but not permanently inverted). Highest quality gelcell batteries can sustain up to a thousand 50-percent-discharge/recharge cycles.
Absorbed glass mat lead-acid batteries share many characteristics with the gel-cell, including sealed cells, similar choice of alloys in the plate structures, and electrolyte chemistry. Unlike the gel-cell, the electrolyte is a fluid; however, it is entirely absorbed within microfine layers of fiber tightly packed between the plates in each cell. The tight packing of large numbers of thin plates increases the area of active material exposed to the electrolyte, allowing the battery to deliver high discharge and accept high charging currents. Charging temperature limits for AGM and voltage limits for gel-cell and AGM batteries must be strictly observed to prevent cell overpressure and venting of gas and electrolyte, which, if they occur, will require replacement of the battery.
Gel-cell or AGM batteries are about twice the price of an equal size standard quality floodedcell battery and about equal in price to a premium flooded-cell battery whose useful life can extend to thousands of cycles.
MAINTAINING GEL-CELL & AGM BATTERIES
Gel-cell and AGM (absorbed glass mat) lead-acid batteries do not lose water from their electrolyte. A pressure-relief valve in each cell retains the oxygen and hydrogen gases emitted during the charging process; the increased pressure forces the gases to recombine into water. The valve opens, venting gas and electrolyte, only if the battery is charged improperly or suffers an internal failure. Any gel-cell or AGM battery that shows evidence of leakage must be disconnected and replaced.
The charging voltages applied to both gel-cell and AGM batteries must be carefully limited to the maximum specified by the manufacturer of the battery. Different types of batteries—floodedcell, gel-cell, and AGM—must not be charged in parallel. If either gel-cell or AGM batteries are substituted for previously installed flooded-cell batteries, it will be necessary to adjust the alternator voltage regulator and shorepower-operated battery charger to match the charging voltage requirements of the new battery. Most battery chargers provide charge settings for each type of battery. The internal voltage regulators in most alternators do not permit adjustment of the charging voltage, making it necessary to fit the alternator with an external voltage regulator.
Gel-cell and AGM batteries lose less stored energy due to internal chemical reactions (self-discharge) than floodedcell batteries and can be somewhat more tolerant of being left in a partially discharged state. However, like all lead-acid batteries, they perform best when kept fully charged, with per-cycle dis charge limited to 50% of capacity.
Gel-cell batteries offer the advantage that they can be tilted or even turned upside down without the risk of spilling electrolyte. This is an advantage for any boat, but particularly for sailboats.
Choosing Battery Size & Type
It’s extremely important to have the appropriate type and size of battery or batteries for the job at hand. Each discharge-charge cycle in a storage battery results in a degree of irreversible chemical degradation of its plates. The “deeper” the cycle—that is, the more the battery is discharged before being recharged—the greater the resultant chemical degradation. For that reason, deep-cycling is one of the major factors in—if not the primary cause of—premature battery failure. It’s necessary to provide a total battery capacity of at least twice the anticipated total amp-hour consumption per cycle to assure acceptable battery life.
However, capacity alone is not the sole consideration in determining what is the right size and type of battery. There are two basic types of lead-acid batteries: SLI (starting, lighting, and ignition) and deep-cycle. SLI batteries are designed to supply the short-duration, highcurrent draw of the engine-starting motor, after which the alternator recharges the battery. Deep-cycle batteries are designed to deliver moderate amounts of power for many hours, powering the boat’s lights, fans, electronics, and entertainment systems, often depleting half of the energy stored in the batteries before they are recharged.
SLI batteries (and many AGM batteries) are built using large numbers of thin plates to increase the total surface area of the active material in contact with the electrolyte, maximizing the available current flow. Although ideal for SLI service, the thin plate structure does not provide the depth of active material and mechanical strength needed to withstand the stress of repeated deep cycling. The AGM battery’s thin plates are tightly constrained between the electrolyte-absorption material layers and are therefore better able to withstand the stress of cycling service, typically providing a 300-cycle life.
Some so-called deep-cycle batteries are little more than relabeled starting batteries. So when buying batteries, check the specifications of similarly sized starting and deep-cycle batteries. For a given overall external size and weight, the deepcycle battery will have fewer but thicker plates.
ABYC Standard E–11, AC and DC Electrical Systems on Boats provides general design and installation guidance for both low-voltage DC (12- and 24-volt) and 120/240-VAC electrical systems. ABYC Standard E–10, Storage Batteries contains information about battery capacity, location, installation, and ventilation practices necessary to comply with both USCG and voluntary industry standards; see Figure 19-07.
Figure 19-07 Batteries must be installed so that they are firmly held in place. For all batteries, ventilation must be provided to prevent the build-up of flammable hydrogen gas, which tends to rise (even “sealed” batteries may vent if abused). Covers are essential to protect the terminals from being short-circuited should a conductive object, such as a wrench, fall on them.
Battery Testing & Charging
After initial selection, the second most important factor in battery performance and longevity is proper control of discharge/recharge cycles. Draining a battery below about half its full charge before recharging greatly accelerates the deterioration of its plates. So does leaving a battery in a partial or fully discharged state for long periods of time.
It is important to monitor the status of a boat’s DC electrical system and the voltage of each battery. Engines equipped with alternators supply at least a modest amount of electrical power even at idle speed and are monitored by voltmeters. The voltage indication may be close to 12 volts when the engine is idling, rising above 13 volts when underway.
Figure 19-08 The state of charge of a storage battery can be determined from its voltage. Specially designed voltmeters are used for this purpose; the lower part of the voltage range is collapsed so that readings can be taken more precisely around the normal voltage range. The switch selects the battery to be checked.
Although the typical instrument panel voltmeter provides some useful information, it is not accurate enough to indicate the state of charge of the battery. The special analog battery-condition voltmeters installed on many boats are designed to more precisely measure voltage over a limited range (often 8–16 volts), making it possible to determine the voltage of each battery within about ±0.2 volts; refer to Figure 19-08. More precise and complete information can be obtained from a digital voltmeter or a battery-condition monitor such as that shown in Figure 19-09.
A 12-volt, deep-cycle battery is considered fully charged when its no-load voltage is 12.6 volts or higher and fully discharged at 11.8 volts; 12.4 = 75%, 12.2 = 50% (the voltage at which it is desirable to switch to another battery or begin recharging), 12.0 = 25%, and 11.8 = 0%. The voltage should be checked when there is no load connected to the battery. The battery’s voltage indicates only its state of charge, not its energy storage capacity. The voltage of a fully charged battery nearing the end of its useful life will still be 12.6 volts or higher. While the only precise way to measure a battery’s remaining energy-storage capacity is with a special test meter, the way a battery cranks an engine or powers some other large load can be a useful indication of its condition.
Figure 19-09 A more precise and useful watch on battery condition can be accomplished with an electronic device designed for this purpose. It will show voltage, current presently being drawn from or charged into the battery, accumulated ampere-hours of discharge, and other data.
The specific gravity of the electrolyte in a leadacid battery varies with the battery’s state of charge. For wet-cell batteries, it is possible to measure state of charge with a hydrometer—in fact, for many years, this was the standard practice. (Using a hydrometer is still an easy way to identify a failing cell within a battery that is in otherwise good condition.) However, it is far more practical to use a voltmeter.
Your battery-condition meters may reveal that your batteries are not being fully charged when you run your engine. Or they may indicate that, while your starting battery comes to full charge, your ship’s service battery, which has been drained overnight by your lights, refrigerator, radio, etc., does not. Such a situation is common and is one of the leading causes of accelerated battery deterioration.
The voltage needed to completely recharge a battery depends on its temperature. The voltage regulator built into the alternator senses the temperature of the alternator and, in addition, senses the voltage at its output terminal, ignoring the voltage drop in the wires to the battery. The internal regulator is satisfactory in a vehicle where the battery temperature is closely related to the temperature of the alternator and the connecting wires to the battery are short. On boats, the typical problem of chronic undercharging of the batteries can be corrected by installing an external marine-service voltage regulator that senses voltage and temperature at the battery.
