At the outbreak of war in 1939, Winn volunteered for service as an interrogator of German prisoners. But he was soon assigned to the Admiralty's Submarine Tracking Room, although he was still a civilian. Though new to the naval environment, he quickly came to understand U-boat tactics, and could frequently predict their actions. As a result, he was promoted to command the Tracking Room, as a temporary Commander in the Royal Naval Volunteer Reserve. It is a measure of his ability that he received this rank and position without formal naval officer training, which was unprecedented at the time, and that he displaced his former superior. Winn's advancement was no surprise to his colleagues and he extended his influence within the OIC. During the German attacks on shipping off the U.S. coast, Winn was sent to America to put the British case. His arguments and expertise proved effective; he managed to persuade Admiral Ernest King, to implement a convoy system. Winn was a keen student of ULTRA intelligence. From ULTRA and his observations of U-boat movements, he deduced that German codebreakers had cracked the BAMS code used by the Admiralty for convoy operations. In 1943 he eventually convinced the Admiralty to make the necessary revisions to BAMS. After the war, captured records showed that the German Navy's Beobachtungsdienst had been reading BAMS since the start of the conflict. In 1944, the Germans equipped the U-boats with snorkels, so that they could operate without surfacing. It was still extremely difficult for a U-boat to navigate without surfacing. But U-boats operating in the dangerous waters south of Ireland managed anyway. Winn guessed that they were using their depth sounders to locate and fix on a particular conical seamount. He arranged for a double agent to send a bogus message, warning the Germans of a new British minefield "where go to fix their position." The Germans soon declared a zone 60 miles square, prohibited to U-boats and centered on that seamount. Winn's war-time work was crucial to the Allied success in the Battle of the Atlantic. Without this success, Britain might have been forced out of the war. By the war's end, Winn attained the rank of Captain. His reputation and influence extended to the United States, where his Tracking Room was the model for a similar facility. He received an OBE in 1943, and the American Legion of Merit in 1945.
