Depth ratings are primary design parameters and measures of a
submarine'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 Kingdom of England, English and Kingdom of Scotland, Scottish kings from the early medieval period, the first major maritime engagements were foug ...
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 Mar ...
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 ma