How´s the weather? If it´s totally clear, my advice would be to stay out of their sight. Move surfaced and dive from time to time to get a new bearing to the convoy.
Other way would be a surfaced run just so that you can see the smoke plumes on the horizon, but this would be more risky.
Oh and by the way, I dont have a clue on what I´m saying-- I havent never shadowed convoy.
