-----Original Message----- From: Matt Broze <mkayaks_at_oz.net> To: 'Paddlewise' <paddlewise_at_lists.intelenet.net> Date: May 28, 1999 12:38 AM Subject: Re: [Paddlewise] Pro's and Con's of the "Swede Form" (SNIP) > >My point of view is from the water watching the boat move by. You seem to be >observing from the boat. The source of the apparent flow is because the boat >is moving. It comes from the front only if you are riding on the boat. Jump >off for awhile. Even if I jump off the source of flow comes from ahead. Whether the boat moves or the water moves the flow still comes form ahead. (SNIP) >Of course the boat creates the wave but that wave sure looks to me like a >hill to climb that is bigger than the hill that would have to be climbed to >reach planing speed if one started from a position where the wave hadn't >already been formed. I am well aware that true planing is beyond human power >except with the use of hydrofoils (at least at the present time and into the >forseeable future). >The powerboat sure looks to me to be climbing out of the hole of the first >trough behind the bow wave when it is moving from displacement to planing >speeds. A waterskier starting from a dock finds it much easier to plane than >one being dragged through the first water does (or to eliminate some >variables --has slowed to the point of sinking before power is again >applied). >Since planing is using an angled plane and speed to maintain a higher >position against the force gravity, it sure seems to me that if you start >from a lower position you need to climb up to the higher plane. If you have >pushed up a wave in front of you you will have to climb even higher to get >it more behind you. This common misconception comes from looking at the waves rather than looking at the CG of the boat. When a boat begins to plane its CG rises relative to the normal water surface (not the waves). The water skier on the dock starts out with less displacement. If we apply a bit of logic to this it should come clear. As the boat lifts due to dynamic forces it produces a smaller bow wave because it displaces less water. If the boat had to climb the bow wave the bow wave would stay the same size while the boat climbed over it. In reality the boat simply produces a smaller a smaller bow wave until it fully planes. 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. 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. Planing is the vertical rise of the CG not the angular climb of the CG (there being no angular climb). I highly recommend Savitsky's papers on planing boats to people interested in this complex but fascinating topic. > >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.;-) 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. (SNIP) > >I don't think I was. Sometimes it is easier to see things when you take them >to extremes. If evidence is being used to show something to be true but if >that same evidence can also be shown to be present due to a different cause >it would cast some doubt on the real meaning of that evidence. That works both ways. If we can show a thing to have validity under normal circumstances we must then cast doubt on whether we have observed the extreme example correctly. In this case we have not. Once we induce the flow to travel over the hull as well as under the hull we have created a submarine. (SNIP) >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. How far away? (SNIP) > >I have read the relavant parts of "Principles" but it has been awhile. I do >have Aero-Hydrodynamics of Sailing by Marchaj in front of me right now >though. If you have a copy check out the picture on page 77. That picture >(of a sailboat pushing hull speed) looks like it might expain why the flow >lines dive under the boat and follow the buttock lines (at least when one is >pushing hull speed). I still doubt we will see this at very slow speeds >though. Don't have aerodynamics of Sailing but do have Sailing Theory and Practice. So will have to get it and see what Matt is talking about here. At very low speeds (Here I would say below Froude number 0.10 for the sake of discussion) I don't think we have much to work with so I can't argue the point. Sea kayaks normally operate closer to Froude 0.2 so maybe it doesn't matter much. > >As long as you have the book open check out the flow lines on the hull of >the boat on page 279. They appear to be diving all along the hull and only >following the buttock lines in the forebody. In fact it looks to me that >they may be moving to fill in the low pressure area behind the hull like I >proposed might be happening earlier. Of course that huge keel is probably >confounding things. You can bet the keel is mucking things up. I wish I had half the money spent on studying how keels interact with hulls. *************************************************************************** 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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