Probably lots of different tactics, but this is what I do:
*set a paralell course with the convoy at flank speed, but clearly out of their field of view.
*make a 90 degree turn towards the path of the convoy some way ahead of it
*submerge approx 3000 meters away from the interception point
*wait for them to get really close
*fire all tubes rapidly at several targets, make a u-turn, and fire rear (if I have)
*leave without saying goodbye
*flank speed again, and once again overtake the convoy
(at this point it is zig-zagging anyway)
*reapeat all above
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