Dan wrote; > >I am hoping that you can elaborate on the above. I was talking to a >well-respected Seattle boat designer who suggested that carbon/kevlar >weaves are an excellent material for kayaks from the standpoint of >durability. (SNIP) The problem with these mixed weave fabrics is that they produce stress concentrations within the layer. I have examined four boats that had your typical impact with a rock and there were cracks travelling along the carbon fiber fill extending to a point where the stiffness increased rapidly. I think that the hammer test is not very representative of real life . In real life the impact causes large area flexure that terminates at a bulkhead or the sheer, or the turn of the bilge. Builders of my designs also use these materials and I have had little success in talking them out of it because it looks so sexy. In discussions with J.B. Martin, a company that makes various types of combination materials they have always said that the mixed weaves were poor for boats. They recommend stitched materials (non-woven with the Kevlar on one side and the carbon on the other. This was the type of laminate we used many years ago (we did not have carbon then but used S glass) when I was building racing sailboats and we had great success with it. I can recall Darryl Lideigh (SP?) hammering on his chopped strand canoes and challenging builders of Kevlar boats to do likewise. Of course, it was not an appropriate test. One has to compare similar weights etc. O course, hammering on a laminate is not very scientific or consistent. That is why companies use independent labs with appropriate test facilities to verify laminate characteristics. Your friendly boat builder may build fine boats (I suspect he does) but I would be wary of specious comparisons. Without knowing what types of laminates he was comparing it is awfully hard to say much about the comparison. If were seeking the lightest possible boat there is no question that is would have to be carbon fiber pre pregs over honeycomb and heat cured. If I were seeking the lightest boat that would withstand repeated impacts etc. it would be a Kevlar skinned boat with a syntactic foam core using a high impact resistant microballon and a moderately flexible resin carrier, If I were seeking the toughest damned kayak the world it would be a Kevlar skinned boat with carbon fiber cores, epoxy heat cured resin system and all laminates unidirectional and oriented appropriately. What I have is a cheap Fiberglas kayak, a couple of beat-up strippers canoes, One gorgeous Martin Step stripper canoe that I use once a year, two Kevlar syntactic foam canoes that I use all the time boats and one Royalex kayak that I think I am going to like. Obviously I don't always practice what I preach. :-) Cheers, John Winters Redwing Designs Specialists in Human Powered Watercraft http://home.ican.net/~735769/ *************************************************************************** PaddleWise Paddling Mailing List Submissions: paddlewise_at_lists.intelenet.net Subscriptions: paddlewise-request_at_lists.intelenet.net Website: http://www.gasp-seakayak.net/paddlewise/ ***************************************************************************Received on Thu Apr 30 1998 - 17:57:17 PDT
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