flotation foam

I have a bunch of 2 part closed cell foam and I was wondering if there's any reason I shouldn't pour it under the floor after I prime and paint? I figure it should deaden some noise as well as allow for thinner plywood for the floor.

Only problem I see is clean up from over run

While I can't say whether you should paint first or later...... as far as pouring it in, that's what the factory does. However, the factory does not provide for proper drainage, they pour it where it's in direct contact with the floor, not a good thing to do in an aluminum boat.

My suggestion would be to put some type of fairing strips on the floor, made from something that won't break down, such as PVC pipe split longways, then put a thin sheet of closed-cell foam, like 1/2" on top of those strips, then pour your foam.

This will get the foam up off the floor where it can get air underneath, allowing any water to drain from the foam, as well as preventing poultice/crevice corrosion of the aluminum from being in contact with water or waterlogged foam.

Thanks for the advice, I didn't think about leaving "breathing" space. I'll save the pourable foam for compartments and just use sheeting under the floor.

I'm a huge fan of 2 part expanding urethane foam... but I don't like the deck support design you mentioned.

As stated above, it doesn't allow for positive drainage of the bilge. It can also be detrimental for your plywood decking when water has no place to go.

You'll need to seal both side and all of the edges of your decking in case water gets between the foam and decking, but even this won't make your decking 100% waterproof because wherever you have a fastener penetrating your decking you'll have a potential leak to rot your decking from above and below.

I personally don't care for thinner decking either, fasteners in 1/2" decking have little holding power for mounting accessories. I like a minimum of 5/8" decking and 3/4" is better.

I don't mean to sound so negative, but these are findings from my past experience.

I suggest your pour your foam before laying your decking and leave a drainway down the center at the keel and leave a dead air space between your decking and the top of the foam... kind of like this: