Stress concentrations tend to happen where there is a sudden change in stiffness. This can happen with something that is stiff in the area of the load, but is less stiff elsewhere, or if it is flexible at the load point and stiffer elsewhere.

A friend of mine is very successfully making flexible Kevlar composite boats, but he had some failures early on at the deck-hull joint where he taped them together. The flex of the boat was concentrating the stresses at the stiffer joint and causing eventual failures. By changing his joint bonding procedure to make it more flexible (more closely matching the rest of the boat) he fixed the problem.

I think a flexible boat can be built very ruggedly, but the engineering to get the whole system correct tends to be more complex. For example flexing introduces the possibility of fatigue, which epoxy and fiberglass don't tolerate well. The Aleut built extremely flexible boats and we still don't why they did everything they did, but they were working with wood which is very fatigue resistant.

> Hopefully you can draw some parallels to a kayak hull parked on top of a
> rock -- that if it flexes, the stresses are less concentrated in the area
> of contact with the rock, and that failure is less likely.

> In the words of the old jazz standard, "somethin's got to give,
> somethin's got to give, somethin's got to give..."

> End ramble.

> Dean