The theoretical crush depth of the Type VIIC was 280 - 375 meters, depending on which figures are used. Design depths from 120 to 150 meters are published. The Germans generally used a safety factor of 2.5 to get design depth and they tested the hull at a safety factor of 1.5. The maximum safe depth for the VIIC was therefore between 185 and 250 meters. The problem is that most of numbers published do not specify the applicable safety factor, so it is difficult to come up with a precise number for theoretical crush depth. The actual crush depth varied from one boat to another anyway since the quality of the materials and the workmanship varied.

BTW, there was a project for a VII C 42 which was to have a design depth of 200 meters using armor plate as the hull material. This gives us a maximum safe depth of 330 meters and a crush depth of 500 meters. Most of the other U-boat types only had a design depth of 100 meters, so their test and maximum safe depth was only around 170 meters, but many went deeper.

The Type XXI was designed to have a design depth of 135 meters, which gives a crush depth of 340 meters and a maximum safe depth of 225 meters. However, when the finished product was tested, there was a minor problem in the design which reduced the design depth to 120 meters, so we get a crush depth of 300 meters and a maximum safe depth of 200 meters, rather similar to the VIIC. The deepest test dive was to 220 meters.

Regards,
SuperKraut