Hello Jimboy,
Before answering your question, it is important to make a distinction between Enigma decrypts (ULTRA) and other SIGINT. Perhaps you are aware already, but the bulk of the information coming into the Submarine Tracking Room(s) was not from ULTRA, but rather from Direction Finding or any of a variety of other sources. These sources were constantly evolving and provided adequate data to allow a good picture of the situation during times when the KM Enigma settings could not be mastered, with Direction Finding and Fingerprinting being the two on which the Admiralty became most dependant. The KM ran a similar system, using many of the same techniques, to find and track their targets. The Allies would route and reroute their convoys based on the disposition of U-boats as they understood it, and the Germans tried to find and follow the convoys to position the U-boats best. It was cat-and-mouse, and both sides made use of decrypted enemy transmissions.

The Allied Submarine Tracking Room records that would give the definitive answer to which convoys were rerouted due to information gleaned from ULTRA were destroyed. Some of the people directly involved, however, participated in writing the Official Histories, both published and unpublished, and from these, it is clear that (in their opinions) the losses to U-boats would have been much worse in January 1943 had it not been for convoy rerouting based to a great degree on reading KM Enigma virtually concurrently. No specific convoys are named, but their viewpoints are reinforced by referring to post-war investigations of German records and interviews with German personnel.

Hope this helps,
Bruce

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