Sounds as if your only real mistake was not being as godly as Otto Kretschmer.:p
See usually you don'y sink the whole convoy. Even on noobish auto TDC settings. Many real Kaleuns walked away with only a few sunk. However if it's very early in the war you should be able to use all your torpedoes to good effect. The escorts are usualluy inept and useless.
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