George,

I am not sure that your math has much practical application when deciding upon composite skantlings for a kayak. The formulae probably don't even apply to a simple composite test sample. An actual test (to failure) of each kayak would be required (you'd have to build a lot of boats)! Furthermore, each kayak would have to be of identical design, using wood of equal moisture content.

Reference "The Gougeon Brothers on Boat Construction," page 180, figure 11. Simple, 12" X 12" squares of red cedar/6 oz. cloth (how similar can you get?) were tested to failure. They tested four core thicknesses: 3/16", 1/4", 5/16", and 3/8".

The results were anything but mathematically predictable. The 3/8" sample was 1% "weaker" (pounds to failure), but 33% "stiffer" (deflection at failure), and 13% heavier, as compared to the 5/16" sample.

The 5/16" sample was 36% stronger, 33% stiffer, but only 18% heavier than the 1/4" sample.

My point is that testing is the only reliable method to determine the strength of even simple structures, much less a kayak.

Additionally, your use of the terms "stiffness" and "strength" needs to be defined. Are you talking about modulus of elasticity, deflection at failure, or tensile elongation? It is difficult to apply these terms to a structure as complex as a kayak. Stiffer is not always stronger. Indeed, which mechanical attributes are most desirable in a kayak is open to discussion.

Interesting topic, though.

GB