Nick Schade wrote: > > We have a working definition of the paddle moving backwards through > the water. I don't see this a being particularly vague. For the > paddle to move backwards, water must also move. There are other ways > of making the water move such as vortices. But however you make your > boat move, you do it by moving water. The more water you move the > better. The slower it moves, the better. We can drop the term > "slippage" if it makes you more comfortable. > Both lift and drag is a function of a square of the relative fluid velocity over the surface. So the faster you "slip" the paddle through the water, the more force you get out of it, you will not get any thrust without "slip". The slower the stroke the more "slip" you will get, the faster the less. But calling it "slip" implies it is an undesirable motion, which is the wrong idea. The thrust is intrinsic to the movement itself and a result of it, not related to efficiency at all. Because the perceived resistance is a function of the square of the relative fluid velocity, the same paddle pulled back rapidly through the water would have more resistance (less perceived "slip"), and pulled slowly would have less resistance (more perceived "slip"). So it is non-sensical to say the less slip you have in the water the more efficient the paddle design, they are not related. The only practical comparison of efficiency is comparing the total power required to push the same kayak, at the same speed, in the same conditions. The perceived resistance against a solid object is a direct function of the speed you push against it (not a square of the speed as in a fluid), and the slippage against the solid object would indicate a loss of perceived resistance (remember sliding friction is always lower than static friction). This is why the idea of slippage in a fluid, which comes from this model of pushing against a solid object, is invalid. Thrust as a result of fluid motion is not comparable to slippage. Unlike an airplane wing however, both components of lift and drag can be useful in a paddle stroke, and both can also cause wasted effort. the optimum shape for a paddle (and the optimum stroke style), that takes advantage of this would be anybody's guess, and likely only determined with lots of trial and error. Get the idea of "slippage" out of your vocabulary when considering fluid generated thrust. Power is a work rate, to define efficiency with anything other than "power in", or reduce it to something not related to power, is clearly invalid. You can not compare pushing against concrete to pushing against a fluid. Push very slowly against concrete, you get thrust, push fast, you get slippage. Push very slow against a fluid, you get nothing, push fast you get thrust exponential to the speed you push at. Resistance against a fluid is not related to efficiency. "Slippage" in a fluid is a nonsensical notion, and not a measure of efficiency. > > > > >> As long as you move the paddle parallel to the direction of motion > >> desired all the force applied will go directly to propelling the > >> boat. > > > >This is totally false and again based on uninformed intuitive ideas. > > Is it really? Please re read it. Any force you apply parallel to the > motion of the boat will be useful for moving the boat in that > direction. Are you suggesting that forces don't create reactions in > the direction they are applied? My statement is correct, you have not thought this through. Just because you pull a paddle strait back does not mean your effort "will go directly to propelling the boat". You will push water in every direction (including backwards), but you also get turbulence. Turbulence heats the water and pushes it in directions not useful for forward movement, using up power and therefore it is wasted effort, i.e. poor efficiency. You want to accelerate the mass of the water exactly aft so you get forward thrust, accelerating the mass of the water in any other direction (as in turbulence) results in wasted effort. The cleanest way to accelerate the mass of water mostly aft is with the least amount of turbulence, this is done with a high aspect ratio surface with a foil shaped cross section. This is a FACT of fluid mechanics, if you do not understand this, nor simply can not accept it as a fact, than there is nothing else I can do for you. The fact that a propeller moves perpendicular to the line of travel to make forward thrust demonstrates your statement is false. If a general statement is false in one circumstance, than it is a false statement. the way a propeller gets thrust with motion at right angles to the direction of travel is because it pushes the water aft with the least amount of lost power, therefore it is more efficient than a paddle wheel, which is why paddle wheels are not used in commercial ships anymore. > > I know you are suggesting that I don't know that you can make forces > parallel to the direction of boat motion while moving the paddle > perpendicularly. I am aware that you can, but in a paddle stroke most > of the force comes from the fore-aft motion of the paddle, with the > perpendicular motion serving only to increase the efficiency > somewhat. The increase in efficiency is not insignificant, but it > does not materially change my point. Well is not efficiency what we are talking about? You can paddle a kayak by pulling a tennis racket strait back, you'll will make lots of turbulence, i.e. wasted effort, and it would not be efficient. Do not confuse the effort in pulling back the paddle with the motion of the water over the blade. since the kayak is moving forward, you have to pull back on the paddle to get any thrust out of it, the question is HOW you are getting that reaction at the handle by pushing on a fluid, and how to get that with the least amount