Depth ratings are primary design parameters and measures of a

submarine A submarine (often shortened to sub) is a watercraft capable of independent operation underwater. (It differs from a submersible, which has more limited underwater capability.) The term "submarine" is also sometimes used historically or infor ...

's ability to operate underwater. The depths to which submarines can dive are limited by the strengths of their hulls.

Ratings

The hull of a submarine must be able to withstand the forces created by the outside water pressure being greater than the inside air pressure. The outside water pressure increases with depth and so the stresses on the hull also increase with depth. Each of depth puts another

atmosphere An atmosphere () is a layer of gases that envelop an astronomical object, held in place by the gravity of the object. A planet retains an atmosphere when the gravity is great and the temperature of the atmosphere is low. A stellar atmosph ...

(1 bar, 14.7 psi, 101 kPa) of pressure on the hull, so at , the hull is withstanding of water pressure.

Test depth

This is the maximum depth at which a submarine is permitted to operate under normal peacetime circumstances, and is tested during

sea trial A sea trial or trial trip is the testing phase of a watercraft (including boats, ships, and submarines). It is also referred to as a "shakedown cruise" by many naval personnel. It is usually the last phase of construction and takes place on op ...

s. The test depth is set at two-thirds (0.66) of the design depth for

United States Navy The United States Navy (USN) is the naval warfare, maritime military branch, service branch of the United States Department of Defense. It is the world's most powerful navy with the largest Displacement (ship), displacement, at 4.5 millio ...

submarines, while the

Royal Navy The Royal Navy (RN) is the naval warfare force of the United Kingdom. It is a component of His Majesty's Naval Service, and its officers hold their commissions from the King of the United Kingdom, King. Although warships were used by Kingdom ...

sets test depth at 4/7 (0.57) the design depth, and the

German Navy The German Navy (, ) is part of the unified (Federal Defense), the German Armed Forces. The German Navy was originally known as the ''Bundesmarine'' (Federal Navy) from 1956 to 1995, when ''Deutsche Marine'' (German Navy) became the official ...

sets it at exactly one-half (0.50) of design depth.

Operating depth

Also known as the maximum operating depth (or the never-exceed depth), this is the maximum depth at which a submarine is allowed to operate under ''any'' (''e.g.'' battle) conditions.

Design depth

The nominal depth listed in the submarine's specifications. From it the designers calculate the thickness of the hull metal, the boat's displacement, and many other related factors.

Crush depth

Sometimes referred to as the "collapse depth" in the United States, this is the submerged depth at which the submarine implodes due to water pressure. Technically speaking, the crush depth should be the same as the design depth, but in practice is usually somewhat deeper. This is the result of compounding safety margins throughout the production chain, where at each point an effort is made to at least slightly exceed the required specifications to account for imperceptible material defects or variations in machining tolerances. A submarine, by definition, cannot exceed crush depth without being crushed. However, when a prediction is made as to what a submarine's crush depth ''might'' be, that prediction may subsequently be mistaken for the actual crush depth of the submarine. Such misunderstandings, compounded by errors in translation and general confusion as to what the various depth ratings mean, have resulted in multiple erroneous accounts of submarines not being crushed at their crush depth. Notably, several

World War II World War II or the Second World War (1 September 1939 – 2 September 1945) was a World war, global conflict between two coalitions: the Allies of World War II, Allies and the Axis powers. World War II by country, Nearly all of the wo ...

submarines reported that, due to flooding or mechanical failure, they had gone below crush depth, before successfully resurfacing after having the failure repaired or the water pumped out. In these cases, the "crush depth" is always either a mistranslated official "safe" or design depth (i.e. the test depth, or the maximum operating depth) or a prior (incorrect) estimate of what the crush depth might be. World War II German

U-boat U-boats are Submarine#Military, naval submarines operated by Germany, including during the World War I, First and Second World Wars. The term is an Anglicization#Loanwords, anglicized form of the German word , a shortening of (), though the G ...

s of the types VII and IX generally imploded at depths of .

References