[Puster Bill]

Originally Posted by Khayman

I think that's underestimating the German code-breaking efforts. A lot is said of the breaking of Enigma, but the Germans had their successes as well. Frequently the patrol line was placed on a known route rather than one that was guessed at. You could say it would then be very unlucky if no U-Boat made contact.

This is true. During early 1942, when the British were reading the 3 rotor Naval Enigma fairly well, they were able to route their convoys such that only 1 in 10 convoys were discovered by u-boats. When the 4 rotor Enigma came in, the Germans found 1 in 3 convoys. That is because they had been reading the British codes. In fact, Wilhelm Tranow's Beobachtungsdienst ("Observation Service", usually shortened to B-Dienst) had been reading British Naval and Merchant Marine codes since the beginning of the War. They were able to read the two main English systems up until about mid 1943 - early 1944. The Allies changed their codes due to definite proof from intercepts of messages to the U-Boats that the Germans had cracked their codes and were routing them to intercept convoys and individual ships.

The British actually knew, or at least suspected, that this was the case for quite some time (in fact, some of the highest level users of the British 'Naval Cypher', which was really a codebook, got the only unbreakable system to superencipher it, the one time pad), but it takes time to compile and distribute a system. You need everyone to start using it at the same time, otherwise you will compromise the new system by having similar or identical messages in both the old and new systems.

The final word must go to Karl Doenitz on the effectiveness of B-Dienst: He said they supplied fully half of the intelligence used by the Kriegsmarine.