A submarine's depth ratings are a primary design parameter and measure of its ability. The depths to which submarines can dive are limited by the strengths of their hulls. As a first order approximation, each ten metres (thirty-three feet) of depth puts another atmosphere (15 psi, 100 kPa) of pressure on the hull, so at 300 metres (1000 feet), the hull is supporting thirty atmospheres (450 psi, 3000 kPa).

Collapse depth, popularly called crush depth, is the submerged depth at which a submarine's hull will collapse due to the surrounding water pressure. This is normally calculated through mathematical means; however, it is not always accurate. Submarines from all sides 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.

World War II German U-boats generally had collapse depths in the range of 200 to 250 meters. Modern nuclear submarines like the American Seawolf class are estimated to have a test depth of 1600 feet (about 500 m), which would imply (see below) a collapse depth of 2400 feet (730 m).

Design depth is the arbitrary 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.

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

Test depth is the maximum depth that a submarine is permitted to operate at under normal (e.g. peacetime) circumstances, and is in fact tested at during sea trials. In the United States Navy, it is set at two-thirds of design depth. The Royal Navy sets test depth at a little deeper than half (4/7) of the design depth, and the Deutsche Marine sets it at exactly one-half of design depth.

References