Matt wrote; >The flow over the hull is the resultant of the motion of the hull and the >direction the water is actually moving. If the water moves 2" down while the >boat moves 200" forward the flow past the hull would appear to be at a >slight downward angle but the water would have moved only 2" downward (and >slightly forward due to the friction with the hull). So the water is mostly >moving up or down (and out to the side during a crest and back in the >trough) This just doesn't follow. If the water moved out to the side in the crest and then back in the trough how does one explain the progressive widening of the transverse wave system? (SNIP) (SNIP of Nick's comments) > >I disagree. Your boat is affected by both the size and the length of the >wave created at the bow. My undersanding is that as the wavelength is >shortened due to the shallow water effect the boats hull speed is now lower >(it gets trapped in a now shorter slower moving wave). For a fast moving >kayak you first feel this in four or five feet of water. Some Canoe racers >can actually plane their canoe if they can keep their speed up (and break >the wave barrier) on suddenly hitting an area of shallow water. The hull speed doesn't get lower (Always Froude 0.40) the resistance increases due to the shallow water effects on the wave system. The effect of higher speed in shallow water has nothing to do with planing. Planing is caused by dynamic lift acting on the hull. The effect in shallow water comes from the change in wave pattern at the critical speed V=(gh)^.5 where h = water depth. Below this critical speed the boat generates two waves (Bow and stern). Above this speed the wave pattern for the pressure point (bow) created by the boat increases in angle until the critical speed gets reached when the wave angle approaches approximately 80 degrees and only one wave gets created. At this point the all wave making energy is transmitted to this single wave called the wave of translation. With added speed the angle of the wave begins to decrease and the wave system consists only of diverging waves with lower wavemaking resistance. In short, the boat does not plane it simply enters a new wave making regime. Nick wrote; >>Water flowing under the boat must be displaced and in turn displace other >>water farther down and out. This chain of displacement eventually will push >>out to the surface. Matt wrote; >Since water is incompressible the first effect of disturbing it can only be >upwards, hence the bow wave an upward motion of the water. I think I can now see where Matt comes from. I think he missed the Bernoulli effect and that has caused him to go astray on this. (SNIP) >>The energy in the water due >>to the motion is KE=1/2 m v^2. Moving a large mass slowly tends to be more >>efficient than moving a small mass rapidly. >> >>The result is when the energy is released at the water surface, larger >>waves are made. If the water was being pushed purely to the side, you would >>not see such a dramatic change in shallow water. The typical wake of a >>kayak is pretty small. Probably small enough to travel pretty easily in 1 >>foot of water. > >A wave begins to "feel" the bottom in water about 1/2 as deep as the >wavelength(the waves orbital motion below the surface is touching the bottom >which slows it down--compressing its energy into a shorter space thereby >making the bow wave higher, steeper and harder to climb). No one climbs their bow wave. It is a bit like lifting yourself by your bootstraps. (SNIP) >It is easy to see in very shallow water why it would be impossible for much >water to go down, it would just bump into the bottom. Due to the >incomressability of water even in the deepest ocean no water can go down >without moving some other water upwards and since the water can't go upwards >though the hull it must go first to the side or raise up in front of the bow >(due to the pressure in front of the moving hull). The water need not go down (or upwards) if it increases its velocity. As Nick and I have pointed out the water eventually does rise but it is pushed out of the way by the water passing under the boat (verified by tank testing and mathematical methods), . >Here is another thought. In areas of lower pressure the flow will be moving >back downward and in this area the flow would be diving under the hull as >the flow past the hull moves parallel to the waters wave angle at the >surface. It would seem the flow line on the hull would move back upward with >the next wave crest but if that wave were well back on the hull the flow >might keep moving down to fill in the area of low pressure behind the moving >hull. I don't know if this is right just some thoughts I'm having in trying >to apply basic principles to the flow question. The flow tests do not support this. The flow quite clearly dives under the hull and follows the buttocks. I would be interested in seeing flow studies that show otherwise. Cheers, John Winters Redwing Designs Specialists in Human Powered Watercraft http://home.ican.net/~735769/ *************************************************************************** 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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