It always pays to read the actual reports before believing the hype. Kretschmer did indeed sink many ships with one shot. A large number of them were smaller (2000 tons or less) but at least one large tanker exploded and broke in half. On the other hand one 6000-to freighter took two torpedoes and 88 rounds from the deck gun, plus an unspecified number of gun rounds from Prien in U-47. It varied a lot in real life, but ships of 5000 tons or more usually needed at least two torpedoes to put down.
http://uboat.net/men/kretschmer.htm