Sure you just found a nice fat convoy and your first attack run sunk 3 good sized ships but after enduring 2 real time hours of being depth charged to hell without hydrophones, busted scopes, a busted diesel engine, forward torpedo tubes wrecked, deck gun smashed... its generally a sign of its time to get the hell outa dodge and make for port.

Although you should still consider targets of opportunity fair game... just watch out for deck guns when trying to use the uzo! and you probably want to try longer range shots at night if possible to allow for an easier escape.

Oh and even thought its late 1941, you just survived being depth charged & bombed to hell by the RAF and your only 100-200km from port dont ASSUME that plane that was just spotted is friendly... *sign* - got the damn bligther at the last moment as he released his bombs... lol killed the my flak crew and officer - i swear it would have been faster to scrap the sub when he made port and build a new one :o

I don't necessarily head home and end the patrol, though. I would usually pop in the nearest welcoming port for repairs if the damage is to a degree/to specific systems, where I feel it's best not to go on as-is.