-----Original Message----- From: Nick Schade <schade_at_guillemot-kayaks.com> To: paddlewise_at_lists.intelenet.net <paddlewise_at_lists.intelenet.net> Date: Tuesday, June 01, 1999 12:32 PM Subject: Re: [Paddlewise] Pro's and Con's of the "Swede Form" >At 3:09 PM -0700 5/29/99, Matt Broze wrote: >> >>I'm still having trouble seeing this.There still must be an altered pressure >>distribution underwater (but without creating any waves) and there is no >>"wave" drag then. One of the reasons submarines can be so fast. If the waves >>created alter that pressure distribution in some other way and cause drag >>(as you seem to have said) that would seem to indicate that the waves >>(altered pressure distribution) was causing the drag. > Nick responded >Submarines do have wave drag, it is just very small. Submarines have the >advantage that they can displace water in every direction. So, they don't >have to displace as much water to acheive the same motion. This doesn't seem to make sense. Liquids like water are virtually incomressable, which is why they do not make good shock absorbers but work well in the brake lines of your car. Water flowing under a sub can not be squeezed between it and the bottom so something must go up. Either the sub and all the water directly above it or all the water the sub is displacing. I don't really know if this reaches the surface in the form of a wake and if so where. Many years ago a friend was working on a defense project trying to find out how the Russians were detecting U.S. atomic submarines. Evidence available suggested they were sometimes able to do so. I suggested that maybe a wake was generated on the surface and the Russians had spy satellites that were linked to computers that looked for the signature a sub left on the surface by its motion Later I was told by this same friend that subs didn't leave any wakes on the surface. Don't know which is right but it still seems to me that they would leave a wake. As to how much water they displace it seems to me it would have to be more than a ship with the same total volume (because much of the ship is above water). I'd guess that if the displacement of the ship was the same as the weight of water that would fill the space taken up by the sub they would displace the same amount of water to achieve the same motion. And they are both really displacing it up rather than in all directions as Nick suggests for the sub. Nick wrote: >But the bottom >of the ocean and the surface of the water effectively limit the amount of >water available to aborb the displacement so more drag is created and a >wake appears on the surface. > >The water displaced by the sub has to go somewhere and the water put in >motion by the moving vessel propogates in a shockwave to the surface where >the waves appear. Momentum (mass * velocity) must be preserved and with >less mass, velocity must increase. Kinetic energy (1/2 mass * velocity^2) >is increased with the increased velocity, and since energy must also be >conserved, the viscosity of the water applies more drag to the boat so the >propeller must input more energy into the system. > I can't understand what you are trying to say here. Maybe if I knew the point you are trying to make I could follow your arguement. >While it would appear that the waves are slowing down the sub, the wake is >a manifestation of a lack of water available to be displaced. This lack of >water can be a result of shallow water or the viscosity of the water not >letting the pressure propogate freely. The pressure seems to progagate instantly in all directions which is why your brakes work. I don't see how the viscosity can slow the pressure propogating. If a wake reaches the surface it would seem to me to reach it directly above the front of the sub rather than trailing further back like the shockwave of an airplane (which is a propogating wave). I don't think a wave can propogate under water (unless their is some overlying water of a lower density--where a wave could propagate along the interface). Once the pressure forces the waters surface to rise then wave propagation could occur. >If you found a way to maintain a hull shape and yet reduce the waves, you >would actually increase the drag on the boat. For example, I expect if you >increase the force of gravity you would get smaller waves, but the drag >would increase. Nature does not take the hardway to do things. Waves are >the path of least resistance. Waves are good :-> And fun to surf too! I think all this talk of submarines has helped me see why the ship is slowed by its waves and the sub is not. The motion of the bow increases the pressure and this forces the water up above the level surface of the water. There is now a "head" of water that because it is being acted upon by gravity soon comes crashing back down into the water it just rose above and this displaces the water in a circle around it. That wave in turn does the same thing and so on and so forth until the energy is disapated over a wide area each new wave created by this process is smaller than the last due to the spread of the energy over a wider and wider area. It takes a certain amount of time for each wave to well up and fall back below the waters surface level. The faster an object moves the longer the wavelength created by that object will be because of the time required for this rebound to occur. A boat is therefore always at the leading edge of this wave propagation so the waves seem to spread out behind it when really they are traveling in all directions just like when you drop a stone in the water. The moving wave generator continously creates a wave on the surface which is moving as fast as it is moving (but in all directions from the point of creation). And like Nick said several days ago the waves are the result of the pressure changes around the moving object. But, contrary to what he said, that the waves don't cause the drag but are the result of it, I think that in the case of a boat on the surface the waves also increase the drag