Hey, Does it Float?

SPRAY FOAM MAGAZINE – Polyurethane foam, far from being limited to insulation, finds its practical applications in various areas. In the marine industry, for instance, it has proven to be a valuable asset, aiding in flotation, sound deadening, and structural strength.

Much of the polyurethane foam used in the marine market is your normal 2 lb/ft³ density closed-cell spray foam. For more support, strength, or added ability to shape or sculpt, higher-density foams can be used. This can be spray-applied or hand-mixed and poured into open cavities. If you’re applying it in an engine compartment for sound attenuation, you may spray the walls separating the engine compartment from the rest of the vessel. If you’re filling a barrel or smaller open compartments of a hull, you may want to pour it. Foam could add structural strength to an otherwise flimsy structure that would collapse if not filled with a solid material. In all cases, the foam is not intended to be in direct contact with water because, over time, the foam will absorb water, take on weight, and lose its buoyancy properties even though it is a closed-cell product.

I wouldn’t be the Foamgineer if we didn’t do a little math, so let’s crunch some numbers. I’m sure you’ve been curious to know how much weight foam can support. Simply put, it depends on how much water it displaces. It’s an easy calculation. Let’s say you fill up a 55-gallon drum with 2 lb/ft³ closed-cell foam. Let’s also assume the drum weighs 25 lbs. You want to calculate the maximum weight this drum can support. A 55-gallon drum can displace 55 gallons of water. Then all you need to know is how much does 55-gallons of water weigh. A gallon of water weighs approximately 8.3 lbs. So a 55-gallon drum full of foam will support 55 gal x 8.3 lbs/gal = 457 lbs. With 25 lbs of that being the drum itself, then you are supporting 432 lbs. Wait, you have to subtract the weight of the foam, right? Yes, and this is the downside of using foam for flotation. Since you want to keep the foam dry, its main purpose is to aid in flotation if you spring a leak or to provide structural strength. A sealed empty drum would support 432 lbs, but would it maintain its shape or structural integrity when loaded? Maybe, but the foam can add that safety factor. Luckily, foam is not very dense, and 55 gallons is only 7.4 ft³. The total weight a 55-gal drum of 2 lb/ft³ foam can support is 432 lbs – (7.4 ft³ x 2 lbs/ft³) = 417 lbs. What if for some reason, you wanted the drum to float about ½ way above the water surface, then you make sure it supports no more than half that weight or 417/2 = 209 lbs.

The only thing you need to know now is how much chemical do you need to fill up a certain volume. Let’s continue with the 55-gal (7.4 ft³) drum example using 2 lb/ft³ foam. Just multiply the volume by the foam density to get 7.4 ft³ x 2 lb/ft³ = 15 lbs of foam chemical. Using a typical 1:1 ratio foam you’ll need approximately 7.5 lbs of ISO and Resin. Now you know with spraying foam, things such as ambient temperature, chemical temperature, and humidity can play a role in how the foam reacts. You will have to experiment to get the drum or void to fill to your specifications, but that is a good way to calculate a starting point.

So, there you have it—a brief overview of the different uses of spray polyurethane foam, how it’s used in the marine industry for flotation, sound attenuation, and structural support, and how to calculate its weight and how much is needed. Reach out if you have any questions, additions, or ideas for future articles.  

For use by SprayFoamMagazine.com & Spray Foam Magazine