Gary wrote; > From the Hobie discussion, isn't it possible that some hull forms might > be more efficient when powered by a pulse (paddle) than continuous > (constant pull in tank) force. so maybe tank tests could be devised to > more closely simulate that? Anyone know a reason why not? Dave wrote; > First, how about we settle on a physical definition of the term "thrust"? > Is it in foot-pounds (e.g., newton-meters), pounds (newtons), or what? I > suspect Peter may be using thrust to mean something different from what I > understand it to be: a force, which would have to be in pounds or > newtons. Most people here in North America understand pounds. Won't that do for a discussion on this forum? We could also use co-efficients so we don't have to worry about units and are not tied down to velocity or paddle size which sometimes seems to confuse the issue here? This is why I suggested that we needed to deal with thrust per unit area (a co-efficient) as it gets around all these variables. Any reason why we can't use co-efficients? > Secondly, if there were two paddles of differing area, producing the same > thrust (force), then the pressure on the surface of the smaller one would > have to be greater. That would imply less slippage of water off the > surface > as the paddle moves though the water (if the paddle is fully stalled). If we dealt with a co-efficent as I proposed then we could compare paddle sof differing areas, No? Does it matter how we achieve the thrust? Is not he objective to determine relative thrust from differing paddle types and stroke mechanics? > If the paddle is not fully stalled (or nearly so), then it must be > producing > a force through the infamous "lift," using some variant of Bernoulli's > principle. Does it matter if the thrust or driving force comes from lift or drag or a combination of the two? Do you not want the most output per input? >As for Marchaj's use of thrust per unit area, if thrust is truly a force, > and the area is sail area (don't have the reference at hand), then > basically > that means the craft is making more (or better) use of the wind than > another > craft of similar sail area. In other words, one sail could be a highly > refined one and the other could be one that is not. Unless the people who did the test are complete idiots I suspect they tetsed for sail configurations not sail quality. Does anyone know otherwise?. > The parallel to a paddle would be a one that provides more rearward thrust > for the same "effort" (e.g., force put in) by the paddler. And, this > could > be achieved by the infamous "lift." It seems that way. However, suppose they tested sails with say, three different aspect ratios and the sail with the lowest aspect ratio produced the greatest thrust per unit area. How would this affect your conclusions about paddle type? Peter wrote; > You do not need any of this information to make a valid comparison. the > same hull, at the same gross weight, at the same speed will take the same > amount of power to push it though the water (presuming wind and currents > are > not a factor). What you get a the same power output with each paddle, > what > would vary is the amount of O2 uptake, or heartrate (once stablized to a > steady state condition). Since presumable the O2 or heartrate would vary, > the lower the O2 uptake, or heart rate, the less work the paddler is > puting > into the paddle to mantain the SAME speed (or power output). You do not > need the to know the speed vs. resistance of the hull to measure relative > paddle efficiency. But don't you need to know that the strokes used were identical or at least very close to identical? Don't you need to know if the strokes used were the best strokes to use with a particular paddle? Don't you need to know if the configuration of the boat favours one type of stroke mechanics over another? Mike wrote; > The key then is to oblige the paddler to a paddle at a specific > speed. It would be wise to choose a speed that is not too high, so > that it can be achieved with a wide range of paddles. This seems like a problem. The boat accelerates and slows down during the stroke. Since the resistance varies at different rates with speed doesn't that mean the strokes have to be pretty precise both in timing and thrust? Suppose a group of strokes were a bit light. Then to maintain "average" speed don't you have to go faster at a higher resistance? Suppose the yaw varies and slows the boat, would you not have to paddle harder to get back to the "average" speed. I also wonder what speedometer you plan to use. How much averaging does it do? Will it accurately reflect changes in speed so the paddler can adjust his tempo or force? Correct me if I am wrong on this but shouldn't you eliminate or at least try to reduce the variables rather than just ignore them? *************************************************************************** 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/ ***************************************************************************
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