Through the spring of 1941, the British had had virtually no luck in solving the German navy's codes. In mid-May 1941, however, the British captured not only a German weather trawler with considerable material detailing the settings for the naval codes but also a German submarine, the U- 110, with its cipher machine and all accompanying material.16 With these two captures, the British held the settings for the next two months for the German navy's enigma machines. Thus, the British were able to break into the U-boat traffic by the end of May. Also, because German U-boats were controlled closely from shore and a massive amount of signaling went back and forth to coordinate the movement of the wolf packs, the British gained invaluable information, ranging from the number of U-boats available to tactical dispositions and patrol lines. Moreover, once they had a full two months' experience inside the German U-boat traffic, British cryptologists were able to continue breaking the submarine message traffic for the next five months.17 The impact that this intelligence had on the Battle of the Atlantic was almost immediate.18 The dramatic, decline in sinkings (compared with those that had occurred during the first five months of the year) has no explanation other than that Ultra information enabled the British to gain a decisive edge over their undersea opponent. There was no introduction of new technology, no significant increase in the number of escorts available, and no extension of air coverage. Ultra alone made the difference.
Ultra: Some Thoughts on its Impact on the Second World War