MATT MARINER BROZE wrote: > > Only at one point is [the blade] moving directly backwards. At all other points of the > rotation it is less efficient as energy is being directed at some angle to the > direction you are trying to drive the kayak. Yes, the paddle describes part of a circle, so only at 1 point is it positioned perfectly. But then again: Don't ALL strokes describe part of a circle? I've tried to find a stroke that keeps the blade in the perfect position at all times, but I found it very uncomfortable. Before this turns into another discussion of greenland/low/boogy paddling vs. the stroke I advocate: I teach only ONE stroke, which is the stroke that's taught by everyone in the Netherlands and many more around the world; is shown in all instruction DVD's I own; and is advocated all over youtube. Even if it's not the best; it's what I teach and what I plan to _keep_ teaching. > In fact, some of that misdirected > energy is taking the load off your "paddle holding up" muscles. At the front of the stroke: Yes. At the back of the stroke: Exactly opposite. In the seakayer I put online ( http://www.nibla.nl/tmp/paddlewise/ExampleStroke/ ) I see the blade going in at 45 degrees and comes out at 45 degrees. The forces up and down through the whole stroke cancel out. > >>>>>> When watching the animation from my link, I see the center of my paddle > rise and fall for about 30/40 centimeters, once more indicating about > 20/30 watts of power from potential energy. I maintain that most of the > energy is going into backward force and movement, having nowhere else to > go.<<<<<< > > Could you run through just how you calcutated the 20 to 30 watts of power due > to lifting the paddle and arms? Assuming his hands and paddle on a scale would measure 7 kilos (70N), and he lifts the center of his paddle (which is the center of gravity) 40 centimeters each second, I calculate that each second he expends 0.4*70 (force * distance) = 28Nm of energy. Since he does that every second (which he does), that 28W. Nowhere else to go? How about it going into > other directions than directly driving the kayak and also going into > turbulence in the water (heat) the stroke and blade are causing. OF COURSE there will be energy loss in turbulance - as in any other stroke. The part I'm talking about is the energy conversion of potential energy into force and movement, BEFORE it is dissipated by the water. All I'm saying is that the 30W of output ends up pushing water, in whatever direction it might be. > I use horsepower, one of which I see equals 745.7 watts. My Vista computer > won't open the chart I made in an Excel spreadsheet back in 1994 due to what > it says are my registry settings (if anybody can tell me how to change the > settings to be able to open my old files I'd sure appreciate it). Anyway I > cranked up my older computer and I see it takes about 9.3 watts to move a > Coaster kayak (13' 5" x 23" x 40 pounds) with a 150 pound paddler at 2.9 > knots, and 18.5 watts to go 4.2 knots. It takes about 56 watts to get it to > 4.86 knots and 85 watts to get it to 5 knots (which is very near hull speed > for the 13' 5" long Coaster--one of Sea Kayaker magazines strongest testers > averaged 5.2 knots in a Coaster for one nautical mile--paddling all out). It > would take 124 watts to drive the Coaster plus 150 pounds 5.5 knots and 150 > watts to drive it at 5.8 knots. That seems to confirm the tables I used. (And damn you for mentioning that coaster. I'd starve myself for a _month_ to get my hands on one of those.) > >>>>>>>> Matt, come on! The slower you go, the more efficient you are in energy > per distance. So, the more often you let your kayak slow down, the more > efficient you get. > Just accelerating and decelerating doesn't cost any energy, unless you > put on the brakes. Look at a weight on a spring: It keeps bumping up and > down forever.<<<<<<< > > We don't live in a vacuum. You'd agree that, in a vacuum, there would be no losses? That's all I wanted to say. Then again: In a vacuum my kayak wouldn't slow down when I stopped paddling, so in a vacuum the question is not valid. > But if you are trying to > maintain a speed (even just 3 knots) with a paddle stroke you don't want to > slow down much between strokes because accelleration is needed to get back up > to above 3 knots again (to maintain the average of 3 knots). Even without the > necessary acceleration effort to regain the lost speed the time spent going > faster than 3 knots uses more energy resisting the greater drag than the time > going slower than 3 knots does when your speed varies. Drag due to wetted > surface friction (which is the vast majority of drag at three knots) increases > at the 1.83 power (nearly the square). I agree that, if I constantly slowed down and sped up