Doenitz evaluated Uboat performance based on the ratio of tons/day at sea.
He wasn't so much worried about the sinkings of a single uboat, but the performance of the Ubootwaffe as a whole.
This is like the way
www.82games.com evaluates basketball teams. Not based on a single players stats, but stats on how a particular group of 5 players performs together. Eric Dampier is the perfect example of this. His last year with the Warriors he was averaging over 30 points per game. But when he was on the floor, overall team scoring was down.
At the beginning of the war, Doenitz estimated he would need 300 submarines to starve out the British. But by the time he got 300, he needed 900.
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U.Kdt.Hdb B. I. 28) This possibility of using the hydrophone to help in detecting surface ships should, however, be restricted to those cases where the submarine is unavoidably compelled to stay below the surface.
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