You could also time your attacks to take place at night, not only does this drastically reduce the chances of getting spotted, you'll also be able to fire off your initial salvo's on the surface. I've often managed to set up a long 90 degree shot from 3000m+ and as soon as the torpedoes leave the tubes turn away and run on the surface. The escorts often concentrate around where the first ship is hit and never see you, giving you time to run ahead and get in front of the convoy for another shot.
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