-----Original Message----- From: Dave Kruger <dkruger_at_pacifier.com> To: Matt Broze <mkayaks_at_oz.net>; PaddleWise <PaddleWise_at_lists.intelenet.net> Date: Wednesday, June 02, 1999 3:55 AM Subject: Re: [Paddlewise] Pro's and Con's of the "Swede Form" >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!) Let's see the sub has aready displaced the water just by being there so the water level on shore is already higher. It is just the disturbance of the water due to the subs motion that must be distibuted and ultimately dispersed into random motion. The subs' speed will determine the amount of energy to be dispersed. Unless the bottom (and sides) of the basin the sub is in can disperse the energy from the pressure in front of the sub it would seam that it would eventually (how soon is unclear but given the incomressible nature of water I would think it would have to happen very quickly) end up at the surface and be used to lift some water above the level of the surface creating waves whether they be detectible or not. On the other hand maybe no wake is really necessary as the pressure at the bow and the lower pressure at the stern may just mean the water moves faster over the hull to fill in the low pressure area behind the sub balancing the system. Not being near on the surface no waves escape the system to transfer energy away. Maybe I should look in a reference book but speculating first is more fun. Back when I was trying to understand weatherhelm I looked in a lot of books and while many of them mentioned it and some even described how ships add funnels and such forward to compensate none of the books I found gave an expanation and I had to devise my own. > >'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. As best I can see what you are probably seeing is the first of the transverse waves (created by the pressure at the bow and lower pressure at the stern) to reach shallow water. The surge you see on the beach is due to that deep water wave reaching shallow water and turning into a wave of translation (or surge wave--like the soup after a breaker breaks on the beach). The bow waves you see peaking up high and angling back from the ship is the interference pattern between the divergent waves off the bow and the transverse waves propogating from the initial pressure at the bow and the less than normal pressure behind the ship. > >Could be just a very low amplitude, very LONG wave-length surface wave, now >that I think about it, I guess. Yes, coming into shallow water. > >Or, am I all wet? > Aren't we all about 78% water. *************************************************************************** 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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