I thought I would add to the confusion and contribute some observations on the fluid mechanics of the paddle. With either potential flow (chord wise flow) "thrust", or vortex generated "thrust", the higher the aspect ratio, the lower the induced drag, and better the efficiency of the stroke. The problem is that the high aspect ratio surface tends to stall much easier, and when stalled it will generate much less thrust. For maximum acceleration, as in WW or surf kayaking, or racing, lower aspect ratios tend to be better for maximum thrust, even is somewhat less efficient. For lower speed long distance paddling, high aspect ratios will be better to allow less fatigue over the same distance traveled. It is like comparing a Honda civic with a drag racer; one maximizes distance over the fuel consumed (max MPG), the other maximizes distance over time (max MPH). So it depends on what you are measuring, and what you want to optimize on which type of paddle you choose. You also see the same thing on propellers on aircraft, high powered W.W.II fighters have low aspect ratio prop blades, like big fat paddles, and the low powered human powered aircraft (Albatross and the Condor), have very high aspect ratio props (like Greenland sticks). The thrust on the paddle is better understood by considering the low and high pressure areas on the surfaces created by the movement of the fluid over the blade. It is the movement of the fluid over the surface that generates the thrust, and it is NOT intuitively obvious how that occurs-- it is a complex interaction between the surface and the fluid. In an airplane wing, the acceleration of the air (a fluid) over the wing surface creates a low pressure area on top of the wing, and a high pressure area on the bottom of the wing surface, generating "lift". On a paddle blade, the useful thrust is created by the low pressure side on the forward facing part of the blade, and the high pressure generated on the aft facing surface. This is true for either vortex thrust or attached potencial flow thrust. Most of the thrust is generated on the low pressure forward facing side, and there may be a small component of the drag that is useful to the stroke as well but it is only coincidental. If drag was the primary means of forward motion from the paddle, you would be better off using a tennis racket for a paddle rather than a smooth blade. A low aspect ratio blade or a high aspect ratio blade can produce either vortex thrust or potential flow thrust, but the higher the aspect ratio, always the more efficient, in terms of energy-in vs. energy-out (i.e. thrust). But the higher efficiency is at the cost of limited maximum thrust, hence the reason racers, WW and surf kayakers use low aspect ratio blades, they require maximum thrust for relatively short distances. And depending on the style of stroke you use, you do get mostly cord-wise flow on a Greenland style paddle. The motion of the water over the blade is not quite obvious because the movement is more of a slicing action, and there is the relative motion of the hull over the water. If you pull a Greenland paddle strait back it will flutter (it is stalled, and you have alternating vortexes shedding off of each edge of the blade causing the familiar flutter motion). You have to either angle a Greenland paddle to trap a vortex in one place (creating vortex thrust), or slice it through the water (causing cordwise flow and generating potencial flow thrust) to avoid flutter. Tank tests will not allow for the relative movement of the hull of 3 or 4 knots over the water, which will very significantly affect the flow over the blade, unless you have the water in the tank moving several knots aftward relative to the paddle. So the same reason a high aspect sail plane wing is super efficient at lower speeds, is the same reason a high aspect ratio native kayak paddle is also efficient, at lower speeds. And the same reason a aerobatic or fighter needs the more stall resistant and higher lift generating low aspect ratio wings, is roughly why the low aspect ratio paddles are better suited for racers, WW and surf kayaking. This stall resistance of the low aspect ratio paddle would also be of benefit to beginners since the euro-style blade is much less sensitive to technique. The same issue applies to oars as well, racers use low aspect ratio oars for maximum thrust, and for long distance rowing at moderate speeds, high aspect ratio blades would be best. Be it paddles, oars, wings or propellers, any time you generate thrust (or lift) in a fluid, the same effects are always at work. It is simply the characteristic of fluid dynamic thrust: the higher the aspect ratio of the surface, the lower the induced drag, but at the cost of maximum total available thrust and stall resistance. I hope this help clarify things. 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/ ***************************************************************************
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