Mike Brown" <embee31_at_hotmail.com> wrote: >>>>>>I must have missed the original somehow. I have a high end Kevlar/Carbon Fiber reinforced boat that had a wave break on the deck and, literally, fold the deck, almost destroying the boat. Ocean going sea kayaks need strength and rigidity in the deck. Carbon fiber is rigid but won't withstand impact. The way most of the Kevlar boats are made, the lay-up is so light that it is too flexible leading to gelcoat cracks. Eventually these cracks will lead to water seepage and blistering besides possibly damaging the underlying fabric. Applying carbon fiber to reinforce a light Kevlar lay-up does not help because the flexibility of the Kevlar can cause the carbon fiber to break. All the above from personal experience with my boat, a Necky, Tesla NM.<<<<<< That was not due to the deck being carbon fiber but due to the laminate being too thin to hold it shape under the load and buckling. Ultralight kayaks should not be taken into the surf or landed in waves on rocky shorelines. Actually pure carbon fiber laminates is stiffer pound for pound than just about any other materials so would be better at preventing folding damage in a given weight kayak than other materials. still at some point the pressure may be too great for a given thickness of laminate to resist folding. When that happens the stiffness that previously helped prevent damage may fail catastrophically due to its brittleness (stiffer is usually more brittle as well). Kevlar on the other hand takes a point blow better than carbon fiber because it stretches more. Unfortunately, unless the Kevlar laminate is thick enough it can me too flexible to stand up to a bending force (like a dumping breaker) and it folds. The tensile strength of the Kevlar fibers may keep the parts from failing catastrophically (given strong seams anyhow) but the more brittle gelcoat is likely to crack where it has been bent too much when it is on the outside of the bend. This leaves stress cracks in the gelcoat. Contrary to what Mike says though, these stress cracks in the gelcoat are for the most part harmless (except maybe if you stored your kayak floating in the water all the time like bigger boats are subjected to). Kevlar is usually mixed with glass fiber layers to take advantage of the far better compression resistance of the glass to lower the risk of folding the kayak. Carbon fiber layers used to reinforce the Kevlar are more subject to delaminating between layers due to their major differences in elasticity. I think mixing the two fibers in the weave of a fabric is the best way to take advantage of the good properties of both materials when trying to make a lightweight kayak with reasonable all around strength. I still wouldn't recommend such a kayak be used in the surf unless extra layers were used to make the laminate thicker (and therefore also both stiffer and tougher), Unfortunately the kayak will also be heavier (but not near as heavy as it would have to have been if made out of chopped strand mat on the deck (so fiberglass could be hand laid over and around the bumps from recessed deck fittings--a major cause of the "heavies" in kayaks). Matt Broze www.marinerkayaks.com *************************************************************************** PaddleWise Paddling Mailing List - Any opinions or suggestions expressed here are solely those of the writer(s). You must assume the entire responsibility for reliance upon them. All postings copyright the author. Submissions: PaddleWise_at_PaddleWise.net Subscriptions: PaddleWise-request_at_PaddleWise.net Website: http://www.paddlewise.net/ ***************************************************************************Received on Wed Jul 02 2003 - 03:13:53 PDT
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