A pressure hull
is a component of submarine
s. Early submarines were built in much the same way as surface ship
s, with a single layer of hull protecting the interior from the sea. However, as World War II
ended, submarines began to be designed and optimized for submerged running, rather than surface sailing, as technology improved. The Type XXI U-Boat
is often thought of as one of the world's first 'true' submarines. Able to run for extended periods of time without fully surfacing due to Schnorkel
engines, the Type XXI eschewed much of the then-standard equipment on the outside of the boat. Railings, deck gun, exterior anchors, ladders, decking, etc. were all deleted in the interest of increasing the submerged speed and quietness of the submarine.
As submarines acquired nuclear propulsion, starting with the USS Nautilus, they could finally spend nearly all of their time in what had become their native element. With the extra power available, they could move faster - and they became more streamlined still, to the point where most modern submarines (nuclear or not) have a similarly cylindrical main hull design. The 'teardrop' hull shape they have assumed sports a mostly hemispherical bow to minimize drag.
The problem is that submarines by necessity contain all manner of equipment that has to either live outside the 'sea level' pressure of the interior, or has to be connected to the ambient water pressure to work properly. For example, ballast tanks need to be attached to the hull to affect it, but do not need to exist at standard pressure. Torpedo tubes, buoy launchers, sonar domes, etc. all need access to open water. However, in order to maximize safety, the number of 'holes' in the hull must be minimized.
Enter the pressure hull. Modern submarines tend to have two hulls. In extreme cases, they are entirely double-hulled, just like modern and safe supertankers. The outer hull, also called the light hull, is not pressure-bearing; it serves to protect the inner hull and systems and provide streamlining. The inner hull is the pressure hull, so named because it is the hull which must bear the load of the pressure differential between the crew spaces and the ocean. Some submarines (modern U.S. attack submarines for one example) are single-hulled in the center of the boat, where their cross-section is circular, and have light hull covering the fore and aft ends in order to provide better hydrodynamics and to cover equipment. Soviet (and now Russian) submarines sometimes have entirely double-hulled designs, and they (along with other boats of the world) sometimes even have multiple pressure hulls inside a single light hull. The most famous example is probably the Typhoon class SSBN - which the Soviets called the Akula. It's famous here for serving as the real-world version of the fictional submarine Red October. The Typhoon has three pressure hulls - two laid side by side in the 'main' hull, and one smaller one within the sail. The submarine's ballistic missile launch tubes are fitted between the two pressure hulls.
Typically, ballast tanks are kept between the light hull and the pressure hull. There is also research and design work being done on fully encapsulated torpedo launchers, with an eye towards eliminating the traditional weak point between the hulls of the torpedo room and its torpedo tubes. These launchers would operate much like VLS tubes, but placed horizontally and firing encapsulated torpedos on electronic command - this way, submarines could be made stronger, and wouldn't have to open doors in order to fire. These launchers would live outside the pressure hull.
Although light hulls are typically made of relatively 'light' metal, the pressure hull is made of thick, high-strength alloy steel or (in extreme cases) titanium. The latter, while lighter and stronger than steel, allowing submarines to operate at much deeper depths, is also frighteningly expensive. The Soviet Union built several titanium submarines, including the Alfa-class SSN and the ill-fated K-278 Komsomolets.