David Carlson wrote: > for a given stroke length the slippage depends only on the resistance > coefficients of the paddle and the boat, so of course,.... distance the paddle > slips through the water with each stroke remains the same. The thrust a paddle generates is a complex interaction between the blade shape, the motion of the blade through the water, the Reynolds number (a kind of measure of turbulence) and the fluid movement, there is no such "resistance coefficient" that could accurately describe all the variables, and it is not really related to the "resistance" you feel in the paddle. > > This leads to a simple efficiency calculation, using Power out/ Power in as the > definition of efficiency The only way to measure "power out vs. power in" in the whole paddler/paddle/kayak/water system is to measure total oxygen uptake for the same paddler going exactly the same speed, in exactly the same kayak, in exactly the same wind and sea conditions. The more O2 uptake at the same speed, the more power is being consumed. "Slippage" of the paddle is not easily measured and irrelevant anyway. > How reasonable are the equations? Your equations are not reasonable at all, it shows your ignorance of fluid mechanics, and how complicated the issue is. The paddle is way more complex, and has way more effect of the whole hull/paddler/paddle system, than the hull shape (yet oddly it is the least considered by recreational kayakers, and the least understood). There is no direct relationship between "slippage" and efficiency, what you describe is like riding a 10-speed bicycle up a steep hill in high gear or in low gear. Then calling the low gear a gear that you have a lot of "slippage" in, and therefore less efficient. Remember basic physics F=Ma, you get force from the paddle by accelerating the mass of the water. The more you accelerate the mass of the water in a direction that is useful to generate forward motion in the boat, the more efficient it is. You can put exactly the same "power in" with various paddle designs, but not all will push the boat at the same hull speed. All will put the same amount of power in the water, but not all will result in useful forward motion, the rest is wasted in canceled effects, turbulence (i.e. friction), etc. And each paddle will have a slightly different motion to get the most efficiency out of it so it would take some time before you determine the optimum stroke with each paddle you try. When a paddle slices (or "slips") through the water, you accelerate the mass of the water and get thrust, hopefully most of it in a direction that is useful to propel the boat forward. If you accelerate a lot of water in a useful direction with a small blade that slices quickly through the water, you will have a very efficient paddle, with large apparent "slippage". You can also force a large poorly shaped paddle slowly through the water and not get a lot of forward thrust for your effort, yet it would appear to have less "slippage" that the small blade. Also "Jack Fu" <SeaDogJack_at_cablespeed.com> wrote: > Subject: [Paddlewise] Technology guides paddle design. > > When the Eskimos (or related peoples) designed their paddles, > what materials did they have to work with? Driftwood? Bone? > From these materials you cannot build a shaft with wide blades, > because the blades would break. The only blades that would > last would be narrow (e.g., Greenland) blades. Oh Jack, totally false! Other native peoples made large low aspect ratio blades shaped like a tennis racket, the native kayaker could have too. Large bones from walrus or whales, a bent wood frame covered with skins, large driftwood logs "shake" cut, all would have and could have been used to make "Euro" style blades, but they did not because they are less effective. > Thus I suspect that the Greenland design came into being not because those > early folks rejected a wide blade design in favor of a narrow > blade, but because they did not have the materials (strong > glues for laminating, plastics, composites, etc - all the products > of technologically more advanced civilizations) needed to make > the narrow shaft & wide blade combination, or what is sometimes > called, condescendingly by some, the "white man's paddle." If they had modern materials they would have used them certainly, but they knew a lot more about what works than modern recreational paddle makers know. Thousands of years of not just surviving, but thriving in one of the harshest environments in the world, they knew what worked and what did not by many years of careful observations. No modern make can claim such credentials, and to my observation, no modern maker even understands the fluid mechanics of the paddle, or they would shape them differently. You assumption that modern paddle makers of recreational paddles know anything about fluid mechanics, and would use this knowledge to make their designs more efficient, is just not so. The forces involved are not obvious or intuitive, it is not something that can be accidentally discovered except by vary careful observations and understanding how changes in design will change the performance of the blade. This is not meant as a flame to you or anyone else involved, it is just a matter of fact. Go out and learn how to properly use a native kayak paddle and see for yourself, there are very sound engineering principles involved on why the high aspect ratio paddles are more efficient. But it is easier for you just to take my word for it and go try it rather than teach you fluid mechanics. all the best, Peter Chopelas *************************************************************************** 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 May 14 2001 - 21:30:34 PDT
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