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Shipping and U-boat sinkings each month

Historians disagree about the relative importance of the anti-U-boat measures. Max Hastings states that "In 1941 alone, Ultra saved between 1.5 and two million tons of Allied ships from destruction." This would be a 40 per cent to 53 per cent reduction. A history based on the German archives written for the British Admiralty after the war by a former U-boat commander and son-in-law of Dönitz reports that several detailed investigations to discover whether their operations were compromised by broken code were negative and that their defeat ".. was due firstly to outstanding developments in enemy radar..." The graphs of the data are colour coded to divide the battle into three epochs — before the breaking of the Enigma code, after it was broken, and after the introduction of centimetric radar, which could reveal submarine conning towers above the surface of the water and even detect periscopes. Obviously this subdivision of the data ignores many other defensive measures the Allies developed during the war, so interpretation must be constrained. Codebreaking by itself did not decrease the losses, which continued to rise ominously. More U-boats were sunk, but the number operational had more than tripled. After the improved radar came into action shipping losses plummeted, reaching a level significantly (p=0.99) below the early months of the war. The development of the improved radar by the Allies began in 1940, before the United States entered the war, when Henry Tizard and A. V. Hill won permission to share British secret research with the Americans, including bringing them a cavity magnetron, which generates the needed high-frequency radio waves. All sides will agree with Hastings that "... mobilization of the best civilian brains, and their integration into the war effort at the highest levels, was an outstanding British success story."