The damage model in this game is buggy at best.
10 Mk14 torps at a Large Tanker, all spread along her starboard side, with 15-20 ft set as the depth setting so that at least 5 were under her keel. She looses her rudder, both screws, listing at a 30 - 40 deg angle AT LEAST, and her deck is underwater.... but keeps going along at 7 kts with the convoy
Yep...happend to me. I have also had this happen with Freighters too.... sometimes 1 torp breaks her back...sometimes a full load of 6 torps cant bring them down....