Nick Shade wrote: - >>> Moving water indicates kinetic energy in the >>> water that could have been kinetic energy for >>> the kayak. > I replied: >> Not necessarily. Water moving in the opposite direction of the >> movement of the kayak indicates that energy was transferred to the >> kayak. We had a lot of discussion some time ago about votex shedding >> off the blade and how that contributes to the lift generated by the >> paddle. And so on. > Nick replied: > Yes, absolutely. Any water motion is energy that in a 100% efficient > system would have been kayak motion. In real life water motion is > inevitable consequence of applying propulsive force against a fluid, > but that does not mean it is not lost energy. Vortex shedding may add > to lift, but the vortex is still lost energy. This not to say that a > paddle that creates a vortex is not more efficient than one that > somehow avoids making the vortex, but lost energy is lost energy > regardless of the form it takes. 100% efficiency is only possible when > pushing against an absolutely stiff object of infinite mass, for our > purposes the solid ground of the earth is a reasonable approximation. > Poling your kayak would be as close to 100% efficient as is possible. You say "Any water motion is energy that in a 100% efficient system would have been kayak motion", but as you note, water in not a perfectly rigid medium. The only way you can generate motion with a paddle is to move water backwards. Water moving in the opposite direction to the kayak's travel indicates energy that was transferred to the kayak. Newton's second law. Vorticies don't necessarily indicate wasted energy either. We talk about vortices "shedding" but the vorticies indicate a low pressure zone behind the paddle which generates lift. If you had perfectly laminar flow around the paddle you'd have no lift, and hence no propulsion. -- mike ------------------------- Michael Edelman medelman_at_ameritech.net http://www.foldingkayaks.org http://www.findascope.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 Sun Jun 08 2003 - 18:49:50 PDT
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