Hull difference....german, american

[Sailor Steve]

I don't know about stronger steel, but I have read that when the Allies captured u-boats they were surprised to find that the pressure hulls were thicker, and of course therefore stronger.

As to the claim of causing serious damage at a range of 30-50 meters, Allied sources claimed a lot less:

The British claimed this DC would split a 0.875 inch (22 mm) hull at 20 feet (6.1 m) and force a submarine to surface at about twice that distance. A minol charge introduced in 1942 increased these distances to 26 feet (7.9 m) and 52 feet (15.8 m), respectively.

This note is for the British Mk VII Heavy depth charge.
http://www.navweaps.com/Weapons/WAMBR_ASW.htm