Nick wrote: >The loading of a kayak in surf is not the same as to load from dropping a kayak 6'. This is not to say that SOF boats are weak, or that they can't be used in surf, only that surviving a drop test is not all that relevant. A better test would be to put the boat across 2 saw horses and have someone hop up and down in the cockpit. At least then you testing with loads similar to those experienced in real use. I agree that dropping a kayak is not a valid test, but neither is bending it across two saw horses. The loading on a hull is more complex and if and when failure occurs it happens for different reasons at different points on the hull. To say one is inherently better is a false comparison, you can design either a SOF or hard shell as strong as you want it, comparing a strong SOF to a weak hard-shell (and visaversa) tells you nothing. The question is how do kayak hulls usually fail? I have heard of only a few failing in buckling in surf, and more often from impacts on rocks or other hard objects. I have also seen fatigue failure in old and abused hulls. I do not know which is more common, and if it is even a big issue with most kayak owners. Ramming rocks is not what they were designed for, and not what most people would consider "normal" use in a sea kayak. >The likely region of failure in most long sea kayaks would be in the cockpit area. I suspect SOF boats will have higher stress concentrations than monocoque boats because all the load will be concentrated in a few small parts: ribs, gunwales, deck beams. This is untrue. The genius of a properly built SOF is that the load is distributed among many smaller parts, reducing the risk of a catastrophic failure. In fact the traditional SOF frame is a fail-safe redundant structure. Very similar design is used on almost all modern air craft structures. If any one member fails in the hull structure, the others carry the forces around the failed part, prevent a complete failure of the structure. Of course a poorly designed frames can have weak points, but this would be true with a hard shell kayak as well. I even known of SOF owners that have contiued using their kayaks with broken guwales or keels for many trips, untill they had the time to remove the skin and repair the damage. In fact the hollow shell structure of a fiberglass or rotomolded hull is not fail safe. Though the hull forces are nicely distributed over the hull skin, the failure mode is usually in buckling (that is when loaded in bending). To prevent the hollow hull from buckling you have to make the skin strong and stiff, adding a lot of weight. The softer rotomolded hulls I think are especially susceptible to bucking failure. And if there are dimples, dings or dents in the hard shell to start out with, the risk of bucking failure is very high on a hard shell, this is not true of a SOF (standing on an empty soda can will demonstrate this very real problem). The SOF construction has discrete individual stringers that resist bending, and they are free to flex a lot, long before failure occurs (and of course the skin can buckle and flex all it wants without failure). This is why SOF hulls, that enjoy some flex without risk of failure, that make them easier to control in rough water, it acts like a suspension of sorts absorbing the impacts, as compared to a rigid hard shell. This is also the reason I think SOF are so much lighter, there does not need to be such thick structure to prevent a failure. Landing on sharp hard rocks is another mode of failure that likely the hard-shells are better at than SOF, especially the roto molded hulls. Though all types of construction will get damaged in such an impact, individual frame members of the SOF would be more susceptible I think. Though as long as the skin does not get punctured (a real risk, but yet rare occurrence), you are still seaworthy. In very serious impacts the fiberglass hull can completely collapse and/or puncture, making field repairs nearly impossible. I think the SOF are superior in this instance since even if the skin is punctured, temp field repairs (if necessary at all) are relatively easy. Of coarse the rotomolded hulls are great for such abuse, they only get scuffed and pop right back, which makes them great for a rental fleet. When designing a hard-shell, they must be designed against buckling failure, and for fiberglass, sharp impact loads. When designing a skin on frame, the frame must be design to resist failure from frame flex, meaning all of the joists must have some "give" in them or the joints will fail and cause a stress concentration. I think the SOF is a superior way to build a kayak, but unfortunately it does lend itself well to production nor to modern desires for bulkheads, hatches, rudders and other modern inventions. I do not think there are any manufacturers of non-folding SOF kayaks, it is a shame since they can perform in many areas superior to hard-shell kayaks, and use much less materials and weigh much less too. But again it takes proper careful and proper design to optimize either type of construction method. And if the consumers of production kayaks are not comfortable with the idea of 25 to 30 pound kayaks being just as safe and durable as 60 to 70 hard-shells, you will not be able to stay in business. Peter *************************************************************************** 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 Mon Sep 20 2004 - 11:58:18 PDT
This archive was generated by hypermail 2.4.0 : Thu Aug 21 2025 - 16:31:17 PDT