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Old 30 July 2012, 06:43   #31
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Looks like we're going down the closed cell foam route which can be poured in....

NOOOOOOOOOOOOOOO!!!!! Don't do it!!!!! One word for you - Dory!



Bottom line is 3 tons is 3 tons. What would youy rather lift - a ton of lead or a ton of feathers?


(Let's ignore inconveniences like the feathers absorbing water etc for the following discussion)

A ton of feathers will take up a load more volumethan a ton of lead. To support a tone of anything in water means slightly more than a ton of water needs to be displaced.

Let's use round numbers.... if a ton of widgets take up 10x the volume of a ton of thingumys, then you only need to submerge 1/10th of the block of widgets to get the same bouyancy. The problem you have with almost anything you can make a boat out of (bar possibly wood - although Th density of wood is so close to that of water the weight of the fittings is usually enough to overcome what little spare bouyancy there is) is a block of most construction material is more dense than an identical sized lump of water. Hence why boats are hollow!

Practical example - How does 300,000 tons of supertanker stay afloat? Replace the air in the tanks, with oil, and it submerges quite a bit. Pump the oil out, and load it with mercury (liquid metal) and it wouldn't still be afloat by the time you finish loading...



So, back to the orginal Q, presumably Chewy wants to keep his sunken 3 ton whatever boat afloat in a worst case scenario. Air is about as light as you can sensibly get (helium notwithstanding), so I would suggest either sealed compartments or airbags as used by sailing dinghies.


The "niceties" in the cals above are a bit like the anoraky pre-shot of oil in my fuel tank. Easy maths says "30L tank, throw in 600Ml of oil". But that oil takes up 0.6L of tank space, so you actually only need to to thow in 560 ml to get a "proper" 50:1 mix.
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Old 30 July 2012, 07:30   #32
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Air tanks are the way forward maybe with removable bladders.....
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Old 30 July 2012, 08:22   #33
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I did read on a yachty website of a round the world type who solved this by stuffing the hull voids with as many empty 2ltr plastic drinks bottle as he could get in. With caps on of course. Worked out as a cheap and effective solution that was reversible. He did tie several together to ensure if the boat got holes they didn't all float out. Nets I seem to remember to bundle them up.
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Old 31 July 2012, 04:25   #34
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Got aload of plastic bottles for that very purpose.... and some plastic balls.
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Old 31 July 2012, 04:36   #35
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That would explain the funny walk......
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Old 31 July 2012, 12:41   #36
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Got aload of plastic bottles for that very purpose.... and some plastic balls.
Mate replaced the waterlogged foam in his Wilson with empty lemonade bottles (with lids)......seemed a good solution to me.....

Failing that, stick some sponsons around it
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Old 01 August 2012, 05:20   #37
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How about a lot of the big-bubbled bubble wrap? Weighs nothing, traps lots of air, stuff in anywhere.

Did somebody say inner tubes
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Old 01 August 2012, 05:58   #38
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Expanding foam is tricky stuff. Not only does it seem to absorb water, despite supposedly being closed cell, but it is very strong when it is expanding and is capable of distorting panels etc.. Personally, I wouldn't go that route.

Mythbusters once raised a sunken boat using ping pong balls. I would think that would be a good way to go? Be a bit of a mess if you ever ruptured the hull though.

I suppose another consideration would be if you increase the buoyancy and the boat rides higher in the water than designed, will stability be compromised?
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Old 01 August 2012, 06:34   #39
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Expanding foam is tricky stuff. Not only does it seem to absorb water, despite supposedly being closed cell, but it is very strong when it is expanding and is capable of distorting panels etc.. Personally, I wouldn't go that route.

Mythbusters once raised a sunken boat using ping pong balls. I would think that would be a good way to go? Be a bit of a mess if you ever ruptured the hull though.

I suppose another consideration would be if you increase the buoyancy and the boat rides higher in the water than designed, will stability be compromised?
Anything you add to the boat will make it ride lower. You are adding mass to the boat, therefore it sinks.
The only way to make the boat (marginally) ride higher would be to fill the void spaces with helium.

However adding mass high up on the boat will raise the Centre of Gravity, Lowering stability, so adding foam high up will not be good, and will probably at best make it "wallow" more.
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Old 01 August 2012, 07:41   #40
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However adding mass high up on the boat will raise the Centre of Gravity, Lowering stability, so adding foam high up will not be good, and will probably at best make it "wallow" more.
The RNLI have they buoyancy high up so in the event of a capsize it makes the boat unstable and self righting.
Looks like plastic bottles could be back on the cards...
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