I think Doenitz is being short changed here and a little unfairly at that. It's only fair to expect that once a code system is cracked your enemy will react to coded messages, but the allies went to great lengths to avoid being seen to do this. This worked particularly well in the Atlantic where contact was often a matter of chance, and lack of contact could be written off as bad luck. Also there was great effort made in supplying the Axis with misleading reports about the morale of the navy, creating a feeling of the navy almost being broken and fooling the Axis into believing it was just a matter of time, good news which they were only too happy to wait to see turn into reality. It was a cunning move, and combined with the overall superior Int. the allies enjoyed plus the massive navies they created a winning position.