The energy-storage capacity and service life of a battery depends on how deeply it is discharged before being promptly and properly recharged from an external-regulator-equipped alternator or an AC-powered marine battery charger. The preferred recharge process will proceed in three steps: a “bulk” charge that restores about 75 to 80% of the energy removed; a lower-current, longer-duration “absorption” charge that completes the charge; and, finally, a low-current “float” charge that offsets self-discharge, keeping the battery fully charged. The voltage applied to, and the current flowing into, the battery at each step must match the battery type: flooded, gel-cell, or AGM. Both external-alternator regulators and AC-powered battery chargers are available with charging-mode selectors to match each battery type.
Many AC-powered battery chargers offer a manually selected or automatic “equalizing” charge mode that applies a higher than normal voltage at a controlled, low current to a leadacid battery. Used about once a month, the equalizing charge will restore the active material in each cell to the maximum possible degree. The equalizing charge is best done when the batteries are not being used to power incandescent lights and must never be applied to gel-cell or AGM batteries.
Multiple Charging Sources & Battery Isolators
On boats with multiple batteries and one or more sources of DC-charging power (alternators and battery chargers), it is often desirable to be able to use one of the charging sources to serve at least two battery banks. An example of this is where one alternator is used to charge both the cranking battery and the “house battery”; see Figure 19-10. (Other systems might have two alternators and more than two batteries.) In order to accomplish this, battery isolators are available from a number of sources. These are devices containing diodes that permit the flow of current in one direction only so that the output of the alternator or battery charger can be directed to both batteries simultaneously. The isolator prevents one battery from being charged or discharged by the other battery. It is important to note, however, that the use of isolators introduces a slight voltage drop in the charging circuit and, unless the charging device’s output is adjusted higher, a little less than full charging will occur to the batteries.
Auxiliary Generating Plants (Gensets)
In addition to alternators that are belt driven by a propulsion engine for the production of DC power, a vessel may have one or more auxiliary generating plants, known as gensets. These supply AC power in either 120- or 240-volt systems; the normal frequency is 60 Hz, but some units can be operated at either 50 or 60 Hz.
Gensets are not connected to the vessel’s drive train, but comprise a large alternator that is close-coupled to a dedicated engine, which may be either a gasoline or diesel unit, chosen to match fuel used by the propulsion engines. Engines must operate at precisely controlled speeds to ensure that the AC current frequencies are constant and correct. AC load-sensing systems can provide automatic start/stop operation. Monitoring systems check both engine and electrical system operation and will shut the unit down if a fault is detected. Sound shield enclosures significantly reduce noise level, but their presence must not be allowed to interfere with frequent visual inspection and maintenance. Out of sight must not equal out of mind.
The advantage of a genset is that it can supply power for large, house-type electrical loads, while at the same time allowing a vessel to be independent of shorepower connections. The capacities of most manufacturers’ lines of gensets typically start at about 2.2kW (2,200 watts) and extend up to 30kW and higher; refer to Figure 19-05.
Although the AC produced by a genset cannot be used directly to charge a vessel’s storage batteries, it is possible to wire the genset so that it powers the same battery charger that would otherwise be driven by shorepower. In this way, any genset capacity in excess of the vessel’s immediate needs can be used to recharge its batteries at times when its propulsion engines are not being run.
There are also DC gensets that can be run to directly charge the vessel’s storage batteries.
Alternative Energy Sources
Using a source of energy that does not require the combustion of fossil fuels can both extend a skipper’s cruising range and make him or her feel good for having relieved the environment of at least some hydrocarbon pollution. For boating applications, there may be some cost savings also from using the free energy of the sun and wind.
Solar Power
Sunlight can be converted into direct electricity by photovoltaic cells. The low voltage produced can be used to operate small electrical appliances—for instance, calculators and small ventilator fans. And a number of photovoltaic cells can be connected together in cased arrays called “solar panels” to charge, albeit in a limited way, a vessel’s storage batteries; see Figure 19-11.
Figure 19-11 Solar panels will provide small amounts of DC electrical power quietly and without the consumption of fuel. Flexible panels can be used in areas that are both convenient and out of the way.
In order to charge batteries, the voltage supplied by the solar-cell array must be about 3 volts higher than that of the boat’s battery system. In solar panels, an appropriate number of individual photovoltaic cells are, therefore, first connected in voltage-additive series. Then a number of these cell groups may be arrayed in current-additive parallel to provide greater amounts of energy; panels are normally rated in watts. It is necessary to install a regulator to control the charging of batteries.
Figure 19-10 Shown here is a basic wiring diagram for two batteries charged from a single engine alternator.
Unfortunately, it’s often difficult to find enough deck space for enough panels to provide sufficient current for most shipboard devices. And most appliances whose current demands could be so satisfied directly do not find their major use at times when there is sunlight to be converted. For example, a solar panel of practical size for, say, a 35-foot sailboat, could run an electric lamp. But the lamp would be needed most at times when there is no sunlight available to power the solar panel. For that reason, and because solar panels are still relatively expensive when measured against unit output, the practical onboard application of solar power is limited and usually relegated to supplementary charging of storage batteries.
The initial high cost of solar panels is partially offset by the fact that they require little maintenance other than occasional cleaning. They are available in several types at different efficiencies and costs. Solar panels are rated for the most favorable conditions: cloudless skies, perfect angle of incidence, sun well above horizon, proper ambient temperature, and clean panel surface. Significantly less energy will be developed in more typical conditions. The location of panels is usually fixed, but it is often possible to adjust their orientation so as to be more nearly perpendicular to the sun’s rays. Typically, solar panels will not produce enough energy to fully replace that used during a 24-hour period, but they will reduce the amount that must be obtained from other sources. Newly developed panels can provide enough power to batteries to drive an electric-powered cruising boat at 5 or 6 knots for 5 or 6 hours.
Wind Generators
In recent years, significant strides have been made in the manufacture of wind-driven generators for boats. Strong, ultralight composite materials have become available for rotor blades and other structural parts, and compact, efficient alternators have been developed. As a result, wind-driven generators are available in sizes and weights that make them practical for installation aboard yachts and other small craft; see Figure 19-12.
As an onboard source of electrical power, wind-driven generators offer important advantages over solar panels: They produce appreciable amounts of energy, and they work at night and on cloudy or rainy days. They are also less expensive than a typical group of solar panels. Most wind generators start to produce a charging current with winds of about six knots, with output at significant levels with winds of 15 knots. However, they also have disadvantages: They’re still relatively expensive for the amount of electrical power they develop, and they tend to be somewhat awkward looking unless their installation is carefully designed and executed. The whirling blades of wind generators annoy some people, but the location on a sailboat—up on the backstay or pole-mounted at the transom—is out of the way. They’re most useful at anchor, when a vessel is away from shorepower, lacks a genset, and is not running its propulsion engine(s). Consequently, they’re usually found on sailing craft, for if properly installed, they can be used underway when under sail alone. After all, there would be little point in utilizing a wind-driven generator on a powerboat underway, as the propulsion engine(s)-driven alternator(s) would provide many more times the amount of current at very little, if any, additional cost in fuel or engine wear.
Closely related, but seldom seen, are small hydrogenerators towed in the water astern. A small rotor revolves in the water flow much the same as the larger propeller of a wind generator; the craft must be making way through the water for power to be generated.
Figure 19-12 Though noisier than solar panels, a wind generator, with its higher output in windy and less-than-perfect sunlight conditions, has become a symbol of electrical selfsufficiency among cruising sailors. They are even seen on some voyaging powerboats.
Fuel Cells
Well established as a source of electrical energy on shore and in space, FUEL CELLS are a promising future source of power on boats as a replacement for gensets. Electrolysis is the familiar chemical process of using electrical energy to break down water into hydrogen and oxygen. Fuel cells work on the reverse process, combining hydrogen and oxygen to produce electricity with water as a byproduct. They resemble batteries in that their DC electrical output is due to an electrochemical process, but they are producing energy, not storing it. Fuel cells get their oxygen input from a continuous stream of air. Hydrogen can be used directly, but is more easily and commonly obtained from fuels such as methane and methanol. Although hydrocarbons are used, there is no combustion and no polluting exhaust. Fuel cells are also unlike batteries in that their active elements are not consumed by the chemical reaction, and thus have a much longer service life. A fuel cell power module that uses methanol as its fuel source and delivers DC power at a voltage suitable for charging 12-volt batteries at the rate of 140 Ah per day sells for about $8,000. Virtually silent in operation, fuel cells may become compellingly attractive if the price drops significantly.
Transforming Power
Electrical appliances have fairly specific power requirements. For instance, a 24-volt DC electric motor will not run properly on 12-volt DC current, but instead will run too slowly, overheat, and possibly even burn up. Similarly, 24 or 40 volts fed into charge a 12-volt battery will overcharge and destroy it. Further, AC appliances will not operate on DC, and vice versa.
Some DC electronic equipment and other appliances are equipped with internal power circuitry that tolerates input voltages from, say, 10 to 40 volts; but this kind of tolerance does not generally extend to AC/DC appliances, except in today’s on-board refrigeration systems and some other units that have the capacity to choose a higher voltage if both voltages are available (a refrigerator will choose 120 AC over 12 DC). And therefore, at times when the frequency and voltage characteristics of source currents do not match requirements of the equipment to be powered, it is necessary to employ a transformer (AC), an inverter (DC to AC), or a DC-DC converter.
Transformers
When a transformer is used simply to step up or step down the voltage of an AC system, without providing complete isolation from the shorepower system, it is known as a POLARIZATION TRANSFORMER. If the transformer is connected so as to provide complete galvanic isolation from the shorepower system, then it is known as an ISOLATION TRANSFORMER. ABYC Standard E-11 contains a great deal of additional information on the subject of both of these types of transformers.
Transformers are most frequently used to step the voltage of AC systems either up or down, but in some circumstances they are used without voltage change in order to isolate a vessel’s onboard power system from a shorepower system. They can be so used because transformers magnetically couple their input and output circuits, creating current in the latter by induction rather than by a direct conductive connection.
Inverters
If a vessel is wired for 120-volt AC or higher voltage, it will likely have onboard appliances that are designed specifically for these voltages, which the crew will undoubtedly want to continue to use when away from a shorepower hookup. If the vessel has an auxiliary generating plant of sufficient capacity, then the necessary supply of 120- or 240-volt AC power is no problem. However, there are cases in which the boat is too small to accommodate a genset comfortably, and there may be times when the crew wants 120-volt AC without the additional noise and vibration of a running genset—which is always present to some extent, however well silenced and resiliently mounted the unit is. And in such instances, an INVERTER really shows its advantage.
The potential difference between the two wires in an AC power system varies continuously at a fixed rate: 60 times a second (60 Hz) in North America and much of the Western Hemisphere. The transition of the voltage from maximum positive to maximum negative occurs smoothly, following a sine wave pattern. The power delivered by most marine generator sets and the AC power from “sine wave” inverters conforms very closely to an ideal sinusoid and can efficiently power voltage waveform-sensitive loads such as required by the AC motors in marine air-conditioning systems. Inverters are also available that deliver “modified sine wave” power, a non-sinusoidal waveform that is less expensive to create than a pure sine wave and is suitable for powering many (but not all) of the AC consumers on boats. Devices that will operate satisfactorily from modified sine wave power include all resistive loads (hair dryers, toasters, coffee makers), and most small-motor-driven appliances, including AC-powered fans, most refrigerators, and small air conditioners. Consumers that may not work properly on modified sine wave power include some variable-speed power tools, sewing machines with microprocessor speed controls, certain medical equipment, and fluorescent lights with electronic ballasts.
Inverters are available with power ratings ranging from 20 to 150 watts: from an inverter module that plugs into a cigarette lighter outlet, to sine wave inverters that can continuously deliver more than 3,000 watts, to modified sine wave output units that can power 8,000-watt loads. Units designed for marine use are available in both pure and modified sine wave versions with automatic load sensing, built-in battery chargers, and automatic AC power transfer switches; see Figure 19-04.
The amount of AC power being delivered by an inverter determines the load being imposed on the boat’s battery, plus a small additional amount consumed by the inverter’s circuitry and a cooling blower (inverters are not 100% efficient; between 5% and 15% of the energy they draw from the battery is converted into heat). It is important to evaluate the impact of all electrical power consumption on a boat’s battery system. A small inverter used to power a TV set that consumes 100 watts of 120-volt AC power will place a 9.6-ampere load on a 12.2-volt (50% charged) battery. This DC power load will allow the TV to be used continually for about 4 hours from a typical, fully charged Group Size 27 deep-cycle battery. The DC current from a battery to an inverter powering a 5,000 BTU air conditioner will be in excess of 45 amperes. Operating a 1,500-watt hair dryer from the inverter will require a current of more than 136 amperes. Virtually all the power you may need to operate smaller AC appliances can be supplied from often surprisingly inexpensive inverters; however, supplying more than a modest amount of power or a large amount for a short time will require very large batteries or a continuous supply of 12-volt DC power from an engine-driven source.
Vessels equipped with shorepower systems can take advantage of combination inverter/battery charger units that will automate the boat’s AC power system. Equipped with automatic shorepower sensing circuits and switches, these units will deliver AC power from the on-shore source whenever it is available, automatically switching to supply power from the inverter if shorepower is not available. The system constantly monitors the state of charge of the battery and will warn if the energy remaining in the battery has decreased to a predetermined level and automatically shut off the inverter to protect the battery from excessive energy drain. The unit’s built-in, three-stage battery charger will automatically maintain the boat’s batteries at full charge whenever shorepower is available. Some units provide five battery charging “stages”: bulk, absorption, float charging, equalization (permissible only for flooded-cell batteries), and battery saver, an operating mode used when there will be little or no drain on the battery for a period of time. In this mode, the normally continuous float charge is interrupted so that no energy is supplied to the battery, avoiding loss of water from the electrolyte. When operating in this mode, the charger monitors the battery voltage, automatically restoring the float charge as necessary to maintain the battery at full charge.
Proper installation of powerful inverter/charger systems should only be undertaken by skilled personnel familiar with the requirements set out in ABYC Standards A-31 and E-11.
DC-DC Converters
Devices that take a higher DC voltage, such as 24 volts, and reduce it to a lower DC voltage, such as 12 volts, are called DC-DC CONVERTERS. These are especially useful in powering certain items of electronic equipment that are generally available only in 12 volts when used on board vessels with 24-volt DC electrical systems. The DC-DC converter is preferable to a simple “dropping resistor” in that its output voltage is maintained at a constant level regardless of the connected load current.
Connecting to Shorepower
The shorepower connection is sometimes derided as the boat’s “umbilical cord.” This is because some boaters (basically, those without gensets or inverters) are loath to break the shorepower connection and give up the convenience of “house-type” AC devices. The very nature of this disparagement indicates just how integrated the shore-to-ship power connection has become in boating today.
In North America, the voltage of shorepower is usually nominal 120, but may in some cases be nominal 240, so diligent care needs to be taken at all times in hooking up and using such power. Shorepower connections should be made only to properly wired and protected outlets.
Further, the vessel should be equipped with proper shorepower cords (with lock-type fittings and water-shedding boots and collars), appropriate inlet fittings, and a correctly configured main distribution panel with circuit breakers. The design of proper cords and inlet fittings ensures that neither skipper nor crew will ever handle a “live” cord with exposed (male end) prongs—as that is a certain invitation to potentially fatal electric shock.
In addition, it is advantageous if the vessel’s distribution panel has both a voltmeter and an ammeter to measure the incoming power in order to avoid overloading of the shorepower cord and connections and the shore circuit.
