RE: What foam to mold boat from
Yokomo Aust,
There are basically two types of foam, open cell and closed cell. The 'closed cell' stuff looks like small solid 'beads' of stuff glued together. The 'open cell' stuff looks like foam with millions of bubbles in it. Either will work but do require different techniques in covering/finishing. The closed cell foam doesn't 'absorb' much of anything, sort of like painting wood. The open cell foam, since it is make up of 'holes', usually 'absorbs' more (have to fill all the little exposed holes). There is some 'out gassing' from both types of foam as 'Umi' pointed out, so some type of 'sealer' is probably a good idea.
Another option when using foam as a 'mold' is to cover it with multiple layers of 'glass', then 'melt' out (disolve the foam with acetone) leaving just the 'glass' shell. (The 'big boys' use this technique to build full sized aircraft bodies, EAA, etc.) Makes for a very strong and light weight hull (which does require some internal bracing, maybe.).
Have fun...
- 'Doc