Originally Posted by chewy
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.