It depends on so many factors. Not least a little luck.
If you have the patience, just use one. You can always attack the convoy again to
finish it off.
If you are in a rush, 2 will cripple or sink most vessels most of the time.
Bear in mind that sining two 16,000 ton ships with 1 torp each may be better than
sinking one 30,000 ton ship with two torps.
Personally I fire one torp on the bow and one aft because my manual targeting
requires me to hedge my bets on which way I made the inevitable error.
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