-----Original Message----- From: John Winters <735769_at_ican.net> To: 'Paddlewise' <paddlewise_at_lists.intelenet.net> Date: Monday, May 24, 1999 6:37 AM Subject: Re: [Paddlewise] Pro's and Con's of the "Swede Form" >(SNIP) > >> >>I think we are looking at this wrong. I don't think the water is doing >>anything but getting out of the way. The waves on the surface aren't >causing >>any acceleration or deceleration of the "flow" back and forth. > >Waves are symptoms of what happens not the cause. By examining the symptoms >one can get at the cause. The wave system (and particularly the transverse >wave system) reveals the energy expended in pushing the water out of the >way. Crests indicate lower velocity and troughs indicate higher velocity >along the hull. By measuring the pressure along the hull one can determine >the local velocity. (Eggert, did this in 1939). You are right. I didn't think that one trough very well. > >The water pushed out of the way by the boats passage piles up in the >transverse wave system and we can see the effects of displacement by >observing the size of these waves as Dr. Inui did in his research. From >this Inui developed ships forms that had almost no wave making resistance. > >>From this we pass on to the other phenomenon that occurs, notably the >sinkage of the hull. As velocity increases the hull sinks (most tank test >series measure sinkage) due to the increased velocity under the hull. Now >we must ask, why? If no increase in flow occurs under the hull, why does >the boat sink? Clearly increased flow does occur under the hull that will >cause an increase in kinetic energy over potential energy thus reducing the >hydrostatic forces supporting the hull. Where does this water come from? >Does it enter in from the sides?" Not if it gets pushed away to the side. >Does it come form the stern? Not if the boat moves ahead. Clearly it must >come from ahead. If we have increased velocity we must also have increased >volume. It seems to me that any speed increase in the flow over the boat speed could be due to the orbital motion of the wave trough and any extra sinkage of the hull (beyond that caused by its own speed) reflects that the boat is in the trough. The boat speed itself should cause some sinkage (the fact there is flow and that it is under and to the sides of the hull (but not on top) would cause the Bernoulli effect to be balanced to each side but the unbalance effect below the hull would make the hull ride lower. I don't see how increasing the speed of the flow implies the need for a greater volume of water from anywhere but above (dropping down to make a wave trough). > >> The hull is >>moving at a relatively constant speed in a given direction and what we >have >>been calling flow is just the indicators of the direction the boat is >moving >>as it scrapes through the water. (your surfboard and rudder examples >helped >>me see this). No water goes down under the surfboard without also >displacing >>some other water out of the way and since water is not comressible the >total >>of all these nudges between molecules is upward (making a wave). > >Yes, the water passing under the boat pushes water to the side but the flow >remains critical. As pointed out previously, that flow causes the hull >sinkage. How one looks at this depends upon how one asks the question. If >we ask, "Does the water get pushed to the side by the boat?" and answer, >"Yes it does." then we can get led to the erroneous conclusion that the >flow under the boat has relatively little significance or that it does not >dive under the boat. If we ask, how does the water flow around the boat and >get the answer described by observing the flow we get led to a different >conclusion that the water flowing under the boat displaces the water to the >side. > >How does this affect design? Consider the distance the water travels. A >poorly shaped hull might have a greater effective length than one that >ignores this information for one example. Also, if the water only gets >pushed aside you might be tempted to design boats like the old plank on >edge sailboats of the 19th century. Do you think a Hobiecat's narrow hull is ineffecient? My understanding is that it is so fast because rather than having to climb over the bow wave it simply cuts through it . Being so narrow and moving water only a little to the side and hardly downward it wouldn't suffer much from the Bernoulli effect either. I think our differences here are because you are looking at what the apparent flow over the hull is as it moves through the water and I am trying to understand what is actually happening to the water as the hull goes by. You are riding on the boat and I am swimming in the water. I realize that skin friction and separation drag is going to be determined by how the water appears to flow past the hull. I also realize it would make little or no difference in the physical effects if the water flowed by a teathered hull or the hull moved through still water. I'm just trying to point out that the telltales are going to mostly point in the direction of travel and if the hull is in the way they will point as closely as they can to the direction of travel. > >>I'm not sure I want to argue with the Admiral but imagine a flat bottomed >>ship built like a cow-catcher on a train. It should be clear that most of >>the water displaced would be lifted and deposited to the side by the >>cow-catcher bow yet if we looked at the little telltales we attached on >the >>bottom of the hull they would point in the opposite direction of the ships >>motion even though no water would have gone downward under the hull at all >>to make them do that. Why then are we or the good Admiral assuming some >>downward flow based on this "flow" direction evidence? > >Well, I can't argue this without seeing what kind of shape Matt proposes. >The cowcatcher example Matt provides may not have much to do with ship >shapes. It may have some value in studying crab scrapes or snowplows >though. We have to choose our examples carefully. Would we assume we know a >lot about the flow around ships by studying the flow of water around a farm >plow? Can we study the flow around a wing by testing a house in a wind >tunnel? > >Nevertheless, there are ships with "cowcatcherlike" bows (mostly bulbs and >such) and they have the same flow Taylor describes. Taylor's ships had bulb >bows. I was merely proposing a shape that would show a flow sraight back on the bottom that clearly did not require a downward flow yet exhibited the same behavior of the telltales as was being claimed as evidence of a downward flow of water. > >The flow indicates where the water goes. To assume that it doesn't go where >it is going kind of boggles. Again you are an observer riding the boat. The flow lines over the hull are 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 would appear to be at a slight downward angle but the water itself would have actually have moved 2" downward and slightly forward (due to the friction with the hull). So the water is mostly moving up or down with the waves and being pulled along at some speed between as fast as the boat (or even faster for spray flung forward at the bow) and not forward at all some distance from the boat (which gets wider as the boat moves past resulting in the wake behind the boat. I don't know why the flow lines around a moving hull are as they are (and I'm not disputing the direction of the lines just maybe the expanations given--that I don't see clearly) I'm trying to understand why they are that way in fairly simple terms and basic principles that even I can understand. It is getting into our busy season and I am spending way too much time in front of this computer so I'm going to try to taper off my responses, but if I can't control myself more I'll have to quit Paddlewise cold turkey. It has been fun. Matt Broze www.marinerkayaks.com > >If this is getting too anal for readers just speak up and Matt and I can >carry this on off-list. > >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/ >*************************************************************************** *************************************************************************** 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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