According to my calculations -- (somebody else check it!) -- you'd need to displace about about 12.5 cubic feet of water to float an M15 -- or a bit over twice that for the M17. Now, if we don't take into account a whole bunch of other variables, like how much water the fiberglass walls and other materials in a submerged boat would displace on their own--or the weight of the foam itself, you would need about 13 cubic feet of foam to ensure a fully water-filled M15 would float. Now, that doesn't take into account the weight of your extra large battery, anchors and other gear. On the other hand, it doesn't take into account the fact that it is extremely unlikely your hull could ever be COMPLETELY filled with water (i.e., no air pockets), so maybe we can let all the other variables slide, and just say that you want about 13 cubic feet of foam. Seems like a lot. Doesn't seem like you could get that much volume of foam just by filling really inexcessible areas with it. Does anybody have that much foam in their M15? That would take 90 1-gallon milk jugs or 351 1-liter plastic bottles. I'm going to take a really wild guess and say that it would take in the neigborhood of 50 to 100 bucks worth of Great Stuff.