Bah! All this tricky little sneaky manuvering. I go for the brute force approach.
In heavy fog, visibility is usually 350m max. Basically a point blank shot. What I do is track the target on hydro till I have a good fix on his course and position. Then surface and go flank speed till I spot him. Sometimes I have to dive to get a new fix again. Anyway, ahead flank till you spot the target. Run right up along side him. At this range it doesnt matter if he sees you or not. No amount of evasive manuvers will save a great wallowing merchie. Once you are along side of the target do a hard rudder turn away till you bring your stern tube to bear. Wait till you are at 300m the "FIRE". Bring yer boat hard around again to line up you bow. Provided you still have visual go to slow speed and see if he sinks. If not, plug him again. Simple, straight forward, quick.
Of course, you wouldnt want to do this on any armed ships.
