Chris, by how much does the pink board deform under various loads? How soft or squmbly (tech term) is it? It doesn't have to be as stiff, tough, and etc as plywood. For an instant boat you may need to add some panel curvature by adding some arc to bulkheads or frames, or just make the foam layer thicker. Use less expensive polyester resin and glass for same to lower the cost. Probably overall similar/lower cost than a plywood boat from better stuff. I don't think you'd need any complicated set up for an instant boat (molds, plugs, etc). Cut the foam panels, hot glue tack them together, round angles off and fillet as the spirit moves, apply glass.
Thinks.. asks.. doesn't Dave Gray knock PDRs (family Brickoboatean) out of foam sandwich to good effect?
Another example, not instant though a Bolger: DOLPHIN #259, surf pulling boat for 4 oars, load waterline from 660lb to 1200lb, 17lb lead ballast in the sailing version centreboard, keel, thwarts, breasthooks, knees, posts, hefty bouyancy FOAM SUPPORT to LARGE stern and fore sheets, 21" amidships keel to gunwhale height, 18'x 4' overall, 0.5" strip plank build, ought to weigh what do you think empty? 250 to 300lbs?
What's that? Heavier?
Well then, try only 100lb got out of 3/8" foam sandwich:
--- In email@example.com, "Christopher C. Wetherill" <wetherillc@...> wrote:
> All this discussion has brought to mind an old question I have been
> pondering. Has anyone built an Instant Boat from 1/2" pink board with
> fiberglass on both sides? Issues I have been considering
> 1) easier to work than plywood
> 2) slightly less costly than plywood
> 3) not as stiff
> 4) possibly lighter than plywood.
> I suspect that the need to glass both sides will result in no savings of
> cost or weight.