•  Sawfish, an unsinkable, lightweight, foam kayak (23 lbs). Free DIY kayak plans, the hardware store boat

    Instead of sheets you could use woven denier nylon or pack cloth, about 7 dollars a yard with a 59 inch width. Very strong, much stronger than sheets. When painted it's waterproof. I would think it would also be structural, so the foam could be even thinner in sections and it would create a very strong structure, assuming the paint or glue bonded well to the foam. This is the same kind of material that's used for skin on frame kayaks. They use silicone waterproofing to make it waterproof. I don't think that would work well as a bonding agent to the foam, a small amount would need to be tested. Perhaps an acrylic or oil based paint?You could also build a wooden hoop and tie the nylon onto it, so instead of having a sit on kayak, you can have a sit in kayak that can be used with a skirt and rolled in the water without getting water into the inside of the kayak. I made a skin on frame kayak, the pack nylon is easy to use and is very strong, very tough. I think it would work well. Anyways, all in all I think your foam kayak is a great idea!

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