Also I've read about a few guys who had there hulls completely filled with expanding foam. They said it leaves the hulls super stiff and only added 20lbs per hull. So it is doable and practical if you have a very old boat, that needs some stiffening.
Filling the hulls with foam is a really bad idea.
1) Adds weight. 20 lbs is significant.
2) You can't fill every void without exploding the hulls.
3) If you can't fill every void, every void will eventually hold water - which adds more weight.
There are other problems, too. This has all been discussed before, and there's even a real world example - the Sizzler.
The Sizzler was made from hydroformed aluminum sheets and filled with urethane foam. They worked great until they invariably leaked. They took water in and never let it out. They got heavier and heavier and heavier.
Sure, "filling" the hulls with foam will work for a while. But it's bad for the boat in the long term - and for resale value.