Figure 19-13 Shorepower cords have a male connector at the landward end, shown here in the far-left column. The boat end is female and has the same wiring arrangement, which permits two cords to be connected end to end if necessary for a longer run. Sometimes, however, an adapter is required to fit between the power cord and a dissimilar shore outlet, such as the combinations shown at the bottom left. The configurations shown are for North America; electrical service in foreign locations may be different.
Properly wired AC outlets are found in a wide variety of configurations, each associated with particular combinations of voltage and current. For example, the size and pattern of 120-volt outlets with 15-, 30-, and 50-amp current capacities, respectively, differ from one another. And they all have configurations different from those of 240-volt outlets; refer to Figure 19-13.
Moreover, because wiring codes are not consistent across the Unites States, and certainly not across international borders, the outlets at a strange wharf or marina may not match the end on your shorepower cord. Therefore, if you travel far afield and wish to connect to shorepower, it may be necessary to have aboard a variety of ADAPTERS that will allow connection of your shorepower cord to different types of dock outlets; refer to Figure 19-13.
CHECKLIST FOR USING ADAPTER
Whenever using adapters, the following three actions must always be followed:
• Outlet and adapter must be of the same voltage rating.
• Total amperage drawn should never be allowed to exceed the amperage of the lowest-rated component of the connection.
• Polarity and grounding connection must be maintained.
Regardless of what adapters you must use in order to accommodate the dock outlets, always ensure that the grounding conductor (green wire) is connected to an outlet or metallic device that truly provides an effective ground. Many, but not all, adapters will do this internally. All too often one sees adapters where a grounding (green wire) “pigtail” is connected via its clip to a piece of plastic conduit or water pipe, or just left dangling. The grounding conductor is essential for the safety of persons on board and those in the water surrounding the vessel whenever the vessel is connected to shorepower; never sacrifice the grounding conductor’s integrity.
There are also adapters available (or which can be made up using proprietary, off-the-shelf parts) that can be used to split or merge circuits. For instance, if your boat has two 30-amp inlets but the shore outlet is a single 50-amp unit, a properly wired adapter can likely be used to split the 50-amp outlet into connections for two 30-amp cords. Or if your boat has a single 50-amp inlet and the shore two 30-amp outlets, the right adapter can be used to merge the two 30-amp outlets into a single 50-amp cord. There are adapters available that will match a 50-amp shore outlet to a 30-amp boat inlet without splitting. Conversely, there are adapters to match a 30-amp shore outlet to a 50-amp boat inlet, or a 15-amp shore outlet to a 30-amp boat inlet. It’s important to remember that there may not be sufficient current available in such a hookup to run highdemand appliances that may be on board.
Another way of protecting yourself and your boat’s electrical system is to verify the electrical connections on shore before hooking up. This is done with an outlet circuit tester; see Figure 19-14. Selling for just a few dollars, the instrument employs a combination of colored LEDs to indicate the following situations: correct wiring, open ground wire, open neutral wire, open hot wire, hot and ground reversed, and hot and neutral reversed. Some power cords now have built-in LED lights that, when the shore end is plugged in, will tell you whether the connection is safe or not.
Figure 19-14 The circuit tester shown here is one of many available types that can quickly diagnose a faulty wiring situation.
DISTRIBUTING ELECTRICAL POWER
It is quite obvious that electrical power generated, stored, or brought aboard via a shorepower connection is of little value unless it can be distributed properly to various onboard appliances. Yet while much time, attention, and money are frequently spent on the source of onboard electricity, the distribution side of the system is almost as often given short shrift. And that’s a shame, because relatively modest efforts and expenditures in this area can go a long way toward ensuring the reliable and safe utilization of onboard electrical power.
Caution: Whenever 120-volt or 240-volt AC systems are installed on board, there is the possibility that, due to the failure of equipment or insulation somewhere in the future, AC voltage can inadvertently be applied to the vessel’s DC system. When and if this occurs, portions of the DC system, especially the engine and its propulsion system (which is connected via the battery negative), can now be energized at a potential of 120 volts AC above “ground.” This poses an imminent danger, not only to persons on board who may inadvertently contact such equipment but also to swimmers in the water, especially in freshwater areas, as they may be in the potential gradient or “field” created around the vessel.
In order to guard against this occurrence, the ABYC and NFPA Standards require the connection of the AC system grounding wire (green) to the DC system negative (ground) on board the vessel. This connection must be made on all vessels equipped with shorepower, other than those utilizing isolation transformers. The location of this connection should be ascertained, and its integrity checked on a regular basis. In years past, there have been articles written about disconnecting this circuit or “cutting” the green wire to shore to reduce the incidence of galvanic corrosion on board. This is not only improper but dangerous. There are other ways to combat galvanic corrosion when using shorepower; these are covered later in this chapter.
Main Switches
The 12- or 24-volt DC and the 120-volt AC (or 240-volt AC, if applicable) subsystems should each be controlled by its own main switch that will cut power off entirely. This is an important protection, both for someone working on the system and in the event of an electrical or other fire, when it can be vitally important to be able to power down quickly and positively.
The one exception to this rule is the automatic bilge-pump circuit. This circuit should be wired so as to remain active even if the main switches are turned off—especially if it’s your practice, as it is with many boaters, to turn off the main switches whenever the boat is left unattended. That way, the protection of automatic bilge pumping will not be lost.
The main switch for the higher voltage subsystems (120 and 240 volts) will customarily be incorporated in the main distribution/circuitbreaker panel. However, since the 12- or 24-volt systems supply current to the engine starter(s), the main switch for these circuits necessarily has to carry very high currents (often 300 amps or more); consequently, it is imperative that the switch be of robust construction. The low-voltage main switch is, therefore, sometimes separated from the main distribution/circuit-breaker panel.
Main battery switches are often constructed to control two separate batteries or “banks.” Two or more batteries permanently wired in parallel can be referred to as a bank, or all of the batteries on a boat may be referred to as the battery bank. A typical switch will be labeled: Bat #1, BOTH (or ALL), Bat #2, OFF; see Figure 19-15. If, as is common, the selector switch is used to connect a battery to the engine’s starter, it should be set to either Battery #1 or Battery #2 when starting the engine. Using a single battery to start the engine will disclose a weakness in a battery that would go unnoticed if two batteries were used for starting. Switch to BOTH only in very cold conditions or if one or both batteries are weak.
Protecting the Alternator
The low-voltage main switches are often also wired to control the propulsion engine charging circuit, that is, starting current runs from the storage batteries to the starter(s) according to the setting of the switch, and charging current from the engine alternator(s) runs back to the battery or batteries according to the switch settings. Such an arrangement affords the advantage of being able to direct charging current to bank 1 or 2, or to both; see Figure 19-15.
Figure 19-15 A battery selector switch provides positive battery disconnect, isolates all circuits, and also protects against the hazards of electrical fire and explosions. It can be separate or combined with the circuit breakers for individual circuits.
The alternator on an operating engine must always be connected to a battery to prevent the alternator from producing high voltages that will destroy its internal rectifier diodes. Alternators must be “excited”—connected to an external voltage source (usually through the engine’s key switch)—to establish the magnetic field required for the generation of electrical power. Battery switches labeled “Alternator Protected” interrupt the alternator excitation power as the selector is moved to the “Off” position, protecting the alternator from damage if the “Off” position is selected with the engine running.
However, most battery switches carry a warning label: “Do Not Turn OFF When Engine Is Running” and are “make-before-break” switches. A battery is always connected to the alternator except in the “off” position. Make-before-break switches may therefore be moved from one battery to another when the engine is operating without risk to the alternator. Additional security for the alternator can be obtained by installing a solid-state voltage-limiting device (usually called a Zapstop) on the alternator. Connected between the alternator’s output “S” terminal and the alternator’s ground “G” terminal, the device will prevent excessive voltage in the event something interrupts the connection to a battery (including a cable failure).
If the propulsion-engine charging circuit is wired in such a way that it bypasses the main switch and goes directly to the storage batteries, the above precautions may not be necessary. However, it is always desirable to be able to select individual batteries for use when no charging system is operating, reserving a charged battery for engine starting service. Battery charging can then be managed manually or with an automatic paralleling system.
