... is saving weight."
In whitewater boats, Kevlar's main advantage is making a boat more impact resistant. Weight savings is certainly nice, but replacing inner Kevlar layers with carbon would mean catastrophic failure. Kevlar keeps the pieces smaller, and keeps them together.
Kevlar is a great "inside" cloth, but deficient in two ways when used for outside layers. First, as already noted, it has mediocre strength in compression, and compression strength is important in outer layers both for stiffness and for resisting blows. Second, though Kevlar fibers resist tearing from being dragged over rough surfaces, that resistance can cause them to pull out of the resin and "fuzz". That fuzz won't sand off (I've tried torching before sanding) and is a nuisance to correct.
Glass as outside layers makes for better compression strength, wears smooth, and is easy to sand before repairs. S-glass is the hardest readily available cloth for outside layers.
An S-glass/carbon cloth has been available, but I haven't heard of it being used in a boat. A Kevlar/S-glass weave may not be feasible because usually the glass has to be coated, in part for weaving purposes, in part for good resin adhesion.
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