Replacement Mast Plug: "Great Stuff" Spray Foam?

Has anyone tried using Great Stuff spray foam as a replacement mast plug? It doesn't seem to be the ideal product for this, but I am in a pinch to get this done and need to find something locally.

The product details include the word "waterproof", but I wonder if it would take the 30' of water head (turtle).

I think it would work, but I used it during my comptip installation and it didnt expand and dry like I thought it would but from what I see it is water proof??

I think it would work. In theory you could fill the whole mast (top to bottom) with the stuff. Might add a 1/2 lb of weight to the mast.

Patrick

Bad, bad idea.

Great Stuff is approximately 2 lbs / cu ft, although in a confined space like that, it's likely to be more dense. Let's call it 3 pcf (2 part urethane foam is available in densities up to 8 pcf)

The internal volume of a Hobie 16's mast is about 2 cf. So you'd be adding 6 lbs of weight to the mast. That's a lot of weight.

And it will still leak - but now the water won't easily drain out.

The foam plug in the mast is really not intended to seal the mast entirely. It's meant to provide a backup to the metal plate that goes on top of it - the plate is sealed in with urethane caulk (like Sikaflex or 3M 5200). That's what provides the water tight seal.

So, I went ahead and bought some "Great Stuff"; the type that cures rigid. I guess we will see how "great" it is for the task at hand. I also bought a pool noodle, trimmed to fit around the mast tang rivets and stuffed it in there to give enough clearance for the comptip fit plus about a 1/2 of extra clearance. I filled in the nooks and crannies of the noodle with the great stuff and hopefully that will help seal the rivets somewhat. Once I am sure that the foam is cured I will put the metal plate back in and seal it. Which leads me to my next potential problem. Instead of buying 3M 5200 or some other marine adhesive/sealant like I probably should have, I bought a couple of tubes of silicone caulk for tub and shower sealing. Since I realized this wasn't the best product for the application, I compensated by gooping as much as would possibly fit (about an inch) in the ends of the comptip. I now am wondering if the caulk at the bottom of the goop pile will ever cure.

Anyone else been down this road and can validate or suppress my fears? I think the worst case is that the mast will at least be MORE sealed than it was before and best case is a total seal (doubtful). After the next time I turtle, I will report back.

BTW, thanks for the replies!

Great stuff is a wall filler I thought?

This is not likely a "Closed Cell" floatation foam and would absorb water.

_________________
Matt Miller
Former - Director of Parts and Accessory Sales
Warranty and Technical Support
Hobie Cat USA
(Retired 11/7/2022)

I did this on an 18 that I owned back in the early 90's. it was great at first, but it made the mast a little heavier...no biggie. After a while the foam gets waterlogged and makes the mast really heavy. Eventually, the boat would turtle everytime I went over. I had to take the comptip off and dig all of this waterlogged foam out. It sounds like a good idea...but...

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Sail Revolution
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Never ever use great stuff, it is not a polyurathane close cell foam, that stuff will soak up water. You must use a 2 part polyurathane foam and it must be poured. You can do a search for closed cell polyurathare foam and a good website will come up. They produce 2lb, 4lb, 8lb, and 16lb depending on the density you wish.

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Coral Reefer
H-16
Flagler Beach, Fl