Thank you, Peter for the detailed and patient explanation. You must have been saving it up for when this topic resurfaced on Paddlewise. I had the same question Wes did though. Are you using the paddle as a foil when comparing AR efficiency or does this still all somehow apply with a stalled foil as well? There seem to be several practical limits to using high aspect ratio paddles. You must either reduce the blade area or make the blades very long to get a high AR and maintain the blade area. Reducing the blade area reduces the thrust. Making the blades longer (2"x54") has several problems I can imagine. Since you are pivoting the paddle during the stroke the pressure at different points along the blade will vary (higher at the tip and less and less as you measure going towards your hand. The pressure could become negative (meaning that nearer the hand the blade might be moving forward under water rather than backward. If that were the case, that part of the blade would be using power to thrust in the wrong direction and be working against you. Not only would a 54" long blade be ungainly to get in and out of the water (and lift the extra blade weight--that also has more leverage being so far away). It will also be a much harder paddle to control during the stroke because another characteristic of high AR is a that it stalls at a shallower angle of attack so blade placement angle will become very critical and the flutter rate of the blade will become uncontrollably fast under higher loads). I suspect that since the foil penetrates the surface at full width you will also likely have an efficiency loss due to ventilation (air getting behind the blade and destroying the lift). The long blade needs to be sliced down into the water--while remaining closer to horizontal than vertical (because it is so long) and the direction (and the angle of attack) needs to change abruptly at some point while the blade is underwater so it can return to the surface and generate lift again on its upward path to the surface. There will be a loss of efficiency from stopping the momentum in one direction and them accelerating again once you have reversed blade direction. A wing paddle stroke starts close to the kayak and moves outward until the blade reaches the surface without any abrupt direction changes (energy robbing stops and starts). I suggest you get the high aspect ratio by using two very small foils, say 24"x1.5", (on each end of the paddle-four foils in all) for your paddle blades. They would lie next to each other separated by half an inch or so. That way you will get two narrow lifting foils on each end of the paddle (so the high AR paddle doesn't have to be so long to get the same blade area) and as long as they are used like a wing to generate "lift" you will also benefit from the "slot effect" between the blades increasing the "lift". I'm hereby trademarking the names "Bye-paddle", "bi-paddle", "slot-paddle", "forked stick", and the slogan "four blades are better than two". You could get rid of the ventilation problem by keeping the blades entirely under water and maybe centered directly below the kayak (perhaps spinning around on a shaft that could be spun continuously in the same direction and powered by your strong legs and feet on pedals while leaving your weaker arms and hands free to perform tasks requiring greater dexterity. Of course, we'd have trouble calling it a kayak anymore. It would be way more efficient that way as it could also take more advantage of the high AR efficiency gains with out the high AR paddles problems. A few weeks ago I saw an ultralight airplane that had two 9.5 HP engines that turned two about 18" long by 1" wide "slot effect" dual propellers. As hard as it was to believe, dat thing could fly, I seen it wit mine own eyes. Matt Broze http://www.marinerkayaks.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 Jul 28 2002 - 05:06:43 PDT
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