From the "Secret in building 26" by Jim DeBrosse and Colin Burke

In Feb 1942 the Kriegsmarine introduced the fourth wheel to the enigma machine ( named Triton by nazis and M4 Shark by the British ) .

In the spring 1943 as the U-boat offensive opened, the Germans changed some of their codes and tightened up their procedures so that the Allies were again shut out of the submarine code systems. They remained blind for more than a week what became the worst month for the Allies in the battle of the Atlantic. More than twice as many Allied merchant ships ( ninety-five ) went to the bottom in March as February.

The destruction might have been even worse had another surge of insight among the British codebreakers not allowed the three-wheel Bombes to be useful again. U-boats were required to report the sighting of any Allied convoy to Admiral Dönitz´s headquarters, using a special short-signal code so that Allied direction-finding equipment wouldn´t have time to home in on their transmissions.However, the British had a copy of the latest codebook for those short signals, captured from U-559 in Nov 1942. The codebooks, plus an order to U-boat skippers to report weather conditions and convoy sightings in the Enigma´s simpler three-wheel mode , allowed Bletchley´s Hut 8 to solve Shark for 90 of the 112 days between March 10 and June 30 1943.

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November 1943 was when the US Bombes at last began breaking the keys to the Shark code on routine basis. By December, the average time for gaining entry into Shark was thirty-six hours- down dramatically from the embarrassing early months of 1943 when the Americans had needed an average of twenty-five days to break Shark, mostly by hand.

By the summer of 1944, hundreds of submarine messages were being read the same day, some within minutes of their transmission, giving Allied antisubmarine forces a fresh bead on the subs´whereabouts.

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