My experience has taught me that its not tracking the convoy thats the hard part. As most people will tell you its getting away from the escorts. I have devloped a tactic that is both effective and highly dangerous which is to approch a convoy and destract the escorts on one side of the convoy by running at a speed that they can hear. When they turn and start towards you go slient and deep FAST!! When the escorts are destracted move in and make your kill. Then go back down and make your escape. This does work well on the early convoys on '39 and '40. Another thing to try is to go and use a ship as a hat like in Down Periscope, it does seem rather silly but it does in fact work. I did use this very successfuly in sinking a RN Battleship that was in the middle of a convoy and eluded the escorts long enough to fire three torps into the ship at it keel depth. Try them some time and see how it works. And it helps to have an outside view to see whats in the convoy. Remember the bigger the better!!!
