Expanding foam as a class of stuff has remarkable properties--and not just for sealing mouse holes under the refrigerator. It was six years ago I made a floatation plug for my pram. I approached the expanding foam project with trepidation, was satisfied with the result. That ridiculous plug is still behind my garage, semi-exposed to weather. It remains hard as a rock and I;ve never had the heart to saw it up. Not that this has anything to do with Injectadeck, but it amuses me to remember....(excerpt from from a blog entry)
More flotation was definitely needed for the dinghy's lifeboat role. It came in two cans--as two-part, 2-ounce expanding foam from U.S. Composites, at about $85 the gallon.
Although the chemical reaction causes heat, it's not all that much. So I just lined the forward well of the dink with cardboard, taped black plastic trash bags to contain the expansion in the necessary shape, mixed the components for the recommended 15 seconds, poured like it was nitroglycerin and ran behind the blast wall.
For the foam to rise the ambient temperature needs to be 70F, and 80F is better. Every ten degrees below 80 means 20 percent less expansion. I did the fill in three pours, since it is difficult to calculate the result. Urged by a heat gun, the foam rose in minutes and was hard and smooth in half an hour. When done in close sequence, each new layer adheres to the old. In three hours out popped a perfectly sized plug of foam that looks like it will support 200 or 300 pounds. Great fun.
I screwed a piece of 3/8 inch plywood over it to hold it firmly down. Added weight, about 8 pounds. Some lines looped around the gunnels give a lifeboat-like appearance and place to clip on a safety harness.