I posted this some months ago:

">Safety factors

German U-boat engineers used 4 factors to describe diving specifications:

Name safety factor vs. crush depth % of crush depth
Design depth 2.5 40%
Test depth 1.67 60%
Maximum safe depth 1.5 67%
Crush depth 1.0 100%

The typical design depth for IX and early VII types was 100 meters, which gives us a maximum safe depth of 167 meters and a crush depth of 250 meters (820 feet). The VII C had a greater design depth, but I have not been able to resolve which safety factors were applied. Design depths of both 120 and 150 meters have been published in connection with the VII C, however, the 150 meters would give a crush depth of 375 meters which is a bit ambitious for the St 52 KM alloy used. On the other hand, the 150 meters fits the 300 m crush depth if one allows a safety factor of 2, which may have been the factor used by the Americans.

The other U-boat with a design depth greater than 100 meters was the XXI with 135 meters design depth, but due to some uncertainties with the lower hull in the figure 8 design, the design depth was reduced to 120 meters. The maximum experimentally achieved depth of an XXI was 220 meters or 73% of crush depth.

These figures should be considered minimum specifications. If the shipyard did it's job extra well, these design specs could be exceeded, as they were in several cases when crush depth was exceeded.<"

Regards,
SuperKraut