The Bletchley Park Codebreakers by Michael Smith Page A

and reduced the number of false stops by 99 per cent – from 676 to four with an eleven-letter menu giving two ‘closures’. Depending on the composition of a menu, up to three rotor orders could be run on a single bombe. Setting up a bombe menu took thirty-five to fifty minutes, and changing a wheel order, ten minutes. A complete bombe run lasted about fifteen minutes, after the bombe was set up. Each bombe had two Wrens assigned to it: an operator and a tester, who made a preliminary check on the stop data to see whether they should be sent to a ‘testing party’ in Hut 6 for further examination. The two bombes in service in 1940 broke about 180 daily keys, all but a few (probably under ten) being
Luftwaffe
Enigma – mostly Red, which comprised about 120 keys. The daily keys (including naval Enigma keys) solved by the British bombes rose to a peak of 9,064 in 1943, and declined slightly to 8,444 in 1944, when US Navy bombes also solved a substantial number of keys.
    From about the end of 1940 onwards, the bombes were the essentialbasis of Hut 6’s solutions of
Heer
and
Luftwaffe
Enigma. Bombes at Bletchley Park were an inter-service resource, and were not ‘owned’ by any one Hut. Demands for bombe time by Hut 8 therefore had to be fitted in with the much more complex situation on Hut 6 ciphers. When Hut 8 suddenly needed a large number of machines, it could badly disturb the Hut 6 programme. Fortunately, very few issues of bombe priority had to be referred to Whitehall for guidance, since it would have been far too time-consuming a process: one example was at the end of March 1942 when the Y Board (which was responsible for overall Sigint policy) considered a proposal to allot six bombes out of about twenty then available to work on Shark (the four-rotor cipher used by the Atlantic U-boats) for about twenty-five days. In the event, only 7 per cent of the total bombe time was allocated to Shark in April – about forty-two bombe days out of 600. It was just as well, since the original proposal would have severely disrupted Hut 6’s work. Another major decision on priorities was taken in July 1942, when the PQ 17 convoy was under way and the fighting in North Africa had also become critical. It was decided that the threat to PQ 17 justified priority being given to the naval work on Dolphin. There were remarkably few disputes about sharing the bombes, partly due to the close friendship between Stuart Milner-Barry in Hut 6 and Hugh Alexander in Hut 8, and the fact that Milner-Barry fully appreciated the critical importance of the Battle of the Atlantic. A panel of five ‘bombe controllers’ – Milner-Barry, John Manisty and John Monroe from Hut 6, and Shaun Wylie and Hugh Alexander from Hut 8 – was established in mid-1942. The controller on duty settled all questions of bombe priority without leading to any friction between the two Huts.
    By May 1943, fifty-eight ‘standard’ three-rotor bombes, which did not print stop data, and fourteen ‘Jumbos’, which gave a printout, were being run by 900 Wrens and maintained by seventy-five RAF mechanics. By May 1945, 1,675 Wrens and about 265 men, virtually all from the RAF, were running and maintaining 211 bombes, including 66 four-rotor bombes for naval work. The bombe efficiency rate (the number of possible solutions, less those missed due to a bombe fault) hovered around 96 per cent throughout the war. Since each bombe contained about eleven miles of wire and 1,000,000 soldered contacts and received no preventative maintenance it was a minor miracle that they worked so well, twenty-four hours a day, seven days a week. A US Army contingent, the 6812th Signal Security Detachment, arrivedin February 1944 to operate a bank of about eight bombes in the Eastcote bombe outstation from October onwards. The 6812th achieved a high degree of efficiency, with a daily average of 71.5 runs in April 1945 (twenty minutes per run, including the set-up time, etc.). It was a