[Heibges]

Kretschmer fired from 400m in the middle of convoys. He also must have had extremely good luck with his combat pistols, and depth keeping.

There are quite a number of factors that can contribute to a torpedo hitting and detonating against its target, yet not doing its full damage potential. There are stories of merchant ships taking a ridiculous amount of successful torpedoes to sink.

Some of those battleships took 20 torpedoes plus aerial bombs before they sunk. It took 9 torpedoes to sink the Yamato in Silent Hunter.

I believe the average for the war is over 2 successful torpedoes per sinking. This is not even counting torpedoes that missed the target altogether, or were premature.

As Dönitz writes in his memoirs:

On April 20, U-47 (Prien) came upon a convoy south-west of Westford, steaming north. Although in favourable position to do so he refrained from attacking because he felt he had lost all confidence in his torpedoes. On the previous day when he had attacked the Warspite he had been subjected to "very severe attack with depthcharges", thanks to the failure of one of his torpedoes which had exploded at the end of its run. Upon his return to port he told me he "could hardly be expected to fight with a dummy rifle".