Cryptographic History of Work on the German Naval Enigma

Wetterkurz schluessel.

15. (1) The tie-ups could usually be done. The Wetterkurz schluessel only permitted of fairly crude information being sent e.g. there were only 8 possible "directions of swell" compared with 32 normally used and latitude and longitude could only be sent correct to the nearest degree. This enabled the Met. section (once they had broken the trigram substitution and decyphered the messages) to pick out the synoptics which were retransmissions of U-boat weather reports. The associating of individual messages could then be done through the times of origin of the messages and by comparing the latitude and longitude given in Met. message with the bearings of the U-boats given by D/F.

16. (2) Archer could normally break the trigram substitution given sufficient volume of traffic. This meant delays during the first month of use of a set of tables but current breaking on his part during the second month.

17. (3) The Wetterkurz schluessel presented the major problem. The first edition was captured early on 1941 and remained in force until the beginning of 1942. During this period however Shark was part of Dolphin and W W's were of minor importance; they were useful sometimes in getting out paired days, since with known W.O., ringstellung and message setting the stecker could be found by hand if the content of the signal was known. On January 1st 1942 a new Wetterkurz schluessel came into force so that in February when we badly needed the W W's we were unable to use them.

18. With the aid of decodes for January 1942 (when Shark was still part of Dolphin) and of W W's sent on Dolphin from June onwards a great deal was found out about the new book. By October the tables for pressure change characteristic and swell had been reconstructed correctly and the time basis of the indicating system had been understood (the indicator was single letter which had four different equivalents per day, the T.O.O. determining which was used; equivalents varied from


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