Do submarines shrink

Answer 1

The actual diving depths of most modern nuclear submarines is highly classified; however, all submarines are constructed to either withstand the pressure (e.g., the titanium hulled Russian ALFA), or "flex" with the pressure. It is the metallurgical properties of the steel used that determines how well a submarine's outer and inner hulls withstand the enormous pressure per square inch at deep depths.

While strong hulls like the ALFA mean they can go deeper, the fact that they don't flex with the pressure (contracting and expanding as it goes deep and then shallow) makes the hull more brittle with each dive. It is similar to an eggshell being repeatedly put under stress - since it cannot flex, it will eventually break.

By contrast, U.S. Submarines are built with a specific type of steel that contracts with pressure, and expands. A little known fact of submarine construction is that the decks are actually "floating" - they are suspended and do not actually touch the hull for this reason.

To illustrate the extreme pressures at deep depths, during the first operations on the wreck of the USS Thresher (SSN-593), a styrofoam coffee cup was placed in a basket of the bathyscape that was used to survey the wreck. Placed outside the hull, it was subjected to the extreme pressures of the deep ocean. When they returned to the surface, it had shrunk from its original size (around 6") to barely the size of a thumb due to the compressive forces of the sea. This is still done on civilian research submarines and ROV's for souvenirs.

A submarine's hull shape is also designed specifically to deal with deeper depths. The submarines of WWI and WWII were not considered true submersibles by today's standards - they were surface ships with limited submerged operational capability. As such, they were designed to run faster on the surface than submerged, and their design wasn't truly made for extreme depths.

It has been long known that a spherical, or round shape, is the best shape for withstanding pressure. This is why the earliest diving bells, diving hard hats, and rescue chambers all had spherical designs. Today's rounded hull shapes of modern submarines allow them to go much deeper and withstand a lot more pressure than their older counterparts.