Beetle, just a Fishu said, finish your 24 hour patrol in the assigned sector and after that move on to the closest convoy lines.. find them on the map that came with the game.
Imho, single missions take away most of the fun of the game.
Receiving a radio message about a close by enemy warship convoy while you're in the midst of your patrol really makes up for the realistic, yet admittedly sometimes boring lack of action.
I ran into the Illustrious once, in about the worst weather conditions the game engine can create.
She basically appeared out of nowhere, maybe 100 meters in front of me. She ran full flank and was getting out of sight fast, so I had to calculate a torpedo solution all by sonar as she got away. It actually worked, 2 out of 4 torpedos hit their mark and obviously even the magazine. I didn't see her again, all I heard was the sounds of bursting compartments as she went down.
After that I was DC'd for 7 hours game time, the longest DC pursuit I ever experienced in the game.
Needless to say, that made up for the otherwise boring patrol.
Additionally I love the ability to upgrade your boat. Especially when the new sonar equipment becomes available for exact distance measuring.
I use RUB 1.44