Depth ratings are primary design parameters and measures of a

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

'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 10 metres (33 feet) of depth puts another atmosphere (1 bar, 14.7 psi, 101 kPa) of pressure on the hull, so at 300 metres (1,000 feet), the hull is withstanding thirty atmospheres (30 bar, 441 psi, 3,000 kPa) of water pressure.

Test depth

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

sea trial A sea trial 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 open water, and ...

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 maritime service branch of the United States Armed Forces and one of the eight uniformed services of the United States. It is the largest and most powerful navy in the world, with the estimated tonnage ...

submarines, while the

Royal Navy The Royal Navy (RN) is the United Kingdom's naval warfare force. Although warships were used by English and Scottish kings from the early medieval period, the first major maritime engagements were fought in the Hundred Years' War against Fr ...

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

German Navy The German Navy (, ) is the navy of Germany and part of the unified ''Bundeswehr'' (Federal Defense), the German Armed Forces. The German Navy was originally known as the ''Bundesmarine'' (Federal Navy) from 1956 to 1995, when ''Deutsche Mari ...

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 ever-so-slightly exceed the required specifications to account for imperceptible material defects or variations in machining tolerances. Since the crush depth is the depth at which the submarine is crushed, 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 a more general confusion as to the meanings of the various depth ratings, 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, often abbreviated as WWII or WW2, was a world war that lasted from 1939 to 1945. It involved the World War II by country, vast majority of the world's countries—including all of the great power ...

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

U-boat U-boats were naval submarines operated by Germany, particularly in the First and Second World Wars. Although at times they were efficient fleet weapons against enemy naval warships, they were most effectively used in an economic warfare ro ...

s of the types

VII VII or vii may refer to: the Roman numeral 7 Art and entertainment * The Vii, a video game console * vii, leading-tone triad, see diminished triad * ''VII'' (Blitzen Trapper album) * ''VII'' (Just-Ice album) * ''VII'' (Teyana Taylor album) * ...

and IX generally imploded at depths of 200 to 280 metres (660 to 920 feet).

References