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Speaking of a boat made from recycled bottles..... Answered

Here is an article and a video about a couple that wishes to sail their boat made of plastic bottles, from San Francisco to Australia.

David de Rothschild is preparing to sail from San Francisco to Australia in a raft made of 12,000 recycled bottles. Among other things, he is hoping to convince the world to take better care of its waste.

The Link and video are here

Discussions

Hmm... rich kid, albeit environmentally aware rich kid, makes boat filled with CO2. Convince me. :-)

It's actually sort of nice to see some progress being made on this. So far I'd only been seeing mock-ups. The dry ice idea is pretty clever, as well. :D

Wot's the dry ice idea? (video started off with a commersh, got bored, clicked back)

The idea is to put a lot of dry ice into the bottles, seal them and inflate/pressurize them to enhance their floatability.

I think the real purpose of the dry ice is to increase the rigidity of the bottles through an increase in the axial and hoop stresses, in reality, would not a pressurized bottle float slightly lower than an unpressurized bottle of the same volume?

Hmm, I would think the greater volume of gas in the bottle would make it float better, but I am, by no stretch of the imagination, any kind of expert on gas and the physics surrounding it.

A quick google shows that the density of air @STP is 1.29 g/L and carbon dioxide is 1.98 g/l so even at the same pressure the co2 bottle is already heavier, when you pressurize the bottle you are additionally increasing the density of the gas and consequently the bottle gets even heavier.

Um, have you drank any water from one of those little bottles recently? If you increase the pressure by very much, it would easily burst. Still, for some reason, I didn't think buoyancy depended on weight.

In this case I don't think it does significantly- if he's using 12,000 1 litre bottles that amounts to a maximum difference of 8.28 kilograms (probably less because I suspect the dry ice will pressurise the air already inside the bottle), in a boat with a maximum displacement of 12 tonnes. I don't think the pressure will be significant, a few PSI to make the bottle rigid but not enough to significantly increase the density of the gas inside. I suspect it won't affect the flotation much either way, sounds like a structural concern to me.

I never said it would be significant, I said the reason was to make the bottles stiffer, but by doing so you inevitably make the bottles heavier. In fact I noticed I used the word "slightly". Goodhart, the 2 L Pet bottles shown in the video have a burst strength of about 180 psi. Buoyancy is related to the weight of the fluid displaced, and if an object is less dense than water it will float, the lower the density the higher it will float, since density is mass/volume and weight is mass times g, than bouyancy depends on weight.