Date: 8/29/2007
Name: Bob McGovern
email: bobmcgov@wildblue.net

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subject: Spray foam recore

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Think your idea is still valid -- "end grain wood" and "water" just don't belong in the same universe. What you are trying to create is a core-box, one of the strongest and lightest construction methods. But for that to work, the two skins need to be held rigid relative to one another, and I suspect your foam layer is allowing enuf lateral "shear" that rigidity is compromised. Hence the flex.

I'd suggest inexpensive aluminum I-channel or C-channel, available in many hardware stores. It can be flexed over your knee to fit the deck profile, then roughed-up & lightly epoxied to the deck underside. Butt the pieces together -- no fancy joinery needed. An X shape might work best, or maybe a grid that runs to the vertical edges of the cabin. Some infill here and there.

THEN add the expanding foam. In my head, I see turning the boat turtle and pouring in a 2-part, minimally-expanded polyurethane. It will lock the aluminum channel into place; then level to the surface of the channel (nasty work) and glass over it. The channel will provide a *longitudinally* rigid gridwork that ties the outer fiberglass skins to each other. That's what makes a core-box so strong. Good idea, and good luck!

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