flatwater or whitewater, stiffness is best. For durability, it's more complicated. In some areas of a ww boat that get hard-thumped most often (such as the stern when crossing ledge drops), some flex may reduce damage. But when a composite laminate is bent too sharply, delamination damage accumulates, whether obvious or not.
Exterior cloths, such as glass and carbon, that stiffen the hull, may reduce distortion and show little local compression damage. But if glass or carbon let go, brittle propagation may be extensive. Inside Kevlar layers stiffen also, and may refuse to allow propagation of damage. But Kevlar may show some delamination when the laminate is repeatedly distorted.
I have a super-stiff boat where the hull is S-glass outside and carbon inside. It has taken a lot of moderate whacks with only superficial chipping. I have an old, all Kevlar kayak that flexes easily, and the laminate has never cracked or been torn, but there is some delamination in the chines, which have been repeatedly flexed when the bottom was thumped upward. For long use and long life, the stiff boat would benefit from some Kevlar inside, and the flexi boat would be better with S-glass outside.
Actual lab tests done long ago showed that SS/KK was the best performing combination. Carbon was not included, but probably an SC/KK laminate would be even better.
GO Easy Utility Trailer
Kayak Motor Kit
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