2 part polyurethane foam (mix and pour) use in flotation tanks

Builder's Forum
1

Making slow progress on our Jimmy Skiff.  I have been looking at youtube videos using closed cell polyurethane foam kits to fill up flotation tanks. 

 

It appears this gives you closed cell foam that fills the space efficently and easily, adds some rigidity in these spaces, gets about 60 lbs per cubic foot expanded foam.  

 

Downsides--messy, has to be done before top of tank is put in, possibly more expensive than cutting foam to fit

 

Any thoughts from the experts on the wisdom or idiocy of doing this?

 

Thanks, Henry Clemo

2

   As I recall it checked it out and the two part foam was heavier and more expensive.

3

 First:  If you've not played with that 2-part foam, know that it is more dense and wildly sticky and messy, and hard to control/estimate the volume you'll get upon expansion (and therefore how much over-fill and mess you'll have to be ready to either pre-mitigate or deal with afterwards.  I just used it to set some chain link fence posts to good effect, and very much easier than concrete.  But it gave 30 seconds to mix and pour, and spouted out of the post-hole like a vulcano if overfilled.

Second: Though probably not a problem in a tank/compartment, the foam has very poor durability if exposed to U/V. If it is the stuff I'm thinking of, the surfece will turn to a grainy, sany powder after only months of exposure.

Third: Without Hurculean effort, you'll never access the actual epoxy/wood surface coverd by (bonded to) the foam in the future, yet the foam might not provide a perfect water barrier.  So if you ever have a crack or flaw or whatever that allows water to reach wood inside the foamed compartment, rot will commence, you'll never see it, and probably never be able to get to it to fix it.

Therefore, I wouldn't do it to a wooden boat.  But in other applications, like inside the sandwich of a Boston Whaler hull, I think this is type of stuff they use, to good effect.

4

First, the Coast Guard approved foam weighs 2 lbs per cubic foot so it provides 60 lbs of buoyancy per cubic foot. It does not weigh 60 lbs per cubic foot.

I used it in my Brand-X dinghy and I was happy with it.

As you can see, I overfilled it, just as Bubblehead predicted, but this was my first time using it. In subsequent attempts I've come closer. The  exact expansion is temperature-controlled so it's important to allow for that. Note that it is always possible to do multiple small pours instead of 1 large one.

The overfill is easily cut with a saw, razor knife, etc., and sanded to the final shape. I actually find it easier to work with than the hardware store insulation sheets in that it's easier to control the shape. But it does make lots and lots of dust that sticks to everything. A respirator, eye protection, clothing that you're willing to dispose of and working outdoors are all absolutely necessary if you use mechanical means (saws, sander, rasp, etc.) to shape the foam.

I find actually mixing and pouring it no problem. At its nominal temperature it makes 1 cubic foot of foam per quart of mix. so the first job is to estimate the target volume. You can get creative with irregular volumes and do it by pouring in beans, packing peanuts, etc. and then seeing how much of a regularly-spaced box that all fills and adjusting your mix amounts. It's usually a 50:50 mix by volume so that's easy. And, best of all, as you mix it, it changes color just before it starts to kick (unlike epoxy).

It does expand very forcefully, that's why in my boat I left the top off the compartment until after the pour. The stuff can expand and break a closed compartment wide open.

The inside of the compartment should be epoxy-saturated and glassed, but with the weave unfilled. The foam will grip that roughness like a bulldog. The epoxy will form an internal water barrier, while the glass makes sure that you have an even minimum layer of epoxy everywhere, as well as providing tensile strength to resist any forces from collisions. If the compartment is entirely filled the foam will provide additional compressive strength in case of collisions.

If the outside of the compartment is also glassed the chances of micro-cracks and rot are vanishingly small. In any case, periodic tap tests (where you tap the hull and listen for changes in the sound of the wood) will let you find rot before it gets out of hand.

Bubblehead is absolutely right in that it has no UV resistance to speak of, but it's also never meant to be used unless it's covered with some kind of durable covering like wood, glass, carbon fiber, etc. so that should never be a problem. Plain paint or varnish is not a suitable covering, it has to be a hard surface.

So that's my 2 cents worth. I still have the boat in the picture and everything's holding up fine 12 years later. I used the foam in the bow and under the stern seats, the 2 places where a collision is most likely. If you use it the way it's meant to be used, I think the stuff is perfectly viable for wooden boats.

Laszlo

 

5

   Henry-- I used Total Boat 2 part foam to make the aft flotation block for my tenderly. I put down plastic wrap in the compartment so I could remove the foam fairly easily after it cured. Poured it, let it cure, removed it, shaped it, sanded, covered in epoxy thickened with phenolic micro balloons, sanded, painted, glued to bottom of seat. I think that's a different application than the Jimmy Skiff (foam block vs foam filled compartment), but FWIW it worked fine. That said, it wasn't any easier than just building the foam blocks out of the two inch R10 foam per the instructions. I probably wouldn't do it again-- but it worked, and the stuff is kind of fun to play with.