The Distribution Panel
The electrical distribution panel houses the main power switches for the boat’s low-voltage DC and the AC power selector switches. It also houses the circuit breakers or fuses that protect the branch circuit wiring, which distributes power to all of the boat’s electrical equipment and convenience outlets. Switch-type circuit breakers serve both to control a circuit and to protect against excessive current flow in the circuit. The current rating of each of the branch circuit breakers or fuses is determined by the allowable current-carrying capacity of the wires used in that circuit. Substitution of breakers or fuses with devices rated for higher current than those originally installed can create a serious fire hazard.
Fuses provide overcurrent protection by means of the destruction of an internal metal strip that melts when a predetermined amount of current is drawn through it. Fuses are less expensive initially, but are a one-time-use device. Circuit breakers, although more expensive, perform the same function but are resettable after the problem in the circuit has been corrected. Except in the smallest, least expensive boats, circuit breakers are today almost universally used in preference to fuses, at least when it comes to main distribution panels and when currents of 5 amps or greater are involved. Virtually all DCpowered electronic equipment is supplied with some type of in-line fuse. The fuse serves to protect the connecting wiring, not the device itself, which may have additional internal fuses or other protective devices. Electronic engine management systems and computers are also protected by fuses.
Fuses of the correct size offer safe onboard circuit protection. However, fuses have several disadvantages as compared to circuit breakers:
• When a fuse blows, it is more difficult to replace it than simply to reset a circuit breaker.
It is dangerously easy to replace a blown fuse with an improper one of excess current-carrying capacity.
• If a fuse blows and no replacement is handy, there is the temptation to shunt it with a length of wire, a practice that often leads to fire.
• In damp corrosive atmospheres, fuse clips tend to corrode, offering a high-resistance connection that results in voltage drop.
The main distribution panel is used to distribute current to various onboard circuits. There may be separate panels for DC and AC, or these can be combined; see Figure 19-16. For each of these, heavy conductors are brought to a heavy bus bar on the panel. Branch (or sub-) circuits are then wired on the panel from the bus bar through an appropriately rated circuit breaker, which is often also used as a switch for that branch circuit. In this way, circuit faults usually result in the “tripping” of a breaker on the labeled main distribution panel, and are therefore easier to trace and correct. The use of a main distribution panel also facilitates checking the ON/OFF condition of various circuits. Trip-free circuit breakers are a recommended protective component for onboard application. These are designed so that the reset handle cannot be manually held to override the current interrupting mechanism—this makes “cheating” on the breaker impossible.
An Important Caution: When the lowvoltage and higher-voltage distribution panels are combined into a single unit or located in close proximity to each other, it is critical to make sure that no inadvertent cross-connections are made. Any such cross-connections would result in feeding 120- or 240-volt current into the vessel’s 12- or 24-volt system. This in turn could cause at least the destruction of low-voltage equipment. In the worst-case scenario, it could cause fire or the electrocution of a crewmember or a swimmer.
Figure 19-16 Shown here are front and back views of a combined AC and DC distribution panel. Note the careful labeling of each switch and circuit breaker. This panel also includes a transfer switch for changing from shorepower to the onboard auxiliary generating plant.
The Wiring System
If the battery and alternator are the heart of a boat’s electrical system, then the wires supplying power to the various electrical and electronic loads are the arteries and veins. The wires must be heavy enough—of sufficient cross-sectional area—to carry the current of the loads connected to the circuit.
Adequate wire size is determined by two factors: heating effect and voltage drop. Current passing through a wire increases its temperature; obviously, it must not become hot enough to become a fire hazard. As a general rule, a wire should not become warm to the touch when carrying its full load.
The voltage drop problem is the more common one. The heating losses mentioned above result in a lower voltage being delivered to the load than was put into the circuit at the battery. The voltage drop increases with an increase in the load, being directly proportional to the current in the circuit, and with an increase in the length of the wires.
The Right Size & Type of Wires
Many potential problems can be avoided by ensuring that all wiring is adequate in size for the amperage involved. Although it usually can be safely assumed that a boat will have the proper size and type of wiring when it is delivered from the manufacturer, it is highly likely that additional equipment and accessories will be installed in the craft throughout its active life. Electrical problems encountered with new gear are often traceable to the inadequacy of the original wiring. Thus a boat owner must know the correct wiring practices to support the added devices.
The higher the current (amperage) draw of a device, the heavier the wire that carries the current has to be. Insufficient wire size (diameter) means increased resistance. Excessively high resistance results in greater voltage drop (less voltage at the device) as well as greater heat. This results, at best, in a loss of efficiency (with power being lost in the conversion of electrical energy to heat), and at worst, in damage to electric motors and other appliances—even, in extreme cases, fire. Any wire, when subjected to excessive current, may overheat to the point of igniting adjacent combustible materials. It is for this reason that circuit breakers or fuses are utilized to limit the amount of current that can be loaded on a wire. Voltage drop also increases with the length of the circuit—the distance from the battery (or distribution panel) to the load device and back.
It’s important to keep in mind that wire sizes in North America are generally specified according to American wire gauge (AWG) standards, under which a lower number designates a larger diameter (for example, No. 10 AWG wire is heavier than No. 16 AWG). The system is established so that a change of three numbers, either up or down, changes the cross-section area of the wire by a factor of two; this doubles, or halves, the resistance of the wire (but odd numbers are generally not used in small gauges). A change of six numbers changes the area and resistance by a factor of four; as resistance decreases, currentcarrying capacity increases by the same factor.
When electrical wiring is in a marine environment, size is not the only consideration—wire type is also very important.
Wire with a solid conductor, or less than sixteen strands, should not be used on a boat because it is prone to fracture due to vibration, especially where the conductor may have been nicked when stripping the insulation for connection during installation. Instead, flexible, multistrand wire should be chosen for all systems.
It is recommended that pretinned copper conductors be used in boat wiring, even though it is relatively expensive. Conductor strands that are coated with tin resist corrosion much better than bare copper ones. That’s important not only at terminal fittings, where the copper is stripped of its insulation so that it can be inserted into crimp-type or other terminal fittings, but all along the conductor. The type of corrosion to which copper is subject in a salt-damp environment has a tendency to “creep” along bare copper wires, up under its protective insulation, and so plain copper conductor is potentially subject to severe and hidden corrosion when used in the marine environment.
The type of insulation is also important. In the engine compartment it should be a type that is resistant to oil, gasoline or diesel, and other petroleum products.
Recommended Wire Gauges
In DC systems, typically 12- or 24-volt, the voltage drop that occurs due to the resistance of the wire becomes a critical consideration for certain types of equipment. ABYC recommends that “noncritical” applications (for example, where the voltage drop does not seriously compromise the function of the equipment, such as motors, cabin lights, etc.) be limited to 10 percent. “Critical” devices, such as navigation lights, bilge blowers, and most electronics equipment, should have a voltage drop of not more than 3 percent. The ABYC E-11 Standards provide details and tables for these calculations; see Tables 19-1 and 19-2.
A simple check of equipment “as installed” can be made by using a DC voltmeter; first read the voltage at the fuse or breaker panel, then at the equipment itself. This must be done with all “normal” loads turned on and the DC charging source (engine or battery charger) operating. Additionally, the equipment under test must be running, in other words, pumping water with bilge pump or transmitting on the radio. If 13.2 volts were available at the distribution panel, then 12.8 volts (13.2 minus 3 percent of 13.2) should be the reading at the equipment for no more than a 3 percent drop.
Table 19-1 A 10 percent voltage drop can be tolerated by common electrical components. The American Boat & Yacht Council (ABYC) standards for noncritical loads, such as lights and pumps, are shown in the above table. All wires should be stranded to reduce the possibility of failure from mechanical fatigue.
Table 19-2 Electronic devices, such as radios and navigation equipment, are less tolerant of voltage drops in the wires supplying power to them than are common electrical loads—the ABYC standards call for a drop of not more than 3 percent. The necessary wire sizes are shown above for current drains and various lengths of wires.