of wasted effort. If you can increase the amount > of water you move without increasing the velocity at which you move > it, you will be more efficient. > False, it is unrelated. If this were true why not use giant paddle blades? You would increase the amount of water you push back at the same velocity, but YOU WILL NOT INCREASE EFFECENTCY. Efficiency is a measure of power consumption compared to useful work done. Power is a work RATE, i.e. it is time related, units are foot-pounds per minute, velocity is measured as feet per minute. They are not related, and for a kayak the only thing that is useful is to compare the amount of power it takes to move the same kayak, at the same speed, in the same conditions. Paddling fast or slow is not by itself a measure of power consumed. > By slicing the blade through the water perpendicular to the water you > will always be pushing against stationary water so you will be not be > accelerating any given mass of water to as high a speed. As a result > you move a larger mass of water more slowly. This is more efficient > and is often worth the energy required to move the paddle sideways. > Even native paddles get most of their power from fore-aft motion, but > a slicing motion to the side helps as it does Euro paddles. > If you do not realize how irrational this statement sounds to someone who works in fluid mechanics is, I can not clarify it for you. Mass and velocity, by themselves, ARE NOT RELATED to efficiency. Power input is, this is a work RATE. > Once the fluid is set in motion it makes absolutely no difference > what it does. The momentum has been applied and has been conserved > by the resulting reaction of the boat. How that momentum dissipates > itself in the water is irrelevant. THIS IS LUDICRUS!!! Any fluid that is not moving aft, is wasted effort, F=Ma. If it is does not accelerate backwards, it is not useful for moving you forward. All of the power it takes to move the water in any direction beside strait back is wasted, and it comes only from your efforts, wasted effort. The only momentum that is going into your boat, is equal to the amount of momentum of the water going back! Vortexes, turbulence, lateral water movement is LOST POWER. > The bottom line is it doesn't matter what the vortices do once you made > them, the fact that you made them is what is creating the power. The > vortices and other water motion are the source of your momentum. If > you can make the vortices bigger and slower, you will be more > efficient than if you make them small and fast moving. If the > vortices cancel perfectly, then no momentum was applied to the water > and the boat received no momentum. The goal remains the same, move as > large a mass as slowly as possible for the greatest efficiency. Wrong, wrong, wrong, wrong. If you were a student in a senior level engineering class, I would send you back to freshman physics if you made a statement like this. I am not trying to belittle you, I just can not address something so totally out of step with the laws of physics without teaching you from scratch. And I am not about to do that here. > Imagine a parachute and a glider. Hold each 100 feet off the ground > and let go. The glider will hit first. While it is capable of > creating a lot more lift you must first apply a lot of kinetic energy > before it will fly. It will take longer to drop only if you get it > going real fast. So which is more efficient? Wrong, wrong, wrong!!!!! Your logic is faulty. I did a little calculation for you. But to make the comparison valid you have to use the same size wing in the glider as the area of the parachute, and both have to weigh the same. With these conditions, a well designed glider will sink at a rate of 45 feet per minute, an equivalent round parachute will sink at 1500 feet per minute!!!. You do not need to throw the glider, just drop it at the right angle of attack with the tail trimmed properly, it will convert the initial vertical velocity in to horizontal velocity. Do not question these calculations because I am not going to explain them, they are way too complicated to quickly make sense of them if you do not know how to do it. Go take a class and learn something about it and do the calculations for yourself and you will see these numbers are accrete. I used very typical specifications for a sail plane, and for a typical round parachute, that is what you get; the parachute smashes to the ground 33 times faster than the glider would touch down. you need a parachute 33 time bigger to get the same sink rate. The glider is way more efficient no matter how you measure it, either by sink rate, or by maximum distance traveled: in the same hundred feet drop the glider would have traveled 2300 horizontal feet, the best you could get from a round parachute is about 300 feet (of course the "para sail" type would be better, that is why round ones are not used anymore). You are not making valid comparisons, and your measure of efficiency is irrational. The speed the parachute sinks is not related to efficiency, it has nothing to do with a work RATE. > Having a paddle work like a wing will indeed increase its efficiency > but any paddle can work like a wing. Increasing it's aspect ratio > helps when it is used as a wing, and so will increasing the blade > area. > I thought we were talking about efficiency here? If you agree that the high aspect ratio paddle is more efficient, than that was what I was saying since the first post! Increasing the blade area is not related to efficiency. <snip> > You are confusing human efficiency with paddle efficiency. WHAT?!! Efficiency is related to the amount of power it takes to push the same kayak, the same speed. The only source of power is the human, NOT THE PADDLE. The paddle has no source of power other than the human. You