I'm experiencing the same problem.
Last time while "farming"
tonnage for British Supply Quest I get my first very own convoy. It was foggy night so I passed escorts and prepared attack in the middle of convoy. There was about 12 ships (tankers and frieghters) I fired at 4 targets: 2 tankers (2 torps each) 1 frieghter (1 torp) and 3rd tanker from stern tube. After beautyful fireworks show none of ships sunk and they sailed away burning but none of then ever slowed down. External-camera showed me that most of torps exploded directly below ships...
And from my experience none of ships I sunk through about 25 hours of playing sunk due to damage, everytime I had to beat the crap out of them to get info "ship sunk" before it started to go below waterline
