I struggled with that too for quite a while. I sunk a couple of merchants near Malta. It seems that later in the year it was getting more crowded over there.
But it still wasn't enough. So then I did what I usualy do when I need tonnage, and although you tried it already if I'm right, it mostly works for me.
Just sail into a large harbor by night on periscope depth. Once you're in range, surface the boat and shoot every single ship to pieces with the deck gun. Dive again to periscope depth and silently sail away again like nothing happend ^^
|