Wiring Identification
In addition to having the right size and type of wires, the proper COLOR CODING of wiring should be observed without fail to facilitate the identification of circuits and the tracking of wires during troubleshooting procedures; see Table 19-3.
Identification should also be ensured with a complete wiring diagram that can be understood by a nonexpert. Enclose it in plastic and post it near the main switch panel. Keep it up to date as additions and changes are made. Proper terminal connections, bundling, and support of the wire runs should always be provided for in order to minimize the potential for failures due to corrosion (connections lying loose in wet bilges) and failures due to mechanical problems (connections pulling loose under the substantial weight of wiring or due to vibration).
Table 19-3 ABYC Standard E-11 provides a system of color coding for the identification of wiring on small craft.
Wiring & the Compass
Wiring that supplies DC current to the compass’s night-light and all other DC wiring in proximity to a vessel’s autopilot heading sensor is a potential cause of serious magnetic deviation. When carrying current, such wiring can be the source of magnetic fields that will interfere with the proper functioning of the compass.
Even worse, because electrical circuits are sometimes on and sometimes off, and because the magnetic fields they generate change with variations in current, the magnetic interference they produce is sporadic and variable. Therefore, this form of magnetic deviation cannot generally be eliminated by compensating the compass, and so it is necessary to take a preventive, rather than a corrective, approach to the problem.
The primary preventive measure is to keep all DC wiring at least 3 feet—and preferably 6 feet—away from the vessel’s magnetic compass (and the sensing unit of an autopilot). Obviously, that is not always possible, particularly in the case of wiring that runs to the compass’s own night lighting. Wiring that cannot be routed well away from the compass should be run in twisted pairs; that is, each DC feed wire should be paired with a DC return wire and the two tightly (not loosely) twisted round and round together. This procedure will cause the magnetic field of one wire to be “canceled” by the field of the other.
The wiring installation should always be checked as a safety measure. You can do this by observing the compass while you switch the involved circuit first ON, then OFF. If the wires have been properly twisted, the compass card will not react. However, if the wires have not been properly paired and twisted, the card will shift quite rapidly to a new heading each time the circuit is switched.
MAKING THE CONNECTION: TERMINAL FITTINGS
In marine applications, winding a bared, twisted end of a conductor around the terminal screw on an appliance, circuit breaker, or bus bar just won’t do. In fact, this practice is prohibited by industry standards. Such a connection is simply too prone to corrosion and to loosening due to vibration. Instead, some form of terminal fitting should be employed, one which provides a flanged fork (spade)—or even better, a ring—end that makes for a secure mechanical connection.
At one time, terminal fittings were soldered to the wire, but that procedure is both time-consuming and expensive; crimp fittings are now almost universally used. These are tinned copper fittings consisting of a tubular shank with a plastic sheathing at one end of which is a fork or a ring. The sheathing extends up over the wire insulation. The best-quality sheathing is nylon. Unlike the cheaper PVC, nylon will not crack or punch through when crimped, and resists the corrosive effects of ultraviolet (UV) light, oil, or fuel.
1 Strip the wire of its insulation and slip it into the crimp fitting—a ring or a fork. Then squeeze the crimp with a special tool until exactly the right dimension is achieved.
The stripped end of the wire is inserted into the tubular shank of the fitting, and a special tool is used to squeeze the shank tightly around the conductor. The fitting must be of the proper size for the wire used, and the plastic sheath must not be broken when squeezed.
If properly done, the connection, although mechanical, is excellent and long-lasting. And if pretinned copper wire is used, and the terminals are tinned, the potential for corrosion between the wire and the crimped shank of the fitting is minimal.
It is possible to solder a crimped fitting after it has been crimped. But this is not preferred practice, as the heat will usually destroy the insulation that protects the shank on the crimp fitting. Additionally, the solder will “wick” part of the way up past the crimped fitting and, effectively, destroy the flexibility of the stranded wire, tending to make it stiff and prone to breakage from the vibration that exists on all boats. And where enhanced resistance to corrosion is desired, it is beneficial to coat the terminal with one of the liquid vinyl compounds available for that purpose, or to employ modern vinyl heat-shrunk tubing.
2 Slide a length of heat-shrink tubing (available in several sizes) over the crimp.
Shrink tubing is applied by sliding a piece of appropriately sized material over the wire before crimping the terminal on. After crimping, the tubing is slipped into position so that it covers the shank of the crimp fitting and overlaps onto the wiring insulation. The assembly is then heated with a heat gun or a portable hair dryer, whereupon the tubing shrinks tightly to the contours of the wire and fitting, at the same time exuding a sealant/adhesive that seals the underlying connection from moisture intrusion. A more recent (and convenient) variation of this approach is a combination crimp and heat-shrink terminal fitting, which is first crimped in the normal manner, then heated.
Both standard crimp fittings and these combination crimp/heat-shrink fittings are also available in configurations for joining two, three, or four wire ends; these are commonly called butt connectors. There are also other specialized terminals, such as the male/female pairs termed "quick-disconnect."
3 Shrink the protective tubing with gentle heating. A hair dryer is effective, as is the gentle flame of a lighter.
ELECTRICAL APPLIANCES & EQUIPMENT
Clearly, electrical appliances are a boon for today’s boater. Indeed, the convenience they afford is one of the two major reasons for having electrical power on board—the other being the use of modern sophisticated electronic navigation and communications equipment. Electrical appliances can, however, also be a bane, for unless properly chosen and installed, they will deteriorate and break down quickly, as well as cause problems for critical electronics. The prudent skipper will, therefore, refer to the installation standards promulgated by such organizations as ABYC and NFPA and, in all cases, pay strict attention to the recommendations of the manufacturers of the equipment in question.
Just as for application ashore, choose electrical equipment that bears the listing mark of Underwriters Laboratories (UL) whenever possible. Underwriters Laboratories Marine lists certain devices, especially electrical equipment, intended specifically for installation on board vessels. These listed devices should be used whenever there is a choice of equipment.
Microwave Ovens
For onboard use, microwave ovens offer some significant advantages over conventional electric ovens and ranges because they reduce cooking time, hence power consumption. They also reduce the amount of heat released into the surrounding cabin, which improves the comfort of the crew and, if the boat is simultaneously being air conditioned, further reduces overall power consumption.
Microwave ovens are available in different sizes and wattage ratings. The larger ovens are usually more powerful (faster cooking) and require more current, but not in all cases, so it is important to check wattage ratings carefully when comparing units of the same physical size. Beyond that, choice of preferred size and wattage is determined basically by the same cooking considerations pertinent to shoreside applications, although final selection is limited by a number of other considerations relating specifically to onboard use.
Figure 19-18 Air conditioners, such as shown above, are usually found only on medium-size and large craft operating on 120-volt AC from shorepower or gensets. There are models for smaller boats that can be powered from 12-volt DC, but these place a heavy load on the batteries.
First, on all but the largest yachts, galley space is relatively limited in comparison to onshore applications—so it may not be possible to fit in a full- or apartment-size microwave; see Figure 19-17.
Second, unless the craft is fitted with a genset, supplying AC for the microwave can be a problem when shorepower is not available—for example, at anchor. Yet such times are precisely when microwave cooking affords the greatest advantages. Consequently, it may be important to make sure that the unit can be powered by an existing inverter, and that there is sufficient storage battery capacity to supply current. For example, a 1,000-watt unit set on high power will draw about 8.33 amps at 120 volts. Assuming that the inverter has an efficiency of 85 percent (with 15 percent of the input energy lost to heat), the current draw on a 12-volt battery will be about 96 to 98 amps—a pretty heavy load that would deeply discharge the average deep-cycle battery in less than half an hour.
In contrast, if the microwave were a 700-watt unit used for appropriately light duty, the battery draw would be about 58 amps, and since microwave cooking is, happily, fast, the 10 or so minutes needed would probably only discharge the vessel’s batteries by about 7 to 8 amp-hours. And naturally, if the unit were in use on a powerboat underway, that amount of power would easily be supplied by the propulsion engine alternator( s) without a drain on the storage batteries.
