-----Original Message----- From: John Winters <735769_at_ican.net> To: 'Paddlewise' <paddlewise_at_lists.intelenet.net> Date: Wednesday, May 26, 1999 5:34 AM Subject: Re: [Paddlewise] Pro's and Con's of the "Swede Form" >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) > John wrote: >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? I'll try. Waves move through the water. The water doesn't move much and mostly ends up back where it started before the wave went by. Once the water is disturbed gravity acts on the water raised by the disturbance and it falls back to a point lower than it started only to be lifted again and again overshooting the mark of a calm equalibrium each time by less and less. Waves thus propagate outwards from the disturbance in ever diminishing size until the energy from the disturbance is finally disappated and spent. If the disturbance (the boat) is moving as fast as the wave propagates the transverse wave will appear to be widening towards the stern due to the energy spreading away from the disturbance. Why I said the water might be going back and forth (as well as up and down and forward and back) is that if the hull is flared outward (as most that don't look like a plow are) the hull would be in the way of pure up and down motion of the individual water molecules. It would force them to the side as the crest passes through them and they would fall back next to the hull with the next trough. To see this more clearly (my way) imagine a long flared boat with many crests along its side at the same time. > > >(SNIP) > >(SNIP of Nick's comments) > I said: >>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. > John said: >The hull speed doesn't get lower (Always Froude 0.40) the resistance >increases due to the shallow water effects on the wave system. Perhaps we have a different definition of hull speed. I see it as the speed at which massive increases in power create little additional speed due to the limitations on the speed of a wave. If the speed a wave is able to travel is reduced then by my definition the speed that a hull is able to easily move is also reduced accordingly for the same reasons that "hull speed" exists in the first place. John continues: >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. What you just described sure sound to me to me to be very similar to whate happens to a planing boat. This change in regime John talks about doesn't happen all at once (unless the water suddenly steps from deep to very shallow). I think if you check my math concerning how the wave speed slows in shallow water you will find that I took the different regimes and the transition between them at varous depths all into account. I'm not so sure that the fast canoe doesn't plane some since it is in a regime that may make exceeding the speed of the wave possible. May depend on our definition of planing. The speed of a wave of translation (in feet per second) is the square root of the ((height of the wave + the depth of the water) times 32). So for a 4" high wave in 6" of water the wave speed would be about 5 feet per second (or 3 knots). The canoeists are definitley going faster than three knots but maybe that just mean they raise a higher wave of translation out in front of them. I don't know. I do know it is sometimes possible to see this stange wave in front of your bow (that extends almost straight to the side for a great distance) if you paddle as fast as you can in very shallow water (with a reasonably level bottom). Another question: Is there anything happening in this shallow water regime that is akin to "ground effect" (the reason Pelicans can glide for so long with their wingtips just above the waters surface)? > > >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. I have no idea what you are trying to say here but I am very familair with the Bernoulli effect thank you. I'm wondering if you are making it the cause of wavemaking when I see it as the effect. > > >(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). > >John said: >No one climbs their bow wave. It is a bit like lifting yourself by your >bootstraps. We can try to climb it but we will just waste a lot of energy in the attempt because we lack the horsepower. That is what outboard motors are for. (see last post for my take on this) > >(SNIP) > > I speculated: >>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). > John responded: >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), When I throw a rock in the water it displaces some of the water upwards and then gravity brings it back down but it overshoots equilibrium. Are you telling me that the trough along side of a boat moving along at near "hull speed" is caused by the Bernoulli effect rather than normal wave propagation? Seems to me to be the other way around. The orbital motion of the wave (that has already been created and would be there regardless of Bernoulli) could be the cause of the Bernoulli effect on the hull. I said: >>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. > John responded: >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. Check out the picture on page 279 of "Aero-Hydrodynamics of Sailing". It is enough to make me wonder what is happening there. There are possible confoundings due to the keel and I don't know the speed length ratio it was taken at but it seems to support my theory. 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/ >*************************************************************************** *************************************************************************** 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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