Learn Me: Expanding Foam, specifically expansion rates

I'd like to get some extra buoyancy in my boat between the stringers.   I have been looking at expanding foam and lo and behold, this sale came up on my email

https://sellout.woot.com/offers/polymer-planet-liquid-urethane?ref=w_cnt_wp_0_33

From my rough calculations I need approx 10-12 cubic feet of the stuff when fully expanded.  How do I know how much the stuff above expands to?  Should I bother with this, or should I go find something other brand/density/whatever?  Should I even bother with expanding foam?

I should add some to my Ranger Bass Boat knockoff, but am a ways from going there... looks like a good starting point to learn somptin’ though!

Some quick research of the product pictured says that kit should fill to a volume of ~4cubic feet (almost 30 gallons). It is also available thru other vendors in different densities, which should be the weight (mass) of the expanded product per cubic foot. Go with the lightest you can for boat applications.

Urethane expanding foams are sold by the finished pound/cf.  For Bouyancy, you want the lightest stuff.  Most OEM boat manufacturers use 2lb foam.  If you need additional floor rigidity you could step to 4lb.

So, you would calculate your cubic feet (12) and multiply it by the density of the foam.  2lb foam means you'll have 24 lbs of foam.  You need 24 lbs of raw material.

According to that website (if you click the last photo), one of those one gallon kits does 4 cu ft, so you would need a three gallon kit.  In nearly all cases, they are sold as equal parts, so a 1-gallon kit would be 1/2 gallon each of part A and B, meaning that the finished mix makes one gallon before expansion.

As far as using foam, it depends on the design of the floor.  If your floor is a completely sealed cavity (not common), the foam doesn't add buoyancy, it reduces it.  If the floor is open to bilge water (common) it does provide buoyancy.  The downside to foam is that as moisture soaks into the wood structural pieces, it has less surface area to evaporate (foam touching the wood) and it can rot wood faster.  

Regarding that particular product, I've never used it, but I've heard of it.  I have used a product called Secure Set, but I think they only do 6lb.  It's marketed as a replacement for concrete when setting posts.  I use it for casting props for theater.

Amazon has a ton of options, including the polymer planet brand you linked.  Might be able to research some reviews on Amazon.

Curtis73 (Forum Supporter) said:

As far as using foam, it depends on the design of the floor.  If your floor is a completely sealed cavity (not common), the foam doesn't add buoyancy, it reduces it.  If the floor is open to bilge water (common) it does provide buoyancy.  The downside to foam is that as moisture soaks into the wood structural pieces, it has less surface area to evaporate (foam touching the wood) and it can rot wood faster.  

Not sealed, and the stringers are fiberglass over aluminum strips, so it sounds like there's no downside.

Ding ding.  Do it.