order with a triplet in the 31st day of the month, Porpoise normally had 4 triplets or 3 triplets and 1 quadruplet according to the length of the month.
We left Shark in February 1942 with a brief mention of the introduction of the 4 wheeled machines and of the fact that the existing bombe could not tackle 4 wheeled problems. The effect of a 4th wheel was to multiply by 26 the number of possible positions of the machine for each wheel order and also to make Banburismus out of the question. To have obtained sufficient depth on this much improved machine an approximate equivalent increase of traffic would have been necessary.
The introduction of the 4th wheel did not catch us by surprise. The earliest mention of M4, as this machine was called, is in a captured document dated January 1941 where instructions are given for adapting it to 3 wheel keys. From this it is clear that the machine must have been designed in 1940 or even before the war, but the interest of the document is that it betrayed the characteristic feature of the 4 wheel machine: it was to be an adaptation of the 3 wheel machine and the 4th wheel was to be a Zusatzwalze or reflector wheel, a part of the reflector to be considered apart from the 3 wheels making up the wheel order of the day. The new wheel was called Bruno and its reflector wheel Beta: the two were wired in such a way that if Beta were set at position A (the Ringstellung at this period was always at A) they equalled in effect the 3 wheel reflector B. Thus as long as Beta was kept at A, M4 could be used as a 3 wheel machine and the change from 3 to 4 wheels need cause no complications.
In the course of 1942 there were a number of '4 wheeled duds' - these were messages for which the reflector wheel had been mistakenly set elsewhere than at A and these duds enabled us to break the wiring of the new reflector and its wheel. At first the 4 wheel duds were from U Boats only, but later they appeared on surface craft. It was clear that it was the policy of the German Navy to distribute the new machine to all units, starting with those most often involved in major operations. This last point presumably accounts for the fact that the Mediterranean was equipped with M4 and started to use it as such as soon as February 1944 although a certain number of M4’s were known to have been distributed in the Home Waters area as early as 1941. Dolphin and Plaice did not go onto 4 wheels until November 1944.
Forewarned in this case was not forearmed. When Shark changed
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