This week I experimented to determine how well one-part polyurethane foam flows and expands to fill a mast tube. I used an 8-foot length of common 2-inch PVC pipe with roughly the same inside diameter of a 170 mast, and a 16-oz. can of "Great Stuff Insulating Foam Sealant - Gaps & Cracks" from a local home supply store. With the pipe propped up at 45 degrees, and both ends of the pipe open, I inserted the straw nozzle into the upper end as far as it would go (6 inches). Over the next minute or so, I squirted as much foam into the pipe as I could. When the foam started to expand back out of the top end, I paused until it settled, then squirted more foam. I injected about three-fourths of the can before the pipe wouldn't accept any more. I then capped the upper end of the pipe with a tight-fitting PVC cap (to simulate a mast cap) and pinned it in place with nail inserted through pre-drilled holes. The air temp was 75 degrees F., RH at 48 percent, and I did not rotate or shake the pipe during or after injecting the foam. 24 hours later, I cut the pipe into 3-inch long sections, beginning at the capped end. Below is a photo of the results. From L-R, bottom row are sections at 3,6,9,12,15 and 18 inches. Top row L-R are sections at 21,24,27,30 and 33 inches. The only place I found fully-expanded, hardened foam, was a plug less than 3 inches long, at the farthest limit of the foam's travel -- less than 34 inches from the top. The remaining sections were largely hollow and coated with still-viscous, sticky, unexpanded goop. Once the pipe was cut up and the individual sections were exposed to air, the remaining foam expanded and hardened in about an hour. Before I began, I had expected to find mostly hardened foam with occasional pockets of unexpanded goop. But when I started cutting, I was surprised that most of the pipe remained hollow, as most of the goop remained unexpanded and unhardened. (And I'm still trying to remove a hardened mix of PVC chips and adhesive from the blade and guard of my Skil saw!) Based on the above, I would be reluctant to shoot more than a small amount of this foam into my mast or boom -- just enough to form a plug not much longer than it is wide. Earlier this summer, I shot enough into my mast to form about a 12-inch plug and now I wonder if even that hardened all the way through. The manufacturer of "Great Stuff" makes another variety of this foam, designed for big gaps, and that might work better inside a mast, but it is unavailable where I live. Also, there might be a better way to inject the foam, possibly by extending the straw with clear plastic tubing, but I'm still cleaning up from this mess, so I'll leave that experiment to someone else. I welcome anyone else's perspective on this. Happy sailing.