What version are you playing?
On approaching a convoy, make sure your conning tower is not popping out of the water (broaching). what speed do you use (it shouldn't be more than 2 knots).
As far as selcting targets: I pick myself two nice ones if conditions are favorable and send two torpeoes each. Then dive deep.
If you haven't done so already, visit my kielman website where you will find articles on convoy attack and other interesting subjects.
Good hunting
