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
Human occupants of a submarine would suffer physiological problems if the air pressure inside were simply allowed to be equal to the water pressure outside the hull. So, when the inside air is kept at normal atmospheric pressure, the hull 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 of depth puts another atmosphere of pressure on the hull, so at 300 metres, the hull is withstanding thirty atmospheres of water pressure.
Design depth
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
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 at 4/7 the design depth, and the German Navy sets it at exactly one-half of design depth.
Operating depth
The maximum operating depth is the maximum depth at which a submarine is allowed to operate under any conditions.
Crush depth
Crush depth, officially called collapse depth, is the submerged depth at which a submarine's hull is expected to collapse due to pressure. This is normally calculated. However, it is not always accurate. Submarines from many nations in World War II survived 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. These reports are not necessarily verifiable, and popular misunderstanding of the difference between test depth and collapse depth can confuse the discussion. World War II German U-boats generally had collapse depths in the range of 200 to 280 metres.
