, there is one important
thing you left out. I'm no expert, so I will screw
up all the terminology.
I believe that
the ballast is less important to the climbing and
diving than motion and the "wings" or whatever they
call the fins that stick out of the top conn
tower and the ones in the back. Basically the
submarine is a lot more like an airplane than like
is a diving chamber. As it moves through the water,
the fins/wings generate lift, just as an airplane's wings
do. Since water is much more dense than air
(about 826 times under normal circumstances), you
only need itty-bitty wings to generate lift.
Thus, a submarine which is zooming around underwater
and suddenly loses power is only a little bit less
screwed than an airplane. They do have compressed
air which they can use for an emergency surface
maneuver, but from what I gather, this is not a
Update: 5 August 2005. Yesterday a Russian mini-submarine
sank after its propeller was entangled by a fishing net.\
Also The Custodian, something may have positive buoyancy at the surface,
but still sink at lower depths. Net buoyancy is a function of water
pressure; at lower depths, the increased pressure will overwhelm
the buoyancy. It's a lot harder to float with 500 feet of water
sitting on top of you than when you're at the surface.
Take the human body. Bodies not wearing
cement shoes float under ordinary circumstances. However,
world record holders and seekers for free diving (without air tanks)
pass the point of neutral buoyancy and must actively swim upward, or they
will sink. I read a great article on these guys in The Atlantic
Monthly a few years ago (it looks like it was the May 1997 issue).