Take a look at how I spent my December, 1939. Sailing at 3 knots submerged in a large diamond pattern really seems to work here. In one month, I sank 50,549 tons.
The entire month there was no wind until I reached home port, so I made good use of my the deck gun as well. Take a look at one shot from my deck gun hitting from 45 degrees did to this poor merchant. It seems most of my prey was hauling something very explosive below decks. I capsized 5 of the ships with a single torpedo, but this one takes the cake!
At times I couldn't even sink one merchant before another single would sail into the area. You'd think that after a month, they'd have started to go around. During the whole of this partrol, I saw only two destroyers and only from a great distance. Can't wait to get my IX and play some cat and mouse.
[EDIT to fix images]