John Winters wrote: >Did I read Peter Chopelas correctly that the ratio of thrust to paddle area was unimportant or, at least the wrong issue? Not exactly. What I was saying is we should be concerned with the direct measure of thrust output vs. work input. At best thrust per unit area is an indirect measure. >Suppose we had two paddles one 120 sq. inches and one 80 square inches. both produce the same thrust at the same velocity and the same power input. Does this mean that both paddles are equally efficient or that the difference is unimportant? That depends on what you are trying to measure. If your measure is thrust out vs. power in, then yes they are exactly the same. There are other measures you might want to consider, there are a lot more things we do with a kayak paddle than just move it forward. >Can anyone think of any reasons why a smaller paddle that produces as much thrust as a larger paddle might prove advantageous? sure, but that was not the question. And measuring thrust per unit area does not tell you which paddle would be best in terms in thrust per unit power input. >Also what is one to make of C.A. Marchaj's use of the Driving Force Coefficient ( Equivalent to the Thrust Coefficient or thrust per unit area)? Marchaj is measuring something different because the source of power is different. On a sail, the power input comes from the wind; the bigger the sail area the higher the power input. When deriving the sail efficiency equation the sail area simply falls out because the power input is directly related to the size of the sail (if you go review how Marcharj derived his equation you will see this). Measuring thrust per unit area on a sail IS a direct measurement of efficiency, you do not need to determine what the power input is since that is directly related to the size of the sail.. On a paddle, the power input is fixed by what is available from the paddler, and presuming steady state paddling conditions, either the heart rate or the O2 uptake would be directly related to the power output of the paddler. Unlike the sail area, the thrust per unit area of the paddle blade tell you nothing of the power input it takes to generate a given thrust. 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 Tue Sep 21 2004 - 11:24:42 PDT
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