If you want to get alot out of your patrol/campaign then you have to put in alot of extra effort. Lets say BDU gives your a patrol grid of AM12, you get there and you do your 24 hour patrol and you find no targets. hmmm dont just go home, go to another patrol grid and HUNT for some targets or if ur an experienced skipper who knows the North Atlantic then go to your favorite hunting grounds and start stalking. The best advice i can give to you is to check with Naval Intelligence aka SH3 Forum and talk to your fellow captains. Once you know where the good hunting grounds are then you dont have any good excuses to comming home empty handed.
some personal advice, try sailing down to about 200-250kms west of the Straits of Gibraltar, or the Bay of Biscay (before the Germans Occupied the French ports) or try hunting a little north of Ireland or a little south of Ireland, cause pretty much all of the Allied merchant and warship traffic had to pass through those areas.
Hey i got two questions for those out there. 1- What would happen in real life if a Uboat Captain came home empty handed after a patrol? 2- Im about to start my 8th patrol, its early April 1940, i got the invasion of Norway/Denmark/the Low Countries And France comming up (I cant wait till Dunkirk) Well i still dont have my U-Boat War Badge? I have my 1st/2nd Class Iron Crosses and i have about 230,000 tons sunk.