Microwave ovens can be relatively heavy and need to be securely mounted. It’s also critical to make sure that they have sufficient ventilation, as lack of cooling ventilation is probably one of the most frequent causes of failure in onboard use. Therefore, if the unit is being built in or enclosed in a locker, ventilation holes of adequate size should be cut in its enclosure.
Refrigeration & Air-Conditioning
Refrigerators and air-conditioning units that use sealed, electrically driven compressors are probably among the most, if not the most, reliable pieces of equipment to be found on board. One reason for this is that they incorporate shorebased technology that has reached a high level of refinement. In these units, the compressor motor is sealed inside the condenser casing with the compressor itself, and so is protected from the ravages of the marine environment. If a fresh desiccant cartridge is soldered into the refrigerant circuit when it’s first charged, and the system remains sealed, the basic unit will last for years without the need of attention, even if run continuously; see Figure 19-18.
This is not necessarily true of certain peripheral components, such as the evaporator fans and the electric pumps that supply cooling water to the condenser unit in water-cooled systems. But again, in the better systems, even these components are rated for years of operational life, even under frequent use over extended periods of time.
The important thing to remember about refrigeration and air-conditioning units is that their compressor motors draw starting current (amperage) that is at least twice, and can be as much as three times, their rated current draw when running. This means, for example, that a 12,000 BTU air-conditioning system that is rated at 6.8 amps at 120 volts may momentarily draw 21 or more amps each time the compressor starts up. It’s critical to take this into account when matching gensets, inverters, wiring, and circuit breakers to such systems; it is also necessary to consider which other heavy-drain devices may be running when an air-conditioning system is switched on.
Figure 19-17 A properly selected and installed microwave oven, as shown here above the stove, reduces cooking time, lessens heat generated, and generally enhances life afloat.
Ground Fault Circuit Interrupters
There are occasions when, due to faulty wiring, terminals, or other defects in a circuit, current will escape its normal (safe) path and head directly to ground via some other conductor. The condition is called a “ground fault” in an AC circuit. If the external conductor happens to be your body, you can be in big trouble, especially if heavy current manages to pass through your heart. A GROUND FAULT CIRCUIT INTERRUPTER, or GFCI as it has come to be known, senses such a ground fault before any potentially injurious amount of current can be conducted, and in a fraction of a second breaks (interrupts) the power to the faulty circuit.
GFCIs are available in different sizes and capacities, some portable and some designed to be wired into the electrical distribution system; see Figure 19-19. No vessel with 120 or higher voltage on board should be without appropriate GFCIs in its distribution system. Standards call for GFCIs to be installed in all heads, galleys, machinery spaces, and on weather decks; any unit installed in the engine compartment of a gasoline-fueled boat must also be IGNITION PROTECTED (see below). Portable GFCIs should be employed as necessary on deck and on dock—indeed, anywhere near the water or where one might be showered upon—whenever electrical tools are employed. This caution simply cannot be overstated: Numerous fatalities have resulted from the failure to take these precautions and from electrical tools dropping in water or developing wiring faults while the operator was standing in a puddle or was otherwise well grounded.
Caution: Most GFCIs are not listed as “ignition protected” devices and, in order to avoid an explosion hazard, should not be installed in the engine room or fuel tank areas of gasoline-powered vessels. Additional information about the use and installation of GFCI’s is covered in ABYC Standard E-11, AC & DC Electrical Systems on Boats.
Figure 19-19 A ground fault circuit interrupter (GFCI) is designed to protect people from line-to-ground electrical shock hazards that may develop from faulty appliances, tools, or cords.
PROTECTING YOUR BOAT & EQUIPMENT
Properly directed and controlled, electrical power can be a tremendous asset in the safe operation of a vessel. But stray or uncontrolled electrical currents can be destructive to both hull and gear, as well as potentially very dangerous to the crew and those who may be in proximity to the boat, whether in the water or out.
Ignition Protection
By their very nature, many electrical devices spark or “arc” in their normal operation; switches, relays, engine starters, and many DC motors and alternators are all examples of such devices. In the presence of gasoline or propane vapors, this “arc” can produce sufficient energy to ignite the vapors and produce a violent explosion.
In order to prevent this, the USCG, the ABYC, and the NFPA all require that electrical devices be ignition protected if they are to be installed in engine compartments or fuel tank compartments on gasoline-powered vessels. In addition, NFPA and ABYC have extended these ignition protection requirements to boats equipped with propane under certain conditions. There are various equipment test standards for ignition protection, including the Society of Automotive Engineers Standard SAE J1171 and Underwriters Laboratories Standard UL 1500. When installing or replacing any electrical component in compartments where vapors may be present, it is imperative that the component has been tested under one of these standards and the product itself is marked “ignition protected” or bears one of these two standard numbers.
Safe Battery Installation
Storage batteries must be installed in a ventilated area so that gases generated during charging will be dissipated safely. They should also be protected from extreme heat and cold and from spray. Starting batteries must be close to the engines in order to have short starter cables, thus reducing the voltage drop in the cables. Gasoline fumes are explosive, and any spark from a battery connection is dangerous. Gasoline fumes should be kept away from the battery installation.
Batteries must be secured against shifting as well as against vertical motion that would allow them to pound. They should be chocked on all sides and supported by a nonabsorbent material that will not be affected by contact with electrolyte. Air should circulate all around the battery. A tray of fiberglass or other electrolyteproof material must protect aluminum and steel surfaces of the boat.
Batteries should be accessible for inspection, cleaning, testing, and adding water to maintain the level of the electrolyte (wet-cell batteries only). They should also be covered with a nonconductive material so that a dropped wrench or other tool will not short-circuit the terminals. These covers must have sufficient small holes to allow the escape of any gases from charging; see Figure 19-07.
Caution: Salt water reacts with battery acid to form chlorine gas, which can be deadly.
Preventing Electrolytic Corrosion Damage
A boat owner must take precautions against electrical corrosion, also called “electrolysis.” There are three major forms of this—GALVANIC CORROSION, ONBOARD STRAY-CURRENT CORROSION, and EXTERNAL STRAY-CURRENT CORROSION. Although the electrochemical results are generally similar, these all have different causes and require different protective actions.
Table 19-4 In the galvanic series of metals in seawater, the metals with the lower potential are variously termed the “less active” or “more noble” and act as the cathode in a galvanic circuit. On the other end of the scale (the top of this table) are metals that have higher potentials, termed “more active” or “less noble”; these serve as the anode.
Galvanic Corrosion Principles
When two metals with widely differing electrical potential (voltage) are connected electrically and immersed in an ELECTROLYTE—that is, a currentcarrying liquid such as salt water—a natural battery is formed. When that happens, direct current flows in the circuit comprised of the metal parts, the electrical connection, such as a bonding wire (see below) or a metal hull skin, and the electrolyte.
Different metals have different electric potentials as measured against a standard reference halfcell; the voltages for various metals in seawater are shown in Table 19-4. The difference in potential between any two metals determines the relative strength of the current flow and its direction. The metals with the lowest potential are variously termed the “most noble” or “most passive” and act as the CATHODE in a galvanic circuit. On the other end of the scale are metals that have higher potentials and are termed “least noble” or “most active”; these serve as the ANODE.
The direction of this current has important implications for the longevity of a vessel’s underwater and bilge-mounted metal parts (including the hull if it is metal). In the segment of the circuit that is formed by the electrolyte (seawater), current flows from the anode to the cathode. This causes an anode to progressively disintegrate through the process of electrolysis; this is known as galvanic corrosion.
If the natural current in this circuit is supplemented by the introduction of current from an external source—for example, from an electrical wire that is shorting directly to ground—the rate of the anode’s deterioration will be greatly accelerated. The natural galvanic corrosion is enhanced by the external current. The insulation on wiring should be protected from chafing and maintained in good general condition; wires should never be allowed to be in contact with bilge water. Each DC electrical device should have its own return wire of the same size as the “hot” wire returning directly to the negative bus at the distribution panel.
