Expanding closed cell foam

Builder's Forum
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Hi all, building a Guider. It has 12 separate compartments for built-in flotation. I am planning on filling a few of them (those not really available for storage use) with expanding closed cell foam as “flotation insurance” should something unexpected happen. A couple of them I plan to partition and partially fill. My main question is the stern compartment. It is fairly large, but will not be available for storage. It does have an inspection port built into the last bulkhead. I’m considering just filling the whole thing with foam. Any thoughts on the wisdom (or lack thereof) of doing that? I have read some advise against its use as it can make repairs challenging down the road should something happen to the hull. Another thought was to line the compartment with a plastic sheet so that the foam doesn’t adhere to the hull interior, but will still entirely fill it. That way should I every have to fully remove the deck for a repair I could theoretically remove the foam easily.

Thanks for thoughts

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I wouldn’t use the expanding foam with a liner. The 2-part structural expanding foam is meant to stick. It is meant to provide support for the surrounding structure, like decks or hull panels, etc. If you want the filler easily removable, use blocks of closed cell foam instead of expanding foam. Save the expanding foam for when you want to actually permanently seal a compartment.

Laszlo

1 Like
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i am generally not a fan of foam in flotation compartment of any sort.

if you need foam (or other methods like airbags) to ensure positive flotation, it should be attached to the boat in a way that it is inspectable and removable with little effort.

flotation compartments should also be inspectable in my view.

the problem with mixing the concepts - a flotation compartment with foam that is blown in or not easily removable is that you don’t now have visibility into the compartment to see whats actually happening in there and confirm easily that it is dry and no problem with the structure.

the only time i have personally been sailing when it was required to have positive flotation was when i was a dinghy sailor in a frostbiting fleet where swamping a boat was a distinct possibility. but we managed that with foam blocks that had to be strapped under the seats - no obstruction of the hull and easy to confirm the blocks were in fact in place as required by the rules. they also had to be romovable per the rules so the fleet inspector could confirm your hull was in good repair.

my experience when people or manufacturers put foam into a flotation compartment was typically in the negative trying to sort out how water got in there and now having to remove waterlogged foam with a knife or other tool to get back to the hull. when it was put there by the manufacturer - the manufacturer was not typically a respected brand.

i have also sailed quite a bit in a variety of racing boats and in those cases, foam inside an alledgedly watertight compartment was never part of the safety engineering. wateright compartments were definitely part of it…but not both.

boats like Boston Whalers also have a non-sinkable hull approach….but again, they engineer that foam into the hull….they don’t comingle it in watertight compartments.

anyway….that’s just my thoughts and practical experience/observations on the matter - its not a practice used in industry by sophisticated manufacturers and not one i would pull into the wooden/epoxy builders space.

h

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I would not use any type of almost-permanent foam in a boat mainly because when - not if! - you need to get into that area to repair some damage you will find a real mess. We had an oldish Mirror dinghy that my daughter used to race and where a previous owner had used expanding foam to fill under the seats. maybe in the mistaken belief that this was better than the standard tanks under the seats. When inevitably a hole appeared one day after some spirited youth racing it took me days to hack out the old foam to make a good repair.

In general, I’d prefer not to see any totally sealed areas on a wooden boat, and to use removable flotation bags or blocks.

There are also stories, some apocryphal no doubt, where people have used expanding foam and ended up bursting the tanks.

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I did a repair on a fiberglass canoe that had expanding foam in the end tanks. It was 30+ years old and had shrank. I removed the decks (aluminum) and cut the old foam out. Yeah, you have to be careful with the new stuff as it really expands a lot. I had to trim the overflow before I put the decks back on.

Edit…this was 2 part TotalBoat flotation foam.

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To paraphrase dog trainers, there’s no such thing as bad foam, just bad boatbuilders/boaters.

Seriously, though, if foam is properly used it’s not a problem. The problem is that many builders do not understand its uses and, most importantly, its limitations and use it the wrong way in the wrong boats and that’s what results in the horror stories.

First off, the correct expanding foam for boats is the Coast Guard-approved, 2-part, closed cell, structural, 2 lb/cu ft polyurethane foam. It is is most certainly not the stuff in a spray can from the local DYI store. No one in this thread has mentioned using anything other than the correct stuff, but I see it all over the internet where someone uses the spray can stuff in their boats. That is a recipe for disaster.

Second, builders need to understand that the foam is there to provide emergency flotation in the event of a hull failure until the boat can get back to shore and repairs made. It is not meant to stand up to years of immersion in water. If the foam is getting wet, the emergency has already happened; the hull has failed. Continuing to use the boat in that condition is what causes the problems with waterlogged foam, not the unsuitability of the foam itself.

As Howard mentions, correct engineering is important. You can’t just start filling random compartments with no consideration for the surrounding structure. Ideally, the compartment should be stiff and watertight before the foam is added. That means proper sealing, inside and out, with epoxy encapsulation and even a layer of glass. The compartment should be vented when the foam is added to prevent the blowouts that Peter mentions. But most importantly, the reason for using the foam needs to be kept in mind. Compartments that are not at risk of puncture and not critical to keeping the boat afloat would probably do better with a different solution.

As a real world example, my 16-year-old Brand X dinghy has foam in a compartment in the bow and in a second compartment in the stern. In both cases the bottom of the compartment is also the bottom of the boat. These are the areas most likely to hit rocks and other obstacles. The remaining compartments are air-filled.

Constant maintenance is also a must. Foam-filled compartments can’t just be ignored because they’re sealed. In my boat I inspect the bottom of the boat several times a year, looking for scratches and gouges. I also do a tap test to see if the wood is changing (most likely as a result of water intrusion). This kind of inspection will find problems before they get out of hand.

To sum up, expanding foam is simply another tool. It’s not a panacea, nor does it need a blanket prohibition. Like every other boatbuilding tool, it needs to be understood and used where appropriate, keeping in mind its pros and cons.

Laszlo

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Thanks everyone. I really appreciate all of the perspectives. Sounds like no perfect answer with pros and cons.