EPP Foam Flotation

As I was poking around in my boat today in between rainstorms I noticed a few Styrofoam pellets which I traced to a nearby seat. After a little investigation I realized that what little foam flotation this boat has is made up entirely of expanded Styrofoam. I started looking at options to replace the existing foam and stumbled onto onto a paper that was written buy a Coast Guard representative. It's a very interesting paper that compares aluminum and fiberglass boat construction. The part that immediately caught my eye was about foam recommendations. Here is an excerpt from that section:
 Specific to aluminum boats, flotation must be resilient foam such as solid or beaded EPE or EPP rather than pour in place two part polyurethane. Polyurethane is relatively brittle and the foam against an aluminum shell will crumble as it flexes, leaving a faying surface that absorbs water (and gasoline) and promotes internal corrosion. Polyurethane also is highly flammable, produces toxic fumes in a fire, and must be removed prior to any welding. (Polyurethane can be removed by pressure washing.) It also gives off fumes that can cause respiratory distress to sensitized individuals when it is poured in for repair. Unfortunately, EPE and EPP cannot be blown in place and must be purchased as sheets, stacked and cut to fit. This is a substantial increase in initial cost, but it prevents corrosion and allows the foam to be removed intact for repairs and put back in.
And here is the link to the paper: https://www.sname.org/HigherLogic/System/DownloadDocumentFile.ashx?DocumentFileKey=dbf103fb-4814-f523-4f53-fe15cf989212
I never considered that foam might absorb and retain fuel. That seems a little concerning to me. The paper also talks about other subjects such as corrosion and rust. I have stumbled on a cheap source of expanded polypropylene(EPP) on craigslist and was wondering if this could be used for flotation.
I just wanted to put this out there in case any of you we're interested in some technical reading concerning our hobby.

Thanks for sharing this. I would not have considered side effects of using the two part polyurethane in an aluminum boat. Not too long ago, I was kind of surprised to find a stack of sheet foam in one of my boat's benches. Now I know why.

The styrene foam won't absorb fuel, it will just dissolve away.

That's exactly why I am Planning on removing the expanded polystyrene and replacing it with expanded polypropylene foam. It does not absorb water or fuel nor does it crumble or contribute to corrosion.

Every day I learn something on this site.

richg99

I foamed a 5052 Aluminum tank in with expanding foam once and will never use it again on Aluminum. I pulled the tank out less than 10 years later when I was doing some other projects and found at least 50 pits most of the way thru the tank. The research to figure out what happened showed that foaming in Aluminum tanks is a Big No No! What happens is that water will eventually find its way in and the foam will hold it and it won’t dry out. There is a type if pitting corrosion that takes place in these situations that will eat straight thru 1/8” Aluminum. But foam has its place and I used this stuff I found on Amazon for some projects I think you can get it at Lowes or HD also. It is called Secure Set High Density Polyurethane Post Setting Foam. Used to set fence posts, I used it under my deck on my fiberglass boat. It is a 2 part foam and seems to be the same stuff you would get at West Marine but for a lot less $$$.

I had a bear of a time with drainage after using the 2 part expanding urethane. Next project I was planning on lining the cavities to be foamed with thin painter's plastic so the foam is not able to bond to the side of the boat. Not sure where EPE/EPP would be a benefit in this type of installation but I'm definitely going to look further into this now.

Never thought of pitting being an issue, but if there's water trapped then it stands to reason that there is opportunity for corrosion. Don't know what the advantage to EPP over bordered expanded urethane would be towards avoiding this.

All foam absorbs water, eventually, make sure you allow for complete drainage, any water trapped by the foam will eventually be absorbed by it, can't tell you how many times I have pulled out water proof foam that was soaked.

Actually the white styrene foam is the best I have ever seen when it comes to resisting water absorption, but, it is definitely not fuel proof.

Onthewater,

I don't know of any real advantage to the EPP only that I have found a cheap source for it to replace the white styrofoam sheets that are now in my boat. I don't trust them around fuel and oil and there are not enough of them. I was considering pouring foam but after reading the article I decided against it. I am going to make the seat covers easily removable so that two or three times a year I can open them and allow the compartment to dry out completely.

Cheap is always good! It's definitely more desirable than polystyrene (also used in the boards from home improvement centers) by being resistant to fuel.

Is it an online dealer you've found or something local and limited? I've got cavities between ribs I'd rather cut sheets to fit instead of pouring if I can avoid it. I'm going to use a thinner aluminum for the floor so it will need some support from the foam, but I want to keep the drainage channels underneath the foam free and I'll put a few clean-out drain grates in the floor so I can access them in the future.

I think the irregular cavities along the sides of the hull and in the bow are still better being lined with plastic then filled with expanding urethane foam. I'll probably seal the bottom of the rod locker and fill the area around the rod tubes with expanding foam as well.

I'm not sure if what I did was the right thing, but....

When I put sheets of blue foam under my deck on my tinny, I left the sheets a bit "loose". I did not want to jam the foam in there and trap water.

I believe that, sooner or later, water will work its way below the decks. With the pieces left a bit loose, the water should find its way back and through using the drainage channels in the hull.

richg99

That is probably the best way if you want foam under your deck, plus the hull usually has ribs in it that will allow the water to drain away.