because they interact with the boat. The sub underwater may create a wave on the surface some distance away from the sub but that's old news the damage was done in making the wave and no matter where the wave goes to reveal itself it is usually too far away to cause any trouble. A boat that is on the surface must continue to interact with the waves that it created and it is in this interaction that addidional drag is potentially caused. If one can cancel the waves out they can do no further damage (much like sound waves can be cancelled by generating the same sound 1/2 cycle off can cancel the noise). This all seems to fit pretty well so far but now I must come up with the mechanism by which the waves increase the drag on a hull beyond the energy expended in their creation (which would seem to me to be the same for a ship and a sub of equal dispalcement moving the same speed). One possibility is that the orbital motion creates additional friction by increasing the relative speed of the part of the hull in the trough. Now the crests would be decreasing the relative speed of the part of the hull they are in contact with. I'm mentally on pretty shakey ground here but let's take a flyer just for fun. For illustration sake lets postulate a ship moving at 2 knots and lets say the orbital speed of a molecule in a wave is also two knots (but it could be anything). The area of the ship affected by the trough would have double the speed of flow over it (for roughly four times the drag--2 to the 1.83rd power actually). The area in a crest would have its drag cut to zero. Assuming the areas of forward (crest) and back (trough) flow are equal there would still be a net loss due to the increased friction (viscous drag) created by doubling the speed of the flow over half the area while ending the drag on an equal area of the hull. Anytime waves were cancelled out this effect would go away too. The smaller the waves the smaller would be the effect. Once the boat is going fast enough to stretch out the wave until the second crest is trailing behind the stern, most of the boat would be in a trough with much more water moving back increasing drag than moving forward reducing it and the waves created by this speed are larger and have an increased? orbital speed. Because of the shape of this wave most of the flow is under the boat and the Bernoulli effect is lowering the hull lower than the level it would float at standing still. Moving faster still the second crest has not rebounded in time to support the stern at all, resulting in a stern down trim. The floating hull is now (dis)placed upon an inclined plane which adds a lot of addition resistance (the sub will never see) as added power now must not only overcome the increasing friction of higher speeds but the accelleration downward due to the force of gravity which previously had been neutralized by the buoyancy of the hull in the water. I'm not sure about the added friction of the waves orbital motion. Anybody got any better explanations or criticisms of this one. Things seem a little clearer now that I've finally had enough sleep. Matt Broze http://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/ ***************************************************************************
Matt Broze wrote: > Nick responded: > >Submarines do have wave drag, it is just very small. Submarines have the > >advantage that they can displace water in every direction. So, they don't > >have to displace as much water to acheive the same motion. > > This doesn't seem to make sense. Liquids like water are virtually > incomressable, which is why they do not make good shock absorbers but work > well in the brake lines of your car. Water flowing under a sub can not be > squeezed between it and the bottom so something must go up. Either the sub > and all the water directly above it or all the water the sub is displacing. > I don't really know if this reaches the surface in the form of a wake and if > so where. [big snip] I read Nick's statement: "they don't have to displace as much water to achieve the same motion..." as meaning "they don't have to displace as much water *per unit of surface area of contact with the water* ..., " but now that I've typed that, I'm not so sure. Maybe *Nick* will tell us what he meant! <g> I suspect that submarine "wave-making" has substantially different characteristics than "wave making" by a surface ship, inasmuch as the surface ship's wave-making is mostly (?) at the surface of the water. In contrast, a submarine is neutrally buoyant, so the restoring forces are almost entirely due to the visco-elastic properties of water, and I do not see that there must be a rise in the water surface *fully equal to* the sub's volume as it passes through a water mass. Rather, displacement of the surrounding water is (mainly) normal to the sub's surface, and would propagate to the sides, bottom, and top of the sub. (Agreed the component which goes down can't compress the water there -- so some fraction of the sub's displaced volume has to go up!) 'Bout that component of a surface ship's "wave making" which may not be very evident (in deep water) as a change in vertical position of the surface: you must be acquainted with the "surge" off a ship's bow which reaches an observer ahead of the "main" wake. I don't think I have ever detected that surge when floating in deep water near a ship passing by, but see it every time when the ship passes shallow water as a slow "suck" followed by a huge "surge." You know what I'm talking about, Matt? I think that "surge" wave is a different kind of water wave than the "bow wave" we see as massive vertical displacement of the surface of the water. Could be just a very low amplitude, very LONG wave-length surface wave, now that I think about it, I guess. Or, am I all wet? -- Dave Kruger Astoria, OR *************************************************************************** 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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