Double Hulled Sub

[SJSVOB@aol.com]

I'm new at designing PSUBS and I've just been laying out a design in my head,
slowly figuring how everything will work.  I've ordered some submarine design
books from Amazon.com but they haven't arrived yet.  I've thought of a design
but I don't know if it's been done before.  

How about a double hulled sub?  The inner hull would be a common psub, but the
outer hull also could be pressurized.  By doing so you could travel to a depth
equal to the sum of the depths that each hull could tolerate on their own.
Also the outer hull would serve as your ballast tanks.  Since they can be
pressurized you could pressurize the remaining air volume in the tanks after
they have been filled with water.  So to drain the water you could simply open
a valve and the pressurized air would force the water out.  (Assuming that you
are at a depth with less pressure then the pressure of the air in the outer
hull.)  You could build in sectional dividers into the outer hull, so if you
fractured the outer hull due to an underwater collision, (or another psubber
torpedoed you), you wouldn't lose all your ballast tanks, and the pressurized
air would force the water ballast out and you would immediately rise to the
surface.  By pressuring the outer hull at the surface you could also test for
any leaks in both hulls. 

Is this how any existing subs work?  Granted this design isn't good for having
viewports, but for a psub I've always been more of a fan of using a 3D
Stereoscopic head mounted display connected to a switch box leading to CCD
Cameras and active sonar.