-----Original Message----- From: Nick Schade <schade_at_guillemot-kayaks.com> To: paddlewise_at_lists.intelenet.net <paddlewise_at_lists.intelenet.net> Date: Friday, May 28, 1999 7:48 AM Subject: Re: [Paddlewise] Pro's and Con's of the "Swede Form" Nick wrote: >I have used words to the effect of "climbing its own wave" to describe what >is happening at hull speed myself because, although I knew better, I did >not understand it well enough to come up with a better explaination. As >much for my own benefit as anyone elses, I am going to try again. > >We tend to think of the waves as creating, or being the source of drag, >when they are really the visible evidence that drag has already occurred. >When a hull moving through the water experiences drag, it is due to the >water applying a force to the boat. This force is sapping energy from the >boat. For energy to be conserved, the water must increase in energy. In >other words the water is put into motion by the drag of the boat on the >water at the same time the boat is being slowed down by the drag of the >water on the boat. > >The energy put into the water by the boat must go somewhere. Some of it is >lost in frictional heat in turbulence where the water molecules rub against >each other and heat the water slightly. The rest of it produces waves. If >there was some other way to disipate the energy the waves would not appear, >for example if fish were imparted kinetic energy and got squirted out of >the water, or if little propellors hooked to generators absorbed it all. >Even if the waves never appear, the drag still would have occurred. When the waves cancel each other out (and disappear) the drag is reduced as well. If the drag has already occured and the waves are only the side affect of drag and not also a cause how do you explain the reduction in drag when waves cancel each other? What other form of drag has changed? >Once >you see the waves it is too late, the damage is done. > >When John says "Boats create their bow wave.", it sounds a little obvious. >We didn't think evil demons were doing making them. But, what it means is >that the boat can not avoid the wave. If there is a lot of drag the wave >will be big. If the boat can power its way through the drag and get into >another drag regime, the wave will change. If the drag diminishes, the wave >will diminish. You can't climb over the wave because the wave is basically >innocent. The wave is a side effect, not a cause, of drag. > >This is not completely true because the changing wave shape can change the >flow of water around the boat and this can change the drag, but it is not >really the wave itself creating the drag. Drag is from friction and >viscosity. Same question. When the waves cancel each other out (and disappear) the drag is reduced as well. If the drag has already occured and the waves are only the side affect of drag and not also a cause how do you explain the reduction in drag when waves cancel each other? An alternate explanation for trim change near hull speed I asked for in my last post. The second wave crest moves behind the stern as the boat speed increases (lengthening the wave) above a speed/length ratio of 1.34 wavelength^.5 (Froude 0.4) commonly known as "hull speed". As this crest is no longer there supporting the stern it sinks into the (low pressure) trough now under it, but the bow is still being supported by the bow wave changing the trim to a stern down condition. Higher speeds will now require overcoming the force of gravity (with dynamic lift) as well as the drag due to friction and the inertia of water. It still sounds like one would be paddling uphill to go faster though. Any comments? Matt Broze www.marinerkayaks.com Theories are nets; Only he who casts will catch ---------Novalis *************************************************************************** PaddleWise Paddling Mailing List Submissions: paddlewise_at_lists.intelenet.net Subscriptions: paddlewise-request_at_lists.intelenet.net Website: http://www.gasp-seakayak.net/paddlewise/ ***************************************************************************
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