Locally at least, it's called "Great Stuff." Be advised that it is
incredibly sticky (wear disposable gloves) and that it continues to expand
slowly, with significant force, for weeks after it's applied. It is
waterproof and airtight.

I don't think I'd use it on a mast except maybe for plugs at each end. Less
is better.

Once you've used part of a can, it's really difficult to come back later for
a second task, kinda like 3M 5200 except the problem is the plugged
applicator tube and valve. The solidified foam turns brown and crumbly when
exposed to sunlight for any length of time.

Roger

http://home.earthlink.net/~derbyrm

"Bowgus" wrote in message
.. .

I did it because I was going insane from the constant chiming noise
something inside the mast was making snip styrofoam peanuts.
Bob Swarts

That makes sense ... and I suppose foam would have solved that particular
noise problem. A very long application tube would be required initially I
would think, to be withdrawn as the mast was filled, to ensure complete
filling.

But "Rob or Karen" did then say just to plug the mast. Home Depot here
sells
what's called Hotstuff (I think) foam in aerosol cans for home insulating
... but I don't know that it would provide the desired waterproof/airproof
seal. Easy enough to find out though I guess ... plug the mast, head out
to
deep water and chuck the mast overboard ... "Rob or Karen" ... let us know
how that works out :-)