Adding Foam to a Boat. A Superb Discussion.

I am thinking of building a boat that needs some foam as a part of the building process.  Having never done foam before I did some searching and found some great comments here.  I’ll include a few specific comments, but recommend following the link to the entire forum post.  Note that the post is 8 years old at the time of this writing.

PeterDE said, “I have worked with hundreds of boat manufacturers on flotation, and tested several hundred boats for the USCG to see if they comply with the flotation standard. I also was involved in a research project by the USCG and ABYC to determine why some foams absorb water and some don’t. Frankly, the project came out a nil, but I have formed some opinions about it. All of the problems with water absorption have been with two-part pour (or spray) foams. It appears it is all in the mixing and application of the foam. It has to be done within very narrow temperature and humidity ranges to foam up correctly.

In over thirty years I have never seen a boat with block foam (that is premade foam) have water absorption problems. In fact, we ran a two-year test of block foams to see if they would soak and the most was 2/10% of the weight of the foam. That ain’t a heck of a lot of water.

So if you have the boat taken apart I would recommend putting in block foam. However, if you determine you would rather do the 2 part, the idea given about plastic bags is an excellent one. Seal the foam into plastic and then the water (and oil, gas, etc) can’t get at it.

All the jokes aside, I was the one at USCG HQ that Ok’d the use of pool noodles by a very well known manufacturer and they worked beautifully. But they were used only to add to the foam that was already in the boat. It couldn’t pass the stability portion of the level flotation test. The pool noodles were placed under the gunwales which gave it enough stability to pass.

I have seen milk bottle, ping pong balls, air chambers, air bags and all kinds of stuff used. As long as it works, it’s ok by me.

See the milk bottles at The bottle caps leaked because they rusted. If they had had plastic caps and had been glued shut they would have worked fine.

Here’s a link to Safety Standards For backyard Boat Builders. There is a section on how to distribute the foam to achieve level flotation, or if your boat is an inboard, basic flotation…atbuilders.pdf”


“Actually we looked at all of the possible causes of water absorption and they all play a part. We looked at the chemical process creating the foam, the mixing process in the boat factory, and we looked at freeze-thaw cycles, as well as long-term immersion in both fresh and salt water. Unfortunately, the answer is not one or the other but a combination of all.

If you make this stuff at a factory into blocks, cut or not (no skin or skin) it does not absorb water.
So why when it is made at the boat factory does it absorb water? Because in the foam factory the process is highly controlled. So you get nice perfectly symmetrical cells with skins on each cell that are the same thickness and all nicely bonded together. The exotherm is perfect and there is no contamination of the chemicals used to make the foam.

In the boat factory, you have two processes. Most use spray foam. They have a gun with two nozzles that mix the chemicals from two tanks, supposedly in the right ratio, at the right pressure. The reality is the guns have to be cleaned after every use, not just at the beginning of the day as is the practice in most factories. The ratios have to be checked constantly. The factory floor is rarely at the temperature range required, and the humidity affects it too. So the first shot (usually a test shot into a bag or cardboard box) is great. Everything foams correctly. The exotherm (heat produced) is correct and everything looks good. So the tech starts foaming boats. By the end of the day, the gun is dirty, the ratios are way off, the exotherm is hot enough to burn skin. What you get are fractured cells that aren’t well bonded to each other and look a lot like broken glass (because that’s what they are). Or if the factory is too cold, or the chemicals themselves are too cold ( a lot of places store these outside or in unheated storerooms) you get stuff that doesn’t foam up and looks a lot like bread dough, or cow pies.

The other process is the one you see in most small-volume boat builders. They mix the two parts in a bucket and pour it in the boat. How controlled is that? Need I say more?

As for styrofoam, it is actually great. It almost always comes as block foam. It is usually never in the bilge exposed to gas, cleaners and other toxic chemicals. It is usually (on boats ) in sealed compartments under seats or decks, and never exposed to water until you swamp the boat.

As for freeze-thaw cycles. Freezing and thawing foam itself has little effect on any kind of foam. it’s when you get moisture in it that the problem starts. Foam is very friable. That is. it breaks apart easily. Two-pound density foam is not a good structural material. When water does get in between the cells (not in the cells) and freezes and thaws, it breaks the cells and breaks the bonds between cells. So if you already have bad foam with a lot of unformed cells or broken cells, then water does it’s worst.

As for the pool noodles, The boat maker, contracted with the noodle makers to produce a special run of noodles with no color so they would all be white. It’s Polyethylene foam which is impervious to almost anything. It would cost a lot for a small volume boat builder to do the same. But for a case where you need only a few and don’t care about the color, it’s a good solution.”

Read more on the forum.

Also, it appears that “PeterDE” has also created a site that covers many boat regulations and boat-building ideas.


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