> Robert Lawson asks; > > Tell me more about what you could tell from observing their wake. How > does it look when the bow wake is over run? > Peter A. Chopelas <pac_at_premier1.net> replied: >>Actually quite a lot of information could be gained from the wake. If you could accurately measure the angle of the wake from a photo you could determine the boat's speed. The size of the wake combined with the LWL would also tell you something about the boat's displacement (though I am afraid I am not smart enough to figure that one out, though it is possible I'm sure).<< Actually in deep water the angle of the wake stays the same at all speeds, 38 degrees 56 minutes (at least until the wave barrier is broken and you start to plane--and you don't do that in a human powered kayak). Therefore knowing the angle won't tell you much about the speed of the kayak, but a photo's measured angle might be able tell you about the depth of the water (at least if the angle is greater than 38 deg 56' and you know some of the other parameters in the picture). The amplitude of the waves in the wake of a non-planing hull will be determined by the boat's displacement and boat speed. Moving more water aside more quickly stacks that moved water up higher and results in higher waves. Energy equals mass times velocity squared. More energy imparted to the water makes bigger waves. At the racing speeds the faster kayak will be the one that has managed to climb the furthest out of the hole it made in the water. If all the kayaks were the same and the paddlers and kayaks all weighed the same measuring the amount of the bow that hung out over the water before touching it would probably be the best predictor (for an observe from the side) of the fastest moving kayak at these sub-planing speeds. Measurements of the wave height could also tell which of these identical boats was moving fastest since mass would be controlled and speed would be the variable that created the differing wave heights. The length of the transverse waves (the long ones directly behind the kayak) in the photo will also tell you the speed of the kayak. A wave moves (in knots) at 1.34 times the square root of the wave-length (in feet). Or if you prefer dimensionless (pick your own units) formulas: wavelength = .558 times velocity squared. If you know the length of the kayak in the photo and with Olympic sprint kayaks we do, I believe 5.2 meters (about 17' 1") is the rule. You should be able to use the kayaks known length to measure the wavelength of the long following waves and therefore deduce that kayaks speed. Matt Broze http://www.marinerkayaks.com *************************************************************************** PaddleWise Paddling Mailing List - All postings copyright the author and not to be reproduced/forwarded outside PaddleWise without author's permission Submissions: PaddleWise_at_PaddleWise.net Subscriptions: PaddleWise-request_at_PaddleWise.net Website: http://www.paddlewise.net/ ***************************************************************************
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