Bob Myers wrote: > Slippage and efficiency are related, and what is more, it can be > mathematically related to power required. I do not know on what basis you say this, but with regards to my last posting I suppose this could be argued but it is not nearly enough information by itself to determine efficiency, and all of the other complicated relationships that would have to be included to the equations. By itself it is meaningless, see my last posting. > So if you pull the paddle blade > backwards faster than the kayak moves forward (slippage), you're using > more power than is going into propelling the kayak. (The forces must > be equal by Newton's third law.) That's a direct measure of > efficiency. I am sorry, this is not a measure of efficiency as I would define in my last post. Slippage, whether related or not to efficiency, would be included in the total efficacy if you just directly measured power in at the same speed. Beside, what makes you think a high aspect ratio paddle will have high slippage? If you make two paddles with exactly the same blade areas, one high aspect ratio, and low aspect ratio, the high aspect ratio paddle will consume less power moving you the same speed in the same kayak in the same conditions. There is no reason to assume a high aspect ratio paddle will slip more than a low aspect ratio paddle. It will not in fact. > we can move either a large mass of water > slowly, or a small mass of water quickly. Which is better? > Well, let's look at kinetic energy. Kinetic energy is 1/2 mv^2, > proportional to the square of the velocity. So if we move a smaller > mass of water more quickly, it will have more kinetic energy than > if we moved a larger mass of water more slowly (but using the same > momentum transfer). If we want maximum *energy* to go into propelling > the kayak, and not into pushing the water backwards, we must move > as large a mass of water as possible, as slowly as possible. And > if we do this with the same amount of momentum transfer, we will have > exactly the same thrust. Okay, so what? Again what makes you think a high aspect ration paddle (vs. a low aspect ratio paddle of the same area), will not move a larger mass of water slower? There is nothing in anything I posted that would lead you to this assumption. Your arguments are academic, if you simply compare power consumed going the same hull speed the high aspect ratio paddle will be lower. And your arguments are not true anyway, the most efficient aircraft wing (with the least amount of drag) is the SMALLEST one that generates enough lift to stay in the air. And given wings of the same size, the lowest drag will occur with the highest aspect ratio. There is a complex interaction between tip vortex, skin friction, form drag, induced drag, and a bunch of other stuff that affects both paddles and wings (and propellers too). > > BTW, this same problem comes up in aircraft propulsion - it's much > more efficient to move larger quantities of air more slowly than > small amounts of air very quickly. That is why modern fanjet engines > are so large in diameter - most of the thrust is "bypass" thrust > generated by the very large fans. Ahem, larger diameter fans have higher aspect ratio blades. They have to run slower to keep the tips subsonic, I know I have also done turbine design once in my career (they too have VERY complicated relationships with shape, speed, etc.). If there was a way to get the tip speed up without going sonic, we would do that, it would keep the weight down and improve the efficiency of the engine. Fortunately, paddles move at speeds way below sonic and cavitation speeds in water and this is not an issue. > And you need to give some > support for your assertion beyond just saying it is a "FACT". Well I am not going to teach you what you need to know here so you can know it is a fact, but the information is out there for you to find. I can not prove it to you unless you are willing learn what it take so you can know it too, and I do not have to time to teach you even if I wanted to. If you do not know, then the onus is on you to go and learn it. I want to tell you it was not just my education that taught me this, but also many mega hours of paid experience doing research, wind tunnel testing, and doing computational fluid mechanics (CFD). So have at if you want, but my assertion are not made by speculative suppositions. I also have hundreds of miles in canoes, much less in kayaks, but both with Euro paddles and only recently with native style blades. The high aspect ratio blade is more efficient. > A propeller moves perpendicular to the line of travel, but applies a force > parallel to the direction of travel. Nick's statement is true - he > said force applied parallel, not motion parallel. True enough. But the force applied, by itself, tells you nothing about efficiency. And much of the ENERGY you apply to the water does not go to propelling the boat forward. > > Again, see above. It is entirely related; and giant paddle blades would be > very efficient if you could use them practically. > Again I want to point out I am saying the high aspect ratio blade is more efficient, size is not part of my argument. But you forget something, you increase wetted area with bigger paddles, there is a point were you will lose efficiency, you can not look at only one feature of the design in isolation. Besides I would argue that the high aspect ratio blade will accelerate more water, with the least amount of lost power, than a low aspect ratio