Submarine depth ratings

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. It is important to realize that there is a limit to how high the pressure can build inside the sub, as problems develop. For example, oxygen becomes toxic at high pressures, thus the pressure cannot be allowed simply to equalize. As a first order approximation, each 10 metres (33 feet) of depth puts another atmosphere (1 bar, 14.7 psi, 100 kPa) of pressure on the hull, so at 300 metres (1,000 feet), the hull is supporting thirty atmospheres (30 bar, 441 psi, 3,000 kPa) of water pressure. (Note: The one atmosphere of air pressure at sea level is balanced by the roughly one atmosphere maintained inside the sub, so it does not appreciably stress the hull.)

Design depth is 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. Since the designers incorporate margins of error in their calculations, crush depth of an actual vessel should be slightly deeper than its design depth.

Test depth is the maximum depth at which a submarine is permitted to operate under normal peacetime circumstances, and is tested during sea trials. The test depth is set at two-thirds of the design depth for United States Navy submarines, while the Royal Navy sets test depth slightly deeper than half (4/7ths) of the design depth, and the German Navy sets it at exactly one-half of design depth.

The maximum operating depth (popularly called the never-exceed depth) is the maximum depth at which a submarine is allowed to operate under any (e.g. battle) conditions.

Crush depth, officially called collapse depth, is the submerged depth at which a submarine's hull will collapse due to pressure. This is normally calculated; however, it is not always accurate. Submarines from many nations in World War II reported being forced through crush depth, due to flooding or mechanical failure, only to have the water pumped out, or the failure repaired, and succeed in surfacing again. One of the most popular stories of this occurring was the story of U-96, in the movie Das Boot. Note that these reports are not necessarily verifiable, and popular misunderstanding of the difference between test depth and collapse depth can confuse the discussion. (Planesman error sometimes causes submarines to exceed test depth by a few feet or metres during trials; note that a one-degree up-bubble on an Ohio-class boat indicates that the stern is some ten feet or three metres deeper than the bow.)

World War II German U-boats generally had collapse depths in the range of 200 to 280 metres (660 to 920 feet). Modern nuclear attack submarines like the American Seawolf class are estimated to have a test depth of 490 m (1,600 ft), which would imply (see above) a collapse depth of 730 m (2,400 ft).