Re: [Paddlewise] Pro's and Con's of the "Swede Form"

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
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