In the Doldrums. February - August, 1942.
20. On February 1st 1942, the blow fell and Shark went on an entirely separate key using 4 wheels instead of 3. The new wheel (see Chap. I) occupied a fixed position next to the new reflector which was introduced at the same time. If the fourth wheel was set up with its clip in Z and the letter A showing in the window the effect of the new reflector plus fourth wheel was exactly the same as that of the ordinary 3 wheel reflector. This was done so that while the distribution of 4 wheel machines was incomplete everyone could use a 3 wheel machine. The wiring of the new reflector and wheel had been broken cryptographically as there were a number of cases where operators had set up the new wheel one place out and had afterwards re-sent the message with the wheel in its current position - here is a typical example. (1) Time 14.47 date 17/12/41. From W/T Station Adm, comm. U-boats. E bar 551, Service No. 166 wrongly encyphered. Contents: U.131 reports: Am able to dive. Have been hunted by 4 destroyers. (2) Time 16.50, same date, From Mueller. E bar 551 decyphers with setting B. This security blunder contains quite enough information to give away the complete wirings.
21. Nevertheless even given the wirings the position was quite serious enough. The change meant (a) that we had to break it quite independently of Dolphin (b) that breaking on 3 wheel bombes would be so extremely laborious that unless there was a steady stream of absolutely first class cribs our existing bombe resources would be quite inadequate. Even given this stream so much bombe time would be used that the Air and Army keys would suffer very seriously; an average Shark job would have taken 50 to 100 times as long as an average Air or Army job so that it would have been a moot point whether it would have been worth while even if possible. However as the U-boat cribs were at this time extremely bad this question remained (perhaps fortunately!) a purely academic one. Except for February 23rd/24th and March 14th which were broken - very laboriously - on long R.E's from Dolphin (March 14th came out on a re-encodement of Admiral Doentiz' message to the German Fleet on his succeeding Admiral Raeder - an appropriate celebration on our part) no more U-boat traffic was read until November 7th 1942.
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