-----Original Message----- From: John Winters <735769_at_ican.net> To: 'Paddlewise' <paddlewise_at_lists.intelenet.net> Date: Friday, May 28, 1999 6:14 AM Subject: Re: [Paddlewise] Pro's and Con's of the "Swede Form" >(SNIP) John wrote: >When we see a displacement boat at or near Froude 0.40 we think we see the >boat sinking into its wave because of its change of trim. In reality the >sinkage that occurs is due to the increased velocity of water flow across >the bottom. Does this increased velocity (over the velocity of the boat is moving already I presume) of flow and the resulting decrease in pressure come about due to the longer distance that water must travel around or under the hull (Bernoulli)? Or is it the result of the pressure differential between the front and back of the moving hull (inertial, form or wake drag--pick a name)? Both? Both plus wave drag? Or are waves just the result? Or are these all just different ways of describing the same phenomenom? If the waves are only the result of drag forces rather than a contributor as well how come we don't get the same drag in a uniform fluid (such as underwater). And why does the imposition of another interface between different densities of water (such as fresh water overlaying salt water) create wave at this interface and add more drag to a hull? John wrote: >Once the speed reaches a point where the dynamic forces can >offset Bernoulli's effect the boat begins to climb. This can be seen quite >nicely in the tank. As the good admiral said, the change in trim at high >displacement speeds is a symptom of speed. Can you please explain the reason for the change in trim of the boat using physical principles rather than using vague terms like "symptom of speed"? John said: >Planing is the vertical rise of the CG not the angular climb of the CG >(there being no angular climb). Me, still floating around in the water getting hypothermic, wonders if you are still riding on the boat. As I watch your fast boat move from a displacement to a planing condition it moves a whole lot horizontally and very little vertically the resulting motion of the CG looks to me to be "climbing" at a shallow angle until levelling off once fully planing (I'm talking direction of CG here not trim angle). Now if I was riding on the boat and looking only at a very sensitive altimeter I might be seeing this as a vertical climb. I suspect you mean something else but I can't fathom what that might be. Does an airplane climb only vertically by your definition of climb? Is vertical the only way to climb by definition? >> I said: No, the flow is what happens to the water/moving boat combination (or >>boat/moving water combination). The water itself mostly moves mostly up >and >>down in an orbital motion as the boat passes. If you would jump in the >lake >>it might be easier to see it my way.;-) John responded: >The water moves in circular orbits in the waves but not in circular orbits >along the hull. If it did the flow would not travel along the hull even at >a distance from the hull as is revealed by testing. >I guess what I would like to see is some kind of proof beyond opinion that >the flow does not travel as the tank says it does. I'm not arguing with the tank results you have seen. I was only trying to follow the path of individual molecules of water (in relationship to the center of the earth) as a boat moves by them. You seem to want to think that I am talking about how the molecule moves in relation to the boat's hull (flow). I'm not here. >(SNIP) I said: >>Most telltales are inside the boundary layer. Earlier you mentioned some >>that were not. Hold them far enough away from the boat and they all will >>point only in the direction of motion. > John asked: >How far away? 20 or 30 feet should do it ;-) probably a lot less. > Here is what a text ("The Science of Yachts, Wind and Water" by H.F. Kay page 124) I just reread says: "As the boatspeed approaches the surface speed of its fundamental wave, the drag increases considerably because the boat is trying to go faster than this wave, therefore it is trying to climb uphill out of its own fundamental wave trough in a situation analogous to the high speed aircraft at the sound barrier trying to go faster than the speed of sound. Furthermore the boat tends to 'squat' low down in the water because the buoyancy is operating more at the less effective fore and aft regions than at the bulbous centre region where the water is forced lower." This is obviously one of the places I got my (mis?)education. I sure wish I could understand this subject starting from the basic principles of physics. Waves are a result of drag as Nick says. Is it also possible that they then interact with the hull and each other in ways that add to or decrease the drag on the hull rather than be just a forgotten burden illustrating the drag but not contributing to it? The above "Kay" explanation though not getting down to basic principles is attractive in that it explains the trim change when pushing above hull speed. It also seems to explain why in shallow water with its reduction of wave speed this squatting happens at a lower speed. This explanation may be dead wrong but I have yet to see another explanation for the trim change that takes place. Anyone care to try? Matt Broze www.marinerkayaks.com *************************************************************************** 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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