blade even if it is larger. Lager also gets heavier, and must be stiffer. > > And propellers give maximum thrust for minimum rotational drag with a > high aspect ratio. But it's very unclear to me that a high lift/drag > ratio is important in a paddle - please demonstrate why. For both airplane wings and propeller (and paddles), the component of drag that goes down with increasing AR is called induced drag. this is the drag it "costs" you to generate the thrust, the more thrust you generate off the foil surface (whether it be up to counter gravity as in a wing, or to get forward movement on a propeller or a paddle), the higher the induced drag. This drag on the paddle is not useful to help pull you through the water (the reason is very complicated). The fact is it just takes power to create the thrust off the moving fluid. You have to accelerate the fluid to get thrust off of it, that cost power, and it is a well known phenomenon that high aspect ratio surfaces require less power to generate the same amount of thrust. . You > need a given momentum transfer rate to counteract drag, and what you > need to do for maximum efficiency is move the water as little as > possible to create that momentum - which means moving as large a mass > of water as possible. And given the same amount of blade area, you can move more water, with less turbulence with less power, with a high AR paddle than with a low AR blade. > > > Realize that the idea of the "thrust" face of the paddle is also > > inaccurate, on a well shaped blade 75 percent (or more) of the thrust comes > > from the low pressure side, the face opposite the "thrust" face. This is a > > fact of fluid mechanics, do not argue with it. > > References for this "fact" of fluid mechanics? Where is there a > technical discussion of *paddle* thrust that we can find this > demonstrated for typical paddle usage? > Lots of them, look in any standard senior level fluid mechanics text books. If you do not know this I can not prove it to you, but you may beable to prove it to your self. Take a paddle with smooth surface on both sides and paddle a timed distance with your normal energy output rate for touring (if you could measure oxygen uptake you could verify your power output is the same, heart rate would also work as long as you are in a steady state out put mode). Then cover the low pressure side with something that will spoil the flow over it, say duct tape with a length thick rope under it running along the length of the blade near the edges. Do the run at the same energy out put. Repeat it with the spoiler on the "thrust" face. Compare each with the base line. You must use a paddle with a smooth low pressure face. Most commercial paddles have very poor low pressure faces, they attach the handle on the "back" (side opposite from the "thrust" face) and make no attempt to smooth the attachment. These already have spoiled flow over the low pressure side and the test would not be valid. Also you may want to experiment with the stoke mechanics a bit to get a stoke that feels good before you do the test "run". Also do not attempt to "race", under extreme loading the flow on the surfaces behaves differently, you want to test this at normal "touring" conditions. Do this and you will know for yourself. You could just try it without the timing, you can feel the difference. If the object was just to move water backward why not use a tennis racket, all that turbulence would absorb a lot of energy, but not a lot of it would go into forward motion of the kayak. > I think you're confusing paddles with propellers & wings again. > Nope, not at all. ANY surface, that produces thrust in ANY fluid works the same way: wings, pumps, propellers, turbines, insect wings, bird wings, paddles, swim fins, fans, etc. etc. You are the one that do not understand that I am not confusing anything, because you do not understand the principle. It is not intuitive at all, and from my experience when I started my career, not that easy to grasp either. > I really love all these assertions followed by "do not argue". > If you have the same experience that I do, you would not be arguing. And until you do you are in no position to argue against it. I can not give you that experience, but I commend you go to figure out what I am writing rather than argue from inexperience and ignorance. That is why I say do not argue, but you argue any way. Go do the test I gave you and prove it for your self. One of my engineering professors told me that one simple test is worth 1000 expert opinions. But instead you want to argue. In every post I suggested everyone just go out and try it, (and not to argue). But instead you do not try it, you just argue. Do not take my word for it, go prove it for your self. > Larger blades are more efficient. > A totally unqualified statement, and certainly not one based on taking measurements. > Personally the paddle > I most enjoy using is a Betsy Bay Greenland-style paddle. I just like > the feel of it, and I find it more fun to roll with. Did you ever stop and consider why if "feels" better? Why it is easier to roll with? Perhaps it responds better to your energy input, has better control because it creates less turbulence and generally coverts your power more efficiently in to thrust for both forward movement, and steering and control inputs (and rolls). Just maybe? 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 Sun May 20 2001 - 06:56:04 PDT
This archive was generated by hypermail 2.4.0 : Thu Aug 21 2025 - 16:30:42 PDT