I don't recall numbers of tonnages, but I routinely sink 10 or more ships per patrol from 1939-1940.
I come out of Wilhelmshaven, head to the mouth of the channel, the cut north and follow the English coastline. I usually bag 3-6 ships along the east coast. Sometimes little guys, sometimes larger merchants, all alone. Every once in a while I'll hit a convoy on the east coast. I nailed one last week where I drew off the 4 destroyers, and then wiped out the entire convoy of 12 ships! But that is a rarity.
Then I duck between Lerwick and Scapa Flow, and cut West across the north coast of England. I usually do not find ships there.
I then hit Rockall Bank, and if that's not good for anything, AM52 and AM53 just north of Ireland are always very good hunting grounds.
I almost never come home with torpedoes in the boat, unless damage drives me home.
Also don't forget to use your deck gun whenever weather permits, at least until the merchant ships start being armed.
Now bear in mind that I'm playing the game, and not terribly realistically. I'm set to 100% realism (but I have map contacts turned on). I will reload torpedoes at any time (surfaced or submerged), and I'll pull in external eels at any time, regardless of weather. And I've used the TDC, with map contacts on, to dial in firing solutions while watching the little ship icon move around on the map. And I've been known to press "L" a time or two to lock up ships I could not see, though I'm trying to break myself of that habit.
Steve