ive something simmilar to this, but with big blue plastic jugs/containers? i dont remember that well, long time ago. worked well, till i lost my oar and drifted away to a small islad, lol. i was like 6-7 yrs old. some old guy passed me and tied it to his boat. he offered me to sit in the boat with him but i wanted to sit on the raft instead. he was a cool guy so he sped up. you should have seen my moms face when i came home hauling a plastic raft dragged by some old dude.
man, those were the good days...
Do a google for Richard Sowa, or Spiral Island... This guy took this concept to an extreme. (just noticed that matt has a thread about him, coming number one of the similar threads under this one)
I spent some time building boats, and can tell you this plastic bottle thing is super easy, but also really temporary. The elements will wear out a plastic boat like this in a few months. Even using big 200 litre blue barrels, you should calculate a 6 month life to be safe.
Also a really good thing to know (if you dont already), is Archimedes' principle, short version is that buoyancy is about displacement, and in constructing your boat, you can know that 1 litre of water (its cool, cause its the base measure for all volume) is equal to 1 kilo of buoyancy. A two litre bottle 2 kilos, and so on. So if you make a boat from 500 2 litre bottles, you can effectively float one metric ton (at the surface level of the water. You could safely float, say, 800 kilos, on a calm lake.)
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