Peter wrote; >> > Not exactly. What I was saying is we should be concerned with the direct > measure of thrust output vs. work input. At best thrust per unit area is > an > indirect measure. Fair enough but can you not determine the relative efficiencies of the paddles when the inputs of mass density of water, velocity, and area remain constant but the coeffcient of thrust (Geometric variables) varies? > That depends on what you are trying to measure. If your measure is thrust > out vs. power in, then yes they are exactly the same. There are other > measures you might want to consider, there are a lot more things we do > with > a kayak paddle than just move it forward. True!!!! Now, what are they? And why does thrust only have to do with moving anything "just forward"? > sure, but that was not the question. It was my question. Can you give me an answer? >And measuring thrust per unit area > does not tell you which paddle would be best in terms in thrust per unit > power input. I fail to understand. If paddle A produces x lbs of thrust with a given power input and paddle B produces less at the same input don't you know which is more efficient? > Marchaj is measuring something different because the source of power is > different. Why does that source make any dfference? Did you not say that efficiency was a function of power in versus power out? >On a sail, the power input comes from the wind; the bigger the > sail area the higher the power input. On a kayak the power is human. The stronger the paddler the greater the power. Is that not so? How does this differ from the sail example? If I recall correctly, Peter, you provided an excellent description of how lift works on a paddle based on aero nautical research. Are you saying that that does not apply because airplanes get their power from engines or maybe air currents? > When deriving the sail efficiency > equation the sail area simply falls out because the power input is > directly > related to the size of the sail (if you go review how Marcharj derived his > equation you will see this). Are you sure about this? What do you make of pages 148 through 154 where he discusses the relative power output of differing sail aspect ratios? Where in those pages did he mention area and in what context. >Measuring thrust per unit area on a sail IS a > direct measurement of efficiency, you do not need to determine what the > power input is since that is directly related to the size of the sail.. Are you sure? Are you saying that where all the boats had the same sail area that all should go at the same speed? Suppose you knew the sail area and the wind velocity but not the angle of attack or the relative efficiency of the sail. How would you calculate the thrust? >On> a paddle, the power input is fixed by what is available from the >paddler Is not the power provided to the sail fixed by the wind available? > and presuming steady state paddling conditions, either the heart rate or > the > O2 uptake would be directly related to the power output of the paddler. Are you not assuming a consistent an uniform stroke? Who do you know that can provide that? > Unlike the sail area, the thrust per unit area of the paddle blade tell > you > nothing of the power input it takes to generate a given thrust. Why not? Are not the two similar in that a fluid is acting against a solid body moving through the fluid? Coli wrote; > Ignoring the diversity of paddling styles strikes me as odd ... for me > the motion of the paddle is complex, its also very different with > Rasmunsen > style racing wing blades, greenland sticks, single bladed canoe paddles, > or > flat 'euro' blades. Suppose you knew how each type of paddle would perform under a given set of circumstances (angle of attack, stroke duration etc. . Then could you not relate that information to stroke mechanics? >I can't imagine that it would be > easy to replicate the different motions of just one paddler with different > blades even approximately by a robot in a tank. Do you have to? Why not just know what happens for a range of angles of attack and thrust coefficients and then sum the forces for a net force? Sort of like estimating the resistance of boats where you add all the resistances to get a total? > Even if you manage to build a half decent robot which can move a paddle in > sort of the same way a paddler does, applying power sort of when a paddler > does ... how would you test the rather wild assumption that the robotic > paddle tank data had any validity when extrapolated to human paddling? Is not the purpose of paddle testing in an objective manner to isolate teh characteristics of the paddle? Once you know that can you not use that information to help select a paddle or even improve your stroke? Matt wrote; > I'd like to test kayaks with a paddle that can measure the force applied > to > it and the angle of the paddle at the time the force is applied because > the > paddle force is off center and thus causes the kayak to yaw some back and > forth as it goes through he water (and towing tank tests do not account > for > this and it would be very difficult to simulate the correct amount for > each > kayak and each speed). You and I both. Bruce Winterbon talked to a fellow physicist who figured out a way of doing it that made sense but neither Bruce nor I had time to develop the device. Basically it measured the net force and its direction acting on the boat and recorded it through out the stroke. The seat "floated on sensors in the boat. Sounded good to me but maybe some one knows why it would not have worked and will make me feel good about not trying it. Cheers John Winters *************************************************************************** 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/ ***************************************************************************
On 22 Oct 2004 at 20:40, John Winters wrote: > You and I both. Bruce Winterbon talked to a fellow physicist who > figured out a way of doing it that made sense but neither Bruce nor I > had time to develop the device. Basically it measured the net force > and its direction acting on the boat and recorded it through out the > stroke. The seat "floated on sensors in the boat. Sounded good to me > but maybe some one knows why it would not have worked and will make me > feel good about not trying it. If you make a seat-and-footpeg frame and mount it on three supports (statically determinate), then put appropriate strain gauges on the supports, you could measure the net direction of all forces and moments on the kayak due to the paddler. This would even allow for leg power. With appropriately stiff supports, the paddler would not even feel the difference between this and a normal kayak. However, this would not give much info on the paddle itself. You'd still need to measure its position in time. With the position of the paddle relative to the kayak and the forces on the kayak, you could deduce the forces in the paddle at some point (e.g. centroid of submerged area - assuming calm water). Integrating over displacement would yield work done. Mike *************************************************************************** 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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