Nick Schade wrote: <SNIP>>>>>>>Because Guy B can apply his effort over a longer time he does not need to accelerate the kayak to as high a speed to maintain the same average speed. Since drag increases with speed he is working on moving a boat that has lower average drag. This means less force required and less overall effort.<<<<<< Is this the entire basis for the claims made as the result of the Excel spreadsheet analysis? Why doesn't Guy B have to accelerate to as fast a speed? During the 1 second between strokes a given kayak at a given speed (and all with other variables held constant) is going to slow the same amount and will have to accelerate to the top speed of the previous stroke again to maintain the same average speed (at least it would seem that way to me as long as the acceleration rate was constant throughout both A's and B's stroke. Am I missing something here by trying to do this in my head? If Nick is correct then it will hardly matter to a paddler since the ideal stroke rate will be the one that best finds the point between the stroke being as long as possible vs. it being too long and therefore working at inefficient angles from the direction of kayak motion (the pivoting paddle factor). This would also have to be tempered against the apparently more important factor of being able to minimize the time between strokes. The long stroke may also mean that more distance (time) would have to be covered to get the paddle back into the water again. Also, if the kayaker shifted bodyweight back and forth to make the longer stroke there would be more losses. In either case it would be more efficient to shorten the stroke up some more to maximize the overall efficiency of the stroke. I'm pretty sure that there is something wrong with Peters getting 5% efficiency for a paddle. I like Jeff's addition of paddle slippage speed into the power equation. He wrote: <SNIP>>>>>Thus, the power that a paddler has to put in to keep a constant boat speed is equal to: BOAT_RESISTANCE * (BOAT_SPEED + PADDLE_SLIPPAGE_SPEED) In other words, all other things being equal, greater slippage means more power output by the paddler with no speed advantage.<<<<<Snip> This addition of paddle slippage speed seems to compensate directly for the efficiency factor of the paddle (so you could get the percent of efficiency by dividing that into just the boat speed times the resistance figure. Since the paddle moves very little in comparison to the kayak its efficiency is actually quite high. Mike's example critiquing this while comparing wide and narrow paddles was way out of line in its slippage numbers (having them move twice and 4 times as much as the boat id in the opposite direction) considering the relatively high efficiency for both paddles (but a little higher for the less slippage paddle). The measurement of slippage should also compensate for the lift of a wing paddle (or Greenland paddle if it is acting as a wing like Peter believes). However I don't think it will measure the loss due to drag on the wing as a result of its sideways motion (which is requiring some energy on the paddlers part to move it sideways--that doesn't seem like it will show up as rearward slippage). Maybe this is an area where we need to get into the lift/drag type math Peter was using. We would probably also not be accounting for the energy being wasted in pushing water in directions other than straight back due to the pivoting nature of a kayak stroke (unless we also could measure the total input into the paddle-in which case we could probably skip this slippage measurement altogether in comparing efficiencies). Ignoring the efficiency losses in the human body and the off center location of the pulling point, a perfectly rigid rod at the side of the tank being pulled upon should be 100% efficient at transferring the pullers power output to the kayak. If the center of thrust of a paddle moves as far back through the water as the boat moves forward during the stroke that would be 50% efficiency since you put twice as much in as you get back out in useful work. The rest of the energy didn't disappear it was just lost (for doing useful work) to randomness (or we could substitute "heat" or entropy for randomness). I'm not sure how easily this slippage could be to measured (it is kind of a slippery thing to get a handle on ;-) or even where on the paddle to decide to measure it if we were using, say frames from a video camera to watch and stop the action. We could drop out the time factor (because Distance/Time = Speed and it would be the same interval of time on the video (30 frames per second) and just measure how far the boat went forward and how far the center of thrust on the paddle moved back over the same number of frames (that covered the thrust part of the stroke). By doing this over many strokes and we could likely also include the time between strokes into these efficiency equations. Next question, how can we determine where the center of thrust is on the paddle blade in normal use? Any ideas. Would it bear any relationship to the pivot point in space that will show up on the frame-by-frame screen as a fixed point around which the paddle rotates (in the direction of kayak motion). If so what other points would be involved? I guess I've got more questions than answers again today. 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 Wed Jul 31 2002 - 12:05:41 PDT
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