are the one confused. When I say the high aspect ratio paddle is more effic ient, I am talking about the amount of human power it takes to move the kayak the same speed (using proper technique) is less than when using a low aspect ration paddle. Anything else is meaningless. Human efficiency is related to how much food a human eats compared to how much useful work the human does. What are YOU talking about? > Let us assume that the only difference between a Euro paddle and a > native paddle is aspect ratio (blade area, and foil shape are the > same). You can not make this assumption, most low aspect ration Euro blades have the totally wrong foil shape to generate good thrust. The typical shape on the low pressure side of a Euro blade spoils the flow over it and stalls the blade, the high aspect ratio blade is way more sensitive to stalling, virtually all native style paddles have foil shapes that are smooth on both surfaces, this improves flow and thrust, and reduces turbulence. You also must optimize the foil shape for each aspect ratio to take advantage of it, you can not keep the foil shape the same and have a valid comparison. But most Euro blades do not have a foil shape at all, the shape kills all the useful thrust that you could get off the low pressure side (and some shapes kill the thrust off the high pressure side too!). 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. This is also why most (but not all) recreational Euro paddles are also inefficient, they have VERY poor low pressure surface shapes. This is another one of those areas that "common knowledge" and intuitive thinking about fluid mechanics leads to very wrong ideas. > However: > - Everything else being equal the paddle with the largest blade area > will be the most efficient. WRONG! size is not related to efficiency. > - Everything else being equal the lightest paddle will be the most efficient. True, but we are not comparing weight, we are comparing aspect ratio. > - Everything else being equal the fastest cadence will be the most efficient. This may be true, but has nothing to do with aspect ratio of the blade. > - Everything else being equal the boat with the strongest paddler > will be the fastest. But it does not mean he is more effect, just faster. > - Everything else being equal the paddler with the best technique > will be the fastest. And each size and shape of blade may take a different technique to get the best performance out of it, and fastest is not necessary more efficient. The one with the best technique, using the same paddle, will be more efficient (use less engird to go the same speed and distance). Be again I was comparing aspect ratio. <snip irrelevant statements> > > Generally native paddles are made relatively thick and this lets them > be light weight and still strong. Native paddles are too thick to be > compared with Euro paddles by aspect ratio alone. Their thickness > cuts back on their achievable efficiency when used like a wing. This is not necessarily true. The optimum thickness of the foil is related to the Reynolds's number, a ratio of viscosity, size of the foil, and speed of the fluid. Thicker can be better under some conditions. > However, being thick and light weight are two of the best qualities > of a native paddles. Why lightweight is good is obvious. Being thick > makes them much less dependant on good alignment and position when > rolling and sculling because they are less likely to stall. It also > makes them less touchy about the angle when using them like a wing in > a forward stoke. A thinner blade will be more likely to lose > efficiency if it is held at too steep an angle. So you can gain some > efficiency with a native paddle because it is less reliant on good > technique. There are good reasons to use a native style paddle. They > do not need to be the optimum paddle for forward paddling efficiency > to be good. Another advantage, which has nothing to do with efficiency, is because they are not feathered, and the blade is symmetrical on both sides, and there is no "front" and "back" or "up" and "down", you are less likely to mess up an emergency brace, or a roll, because you have the paddle held the wrong way. > > I find it kind of silly that any comment even vaguely hinting that > native paddles are not the absolute best at everything always draws > some kind of impassioned defense that typically devolves into > questions of the "critic's" competence. I personally think they can > be darned good paddles even if they are not necessarily the most > efficient at one aspect of paddling. I am not dogmatic about native paddles. I actually think there are areas that it can be improved, though you may risk diminishing the paddle's ability to do other functions beside moving the boat forward. Also I do not think I have seen anyone make a high aspect "native" style paddle with light weight composites, with a foam core. Traditionalists would probably cry foul, but it would be interesting to see what could be done with composites. I am not against innovations, and I have seen modern paddles that look excellent. It is just that most of the recreational sea kayak paddles I have seen commonly available have very poorly shaped blades. When I can make a better paddle in a few hours in my garage with a 2x4, I can not spending $200-$300 for something that performs so poorly. That is way more than I ever want to write about that. Forget about fluid mechanics and theory, just make or borrow a native style paddle, learn how to use it, and find out for your self. Peter Chopelas *************************************************************************** PaddleWise Paddling Mailing List - Any opinions or suggestions expressed here are solely those of the writer(s). 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