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: I haven't made a strip built kayak, but I just assume that actually
: fiberglass would not be needed for just paddling. It's needed to make
: protection against rocks and such. I'm building a C&G kayak from foam. The
: foam's lightness can not be utilized because the thin fiberglass (which
: would be otherwise enough) will not protect against rocks or other hits.

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Hi Tony,
On a S&G, you can do without full glassing, just using FG tape at the seams/chines as the ply acts as a contigious material with its own inherent strength and rigidity. Strip built has a seam every inch or so and the grain is all in one direction so has much less rigidity in one direction, laterally. Fore and aft strength is good. The inner and outer glass supplies that strength while the wood core acts to seperate the layers of glass using the concept of an I-beam.
One of the reasons many pre-epoxy moulded hulls were built using double-diagonal stripping, effectively turning plain wood into ply.

Foam use as a core can work, but the foam density must be matched to the expected usage. Higher density foam at high risk locations, lighter at lower risk areas of the hull. Generally, the keel-line is solid FG with the foam feathered so the laminates meet this without a step. Not too practicible for a kayak.
Using two or more densities of foam, the densest would be used for the bottom two panels, around the cockpit and hatches, with less dense foam elsewhere.
Because foam doesn't have grain, more glass is needed to supply strength in all directions than a ply or cedar core. Several alternating layers of uni-directional rovings would give a stronger hull without increasing the resin weight very much.
As others have posted, the weight savings with foam only kick in when the hull is above a certain size, about 25', similar to ferro-cement hulls.
Where foam comes into its own is for FG prototypes or one-off's. No expensive mold needed.

Mike Savage
South West Cork