The pressure hull is generally constructed of thick high strength steel with a complex structure and high strength reserve, and is separated with watertight bulkheads into several compartments. There are also examples of more than two hulls in a submarine, like the Typhoon class, which has two main pressure hulls and three smaller ones for control room, torpedoes and steering gear, with the missile launch system between the main hulls.
The dive depth cannot be increased easily. Simply making the hull thicker increases the weight and requires reduction of onboard equipment weight, ultimately resulting in a bathyscaphe. This is acceptable for civilian research submersibles, but not military submarines.
World War I submarines had hulls of carbon steel, with a 100-metre (330 ft) maximum depth. During World War II, high-strength alloyed steel was introduced, allowing 200-metre (660 ft) depths. High-strength alloy steel remains the primary material for submarines today, with 250–400-metre (820–1,300 ft) depths, which cannot be exceeded on a military submarine without design compromises. To exceed that limit, a few submarines were built with titanium hulls. Titanium can be stronger than steel, lighter, and is not ferromagnetic, important for stealth. Titanium submarines were built by the Soviet Union, which developed specialized high-strength alloys. It has produced several types of titanium submarines.
Titanium alloys allow a major increase in depth, but other systems need to be redesigned to cope, so test depth was limited to 1,000 metres (3,300 ft) for the Soviet submarine Komsomolets, the deepest-diving combat submarine. An Alfa-class submarine may have successfully operated at 1,300 metres (4,300 ft), though continuous operation at such depths would produce excessive stress on many submarine systems. Titanium does not flex as readily as steel, and may become brittle during many dive cycles. Despite its benefits, the high cost of titanium construction led to the abandonment of titanium submarine construction as the Cold War ended. Deep diving civilian submarines have used thick acrylic pressure hulls.
The dive depth cannot be increased easily. Simply making the hull thicker increases the weight and requires reduction of onboard equipment weight, ultimately resulting in a bathyscaphe. This is acceptable for civilian research submersibles, but not military submarines.
World War I submarines had hulls of carbon steel, with a 100-metre (330 ft) maximum depth. During World War II, high-strength alloyed steel was introduced, allowing 200-metre (660 ft) depths. High-strength alloy steel remains the primary material for submarines today, with 250–400-metre (820–1,300 ft) depths, which cannot be exceeded on a military submarine without design compromises. To exceed that limit, a few submarines were built with titanium hulls. Titanium can be stronger than steel, lighter, and is not ferromagnetic, important for stealth. Titanium submarines were built by the Soviet Union, which developed specialized high-strength alloys. It has produced several types of titanium submarines.
Titanium alloys allow a major increase in depth, but other systems need to be redesigned to cope, so test depth was limited to 1,000 metres (3,300 ft) for the Soviet submarine Komsomolets, the deepest-diving combat submarine. An Alfa-class submarine may have successfully operated at 1,300 metres (4,300 ft), though continuous operation at such depths would produce excessive stress on many submarine systems. Titanium does not flex as readily as steel, and may become brittle during many dive cycles. Despite its benefits, the high cost of titanium construction led to the abandonment of titanium submarine construction as the Cold War ended. Deep diving civilian submarines have used thick acrylic pressure hulls.