The air Battle of Britain was crucial, so it was not surprising that the concentration of scarce resources was on "Luftwaffe" and "Abwehr" traffic. It was not until early in 1941 that the first breaks were made into German Army Enigma traffic, and it was the spring of 1942 before it was broken reliably, albeit often with some delay. It is unclear whether the German Army Enigma operators made deciphering more difficult by making fewer operating mistakes. The German Navy used Enigma in the same way as the German Army and Air Force until 1 May 1937 when they changed to a substantially different system. This used the same sort of setting sheet but, importantly, it included the ground key for a period of two, sometimes three days. The message setting was concealed in the indicator by selecting a trigram from a book (the "Kenngruppenbuch", or K-Book) and performing a bigram substitution on it. This defeated the Poles, although they suspected some sort of bigram substitution. The procedure for the naval sending operator was as follows. First they selected a trigram from the K-Book, say YLA. They then looked in the appropriate columns of the K-Book and selected another trigram, say YVT, and wrote it in the boxes at the top of the message form:  They then filled in the "dots" with any letters, giving say:  Finally they looked up the vertical pairs of letters in the Bigram Tables  and wrote down the resultant pairs, UB, LK, RS and PW which were transmitted as two four letter groups at the start and end of the enciphered message. The receiving operator performed the converse procedure to obtain the message key for setting his Enigma rotors. As well as these "Kriegsmarine" procedures being much more secure than those of the German Army and Air Force, the German Navy Enigma introduced three more rotors (VI, VII and VIII), early in 1940. The choice of three rotors from eight meant that there were a total of 336 possible permutations of rotors and their positions. Alan Turing decided to take responsibility for German naval Enigma because "no one else was doing anything about it and I could have it to myself". He established Hut 8 with Peter Twinn and two "girls". Turing used the indicators and message settings for traffic from 1–8 May 1937 that the Poles had worked out, and some very elegant deductions to diagnose the complete indicator system. After the messages were deciphered they were translated for transmission to the Admiralty in Hut 4.