while trying to keep up with a steady paddler, I would expend more energy - but I thought you were saying something else. I thought you were saying that, if both of us kayaked for 20 miles, you keeping a steady pace of 4 knots and me all the time accelerating to 4 knots and then letting my boat drift, I would expend more energy. I think I wouldn't. I would fall back; dinner would be gone by the time I reached the campfire - but I would have expended less energy than you. If you want, I can do the math - but it'll take me at least an hour that I could spend differently. I think the point is mute anyway: I don't advocate taking long pauses between strokes. On the contrary: I advocate to lift the paddle quite fast, so you can slouch down into the vertical rest as soon as possible. > Those muscles get a rest because some of your paddling energy (where ever it > comes from) is resisting the paddle sinking because of the blade angle at > entry. Think of my model. I can keep its arms from falling down by pushing horizontally against the paddleblade. (I'll film it if I have to). There's no need for any upward force from the outside: Your shoulders will take care of it, without the use of any muscles. > But he main reason that paddling is easier than just holding up the > paddle is because you are alternately tensing and relaxing the "paddle hold > up" muscles as you stroke from one side to the other > That's assuming that the water will carry a substantial part of the weight of at least your downward blade. What's the volume of a blade? I hardly think the buoyancy could be significant, compared to the 3 to 4 kilo weight of one hand and half a paddle. >>>>>>> You have to use some muscles in the shoulder of your upper arm, to keep > the paddle from falling over your lap. That force can be directed > strictly sideways. Apart from that, no muscles are needed. While the > arms fall down, the blade moves practically horizontally through the > water, directing the force exactly where we want it. Using muscles to > keep the paddle up in any other way would be counter-productive.<<<<< > > I don't understand what you are trying to say above. Can you give me a better > description and explain why? And where is it we want the force directed and > why there? You seem to think that I'd HAVE to put an upward force on the paddle to keep it in check. I don't: The horizontal force of the water will do fine. Do I need to make a new video, showing how I can keep my model in check? > A buoyant blade would bob up and down during the stroke sinking from the > gravitational energy (then holding up the paddle) and propelling it back up > again to get some more of that potential energy that you seem to like to use > back (and for free). I think such a bobbing paddle would _hamper_ the use of potential energy. The blade _should_ be allowed to go down freely: Only the horizontal force can be put to good use, so all other forces are best kept minimal. > During paddling the blade hardly moves at all once it is planted in the water. > It has so much drag that the kayak is pulled past it while it moves very > little in the opposite direction. Back to my example-paddler at http://www.nibla.nl/tmp/paddlewise/ExampleStroke/ : In the video, there's little reference on the water or in the background to see how far the paddle moves. Judging just from the splashes, I'd say that his hands remain almost stationary above the water, but the blade underneath moves quite a bit (Close to a METER at the tip). > When you hold the kayak from moving (with a > dock at the bow or rope fastened to something at the stern) the paddle blade > slowly slogs through the water and makes the paddle stroke unbearably long > (those muscles remaining tense too long again maybe). The position against-the-dock is introduced for two reasons: - To check if there IS a position of rest to be found; - To give students time to find that rest. There surely will be differences with actual paddling, but no differences that affect the purpose of the dock-exercise. > BTW, I have no trouble going slower if that is what my paddling partners are > doing. I'm curious to hear an explanation as to why from those who seem to > have trouble doing that. Are you all using wing paddles (that just don't work > right at slow speeds) or is there some other reasons? Psychological maybe? If (and only IF) my hypothesis of using potential energy is valid, then _that's_ the reason. To paddle slowly, you'd have to keep that energy in check - by straining muscles that otherwise could be at rest. Niels *************************************************************************** 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 Sat Apr 30 2011 - 11:08:36 PDT
This archive was generated by hypermail 2.4.0 : Thu Aug 21 2025 - 16:31:45 PDT