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Too Close For Comfort: Britain, Ultra, and the Battle of the Atlantic (1941-43) - Page 2

© Joseph Sramek

With these items, Bletchey Park easily broke the German general-purpose naval code Hydra, which was used during 1941 by all U-Boats. [9] This code was being deciphered within an average of 48 hours after it was being received, almost as fast as the Luftwaffe codes. [10] As a result, the average monthly amount of tonnage sunk by U-Boats declined from 282,000 tons between March and June to about 120,000 tons from July to December, a decline of 57 percent. [11] This success, however, did not last long as B-Dienst broke the main British naval cipher (among those changed in August 1940) in September 1941. [12] B-Dienst made a further advance in February 1942 when it cracked the code that was being used by many of the Atlantic convoys. [13] At the same time, Bletchey Park was dealt its most serious setback of the entire war: the changing of the Enigma machine used by U-Boats.

For the next ten months, the British were unable to read the U-Boat Enigma. Coupled with this was the success of B-Dienst in reading the British naval and convoy codes. The results for the Allied convoys was disastrous.

Throughout 1942, the U-Boats sunk over eight million tons of Allied shipping, an increase of more than 80% over 1941 figures. [14] Each month of 1942 saw increased in convoy losses until November, the worst month of the entire war, when 729,160 tons were sunk. [15] And then suddenly, Bletchey Park, through painstaking work, cracked the U-Boat Enigma in December. As a result, convoy losses declined 72% during the next two months from 729,160 tons in November to 203,128 tons in January. [16]

Yet the battle was not over. In March, the machine used by the U-Boats was changed from three wheels to four. The extent of the British nervousness over this was demonstrated by Admiral Edelsten's (the Admiralty's Assistant Chief of Staff) note to Admiral Pound (the First Sea Lord) on March 9: "The expected has happened. The Director for Sea Intelligence announced yesterday that information on U-Boat movements is unlikely to be forthcoming for some time - perhaps even months" {emphasis mine} [17]

Partially as a result of this change, the U-Boats sank 627,377 tons in March, a 75% increase over the total for February. [18] Unlike the ten-month gap that was experienced throughout 1942, however, Bletchey Park managed to break the new code by the end of the month. [19] With Ultra back on track, the Allies decided to finally remove the U-Boat threat from the Atlantic.

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