| From: | Marc Renouf renouf@********.com |
|---|---|
| Subject: | Submarines |
| Date: | Fri, 16 Jul 1999 12:48:38 -0400 (EDT) |
> Actually, IIRC, I believe that submarines are far more efficient
> underwater than on the surface.
That is correct.
<naval architect mode>
The reason for this is because a ship's drag has three components
- friction drag, form drag, and wave drag. Friction drag is exactly what
it sounds like. Form drag is related to how the shape of the vessel
displaces water around the hull (for instance, long skinny ships don't
require as much transverse energy to "part" the water in front of them as
a ship with a flat prow would). Wave drag is the energy that's wasted in
making waves (i.e. the wake). A handy way to think of it is like building
up pressure in front of the vessel and creating suction behind the vessel
as it moves through the water.
As a side note, my esteemed GridSec colleague Graht brought up the
thing about the bulbous bow. Let me explain: A submerged bulb on the
front of a ship is designed specifically to create a bow wave, and is
"tuned" for a specific speed (because the waves caused by a ship propagate
through water at the same speed as the ship - you can tell how fast a
ship is travelling from a photograph by looking at the wavelength of the
transverse waves in its wake). The bulb is designed so that at the target
speed, the bow wave will cancel out the stern wave - basically you still
have the "pressure" of making the bow wave, but you've removed the
"suction" of making the stern wave, and thus reduced your overall wave
drag. Unfortunately, it only works well for a specific speed (related to
the type of bulb and the overall length of the vessel)
At higher speeds, wave drag becomes the dominant component for
surface vessels. However, in the case of submarines, if you're not
wasting any energy making waves, you only have to worry about friction and
form drag. Once a submarine reaches a certain critical depth (determined
by its speed), it no longer causes any significant waves on the surface,
so it loses wave drag all together. Granted, the friction drag is higher
because there's more hull surface area in contact with the water, but the
increase in friction and form drag is more than made up for by the
decrease in wave drag. This is why modern subs are much faster under
water than they are on the surface.
Further, there is the matter of cavitation. Propeller cavitation
is the result of sharp high-to-low pressure transitions across the fins of
a propeller (these pressure transitions are what give you "thrust"). If
the low-pressure side of the transition drops below the vapor pressure of
water, the water turns to steam (though not because it's hot). This
causes local bubbles to form. Unfortunately, since the water hasn't
raised in temperature, as soon as the pressure increases to its normal
level (i.e. as soon as the bubble gets away from the low pressure area),
it collapses. This has two effects: first, the impact of bubble formation
and collapse causes tiny shockwaves to hit the propeller, like hitting
thousands of times a second with a tiny hammer. This causes some prettey
intense wear and tear on a prop. Second, those same collapse-induced
shockwaves also propagate through the water, where they make a godawful
racket to any listening sonar.
Fortunately, the deeper you are in the water, the higher the
ambient pressure. The higher the ambient pressure, the less likely it is
that your propeller pressure transition will result in a sub-vapor level
pressure. It's like the water is "squeezed" harder, so it's harder to
make bubbles. Less bubbles = less noise. Less noise = more sneaky. More
sneaky = happy sub commander.
</naval arcthitect mode>
Make sense?
Marc Renouf (ShadowRN GridSec - "Bad Cop" Division)
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