But it was the Enigma which lay at the heart of the British recovery of summer 1941;
not only in the tactical routeing of convoys, but in making possible action against U-boats, especially their supply system.
Above all the British now had a clear and virtually complete picture of what was happening.
It was through Alan Turing's work that 'in July and August, by which time Winn had got into his stride', losses fell to under 100,000 tons a month.
Overall, the second half of 1941 saw German successes halved despite an increase in the number of U-boats to 80 in October.
By the end of the year it was being claimed that the shipping problem was solved.
Yet the fight was very far from over.
The British improvements were only just keeping pace with the continually growing U-boat strength, and they were at the mercy of the Enigma enciphering system.
September 1941, in particular, saw a dramatic increase in sinkings for the few weeks after a small sophistication was introduced into the U-boat signals.
They had all along been indicating positions by means of the grid references on their maps, and