one other thing i forgot to mention, if you are sending more than two torpedoes at a large merchant ship, you might consider abandoning this practice as two is generally more than enough to get the job done.
remember to adjust your spread angle to account for the range, the smaller the spread the better... you dont want one torpedo going astern and one passing out ahead BOTH missing the target.
smaller merchants will usually only require one torpedo, medium cargos and larger - play it safe and send two shots.
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