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. 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. 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. 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. Imagine a plunger moving in an enclosed rigid cylinder (a shock absorber for example). The walls of the cylinder do not permit waves. If the plunger has a hole in it but otherwise fits tightly in the cylinder, all displacement of the liquid as the plunger moves must happen through the hole. The liquid has to move fast to get through the hole and the plunger is hard to move. Now make the cylinder larger so there is more room for the displaced liquid to move. More liquid will move around, but it doesn't need to move very fast, so the plunger is easier to move. Now, make the walls of the cylinder flexible. The liquid will displace the walls in a wave as you move the cylinder and the plunger will be even easier to move. So, in actuallity the waves actually reduce the drag. 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 :-> Nick Nick Schade Guillemot Kayaks 10 Ash Swamp Rd Glastonbury, CT 06033 (860) 659-8847 Schade_at_guillemot-kayaks.com http://www.guillemot-kayaks.com/ >>>>"It's not just Art, It's a Craft!"<<<< *************************************************************************** PaddleWise Paddling Mailing List Submissions: paddlewise_at_lists.intelenet.net Subscriptions: paddlewise-request_at_lists.intelenet.net Website: http://www.gasp-seakayak.net/paddlewise/ ***************************************************************************Received on Tue Jun 01 1999 - 11:56:53 PDT
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