Protection Using Passive Anodes
The terms “anode” and “cathode” are relative. In any galvanic circuit, the metal with the higher electrical potential acts as the anode, while the metal with the lower potential acts as the cathode. Aluminum hull plating, for instance, will act as an anode relative to a copper radio grounding plate, which will be the cathode. However, a bar of pure zinc would be an anode relative to both the aluminum housing and the copper grounding plate. The traditional method for protecting critical underwater parts is to bond them all together in a single circuit that includes one or more special SACRIFICIAL ANODES; see Figure 19-20. When a piece of zinc of adequate volume and surface area is included in a galvanic circuit, the higher voltage of the zinc overcomes the tendency of current to flow away from other metal parts in the circuit. Instead, current flows from the zinc to those parts that are more noble, thereby protecting them at the cost of the erosion of the zinc. (Aluminum anodes are becoming popular, but are often called zincs.) Magnesium anodes work well in fresh water (do not use in salt or brackish water); never mix anodes of different metals. Caution: Never paint an anode; doing so would isolate it from the water, preventing its protective action.
Figure 19-20 Sacrificial zinc anodes should be placed on shafts, rudders, and the hull. These will do much to prevent galvanic corrosion. They should be expected to progressively deteriorate; periodic inspection and replacement is required for full effectiveness.
A BONDING SYSTEM can be installed to connect all metal hull fittings exposed to bilge water or external water together, and to the external zinc electrode. Connections to the bonding system must be checked regularly; see Figure 19-21. Bonding is a controversial topic, and it may not be justified in a nonmetallic hull if the underwater metals are galvanically compatible.
Since zincs are cheaper and easier to replace than critical metal parts such as propellers, shafts, hull plating, and the like, this preventative for galvanic corrosion has, through the years, been almost universally used. Unfortunately, the use of static sacrificial zinc anodes doesn’t always work as well as intended. The area of wetted surface of the zinc or zincs is critical: If too small, the desired effect of reversing destructive current flow will not be achieved.
Placement is also critical. If the zincs are too distant from the part or parts that require protection, the voltage of the zinc will not be able to overcome the resistance of the current path through the electrolyte; and therefore, the zinc(s) will fail to reverse potentially destructive galvanic currents in the region of the critical part or parts.
For both of these reasons, systems that provide more “active” corrosion protection have been developed in the last few years. These are IMPRESSED-CURRENT systems that sense and work actively to overcome potentially destructive voltage differentials. These systems divide into two types described below: anodic and cathodic.
Figure 19-21 A heavy copper strap run from one end of a boat to the other end will serve as a common bonding conductor. It is not to be used as a part of the return path of electrical power to DC equipment. Individual bonding conductors must be #8 AWG or larger. If copper strap is used, it must be at least 1.32' (0.8 mm) thick and not less than 1.2' (13 mm) wide. In the approach shown here—often recommended—through hulls are isolated from the bonding system, whereas the radio ground plate (also serving as lightning ground) is bonded. Engines are bonded, but prop shafts may be isolated via flexible couplings and individually protected as needed with zincs.
Active Anodic Protection
Anodic impressed-current systems overcome the problems of size and anode-to-cathode distance, which frequently occur in a traditional arrangement of “static” sacrificial zincs. These anodic systems incorporate an electronic controller that senses voltage and/or current flow in a galvanic circuit. The controller then works to ensure that sufficiently high voltage is present on the system’s sacrificial anode(s) by supplying current from an external source (such as the boat’s battery) to the anode(s). And this in turn ensures that critical underwater metal parts are at all times in a cathodic (noncorroding) condition.
Active Cathodic Protection
Cathodic impressed-current systems also incorporate an electronic controller, which senses the voltages and/or currents present in any galvanic couples. However, in cathodic systems, the controller uses external power to boost the voltage of a dedicated cathode to the point where that voltage is equal to the voltage of the anodes in the circuit. Then, since all metal parts in the galvanic circuit are at the same electrical potential, there is no current flow. Cathodic systems thus block the passage of destructive galvanic currents.
One system, such as, for example, Mercury Marine’s Quicksilver MerCathode system, electronically measures the voltage of a boat’s submerged critical metal parts, in particular, the aluminum sterndrive housing found on many smaller craft. Utilizing current from the boat’s battery, it raises the voltage of a permanent, submerged titanium electrode (the cathode) so that it equals the voltage of the endangered part or parts (which become the anodes). With the titanium electrode and the otherwise endangered metal parts at the same electrical potential, there is no current flow, and thus no galvanic corrosion.
Avoid Dissimilar Metals
But whether a traditional system of sacrificial zincs is employed, or a more modern impressed-current system, the sensible boater still avoids, to the greatest extent practicable, mixing galvanically incompatible metals. Bronze through-hulls, for example, must not be used in direct contact with aluminum or steel hull plating. Instead, Type 316 stainless steel is often used, as it is much closer to aluminum and steel on the galvanic scale (thus there is less natural voltage differential between the two); refer to Table 19-4. Even better, nonconductive fiber-reinforced plastic fittings are sometimes used in smaller metal boats to completely eliminate the danger; such plastic fittings, however, are not generally available in sizes suitable for larger vessels.
Internal Stray Currents
Less common, but still possible, is corrosion caused by STRAY CURRENTS within a boat. If two pieces of metal in the bilge are at different potentials, possibly as a result of faulty wiring or poor connection to a bonding system (or lack of such a connection), current can flow from one to the other even if they are of the same metal. This action will result in metal being eaten away from the metal at the higher potential.
Zincs are of no use in such situations. The obvious prevention measures are careful initial wiring of all electrical equipment that might operate in or near bilge water (currents can flow across moist surfaces as well as in liquids), and the continued maintenance of both wire insulation and connections through regular checks.
External Stray Currents
If natural galvanic corrosion is an illness, electrolytic corrosion enhanced by an external current is a plague. The most common offenders are improperly wired shoreside electrical hookups and items of onboard equipment that are leaking current directly to the ground (the water) or to the boat’s bonding system through shorted or defectively insulated internal components and/or bad wiring.
Correct Polarity Many shorepower-related electrolytic problems can be avoided by making certain that there is correct “polarity” when plugging in—that is, that the black wire is hot, the white is the neutral (in a 120-volt system), and the green wire is connected to ground. Your boat’s 120- or 240-volt main distribution panel may incorporate a reverse-polarity indicator, but such a unit provides less information and, ultimately, less protection than a simple and inexpensive handheld indicator, of the type used by electricians; refer to Figure 19-14. What you really need to watch out for is a case in which improper shorepower wiring ends up with the current running to ground through the green wire, rather than through the white neutral wire.
Using an Isolation Transformer Additional protection can be had by incorporating, whenever possible, some form of “isolator” in any ship-toshore electrical connection. In this respect, the best solution is an isolation transformer that will eliminate any direct conductive connection between the shorepower source and a vessel’s electrical system. As previously mentioned, such transformers do not step the voltage up or down. Instead, they produce current for the boat’s distribution system via a magnetic connection (induction) with the shorepower input. Unfortunately, isolation transformers are relatively large and heavy; they are also expensive. For these practical reasons, they are used for the most part only on larger yachts.
Using a Galvanic Isolator Where an isolation transformer is not practical for reasons of space or cost, one useful alternative is to install a “galvanic isolator” in the shorepower connection ground (green) line. These relatively inexpensive units work to isolate the boat’s grounding system from the shorepower ground and, therefore, they help alleviate the potential for electrolytic corrosion. Since the galvanic isolator is connected in series with the AC system grounding conductor (green wire) to shore, it is imperative that the isolator meet certain minimum standards; otherwise the integrity of the grounding conductor will be compromised, if not lost completely. These requirements are reflected in ABYC Standard E-11, AC & DC Electrical Systems On Boats. The only way you can be sure that you are complying with these standards is by using only galvanic isolators that bear the Underwriters Laboratories Marine listing mark.