turned the tide against the U-boats. In the end, the U-boat fleet suffered extremely heavy casualties, losing 793 U-boats and about 28,000 submariners (a 75% casualty rate, the highest of all German forces during the war). At the same time, the Allies targeted the U-boat shipyards and their bases with strategic bombing. Enigma machine The British had a major advantage in their ability to read some German naval Enigma codes. An understanding of the German coding methods had been brought to Britain via France from Polish codebreakers. Thereafter, code books and equipment were captured by raids on German weather ships and from captured U-boats. A team including Alan Turing used special-purpose "Bombes" and early computers to break new German codes as they were introduced. The speedy decoding of messages was vital in directing convoys away from wolf packs and allowing interception and destruction of U-boats. This was demonstrated when the naval Enigma machines were altered in February 1942 and wolf-pack effectiveness greatly increased until the new code was broken. The , a Type IXB, was captured in 1941 by the Royal Navy, and its Enigma machine and documents were removed. was also captured by the British in October 1942; three sailors boarded her as she was sinking, and desperately threw all the code books out of the submarine so as to salvage them. Two of them, Able Seaman Colin Grazier and Lieutenant Francis Anthony Blair Fasson, continued to throw code books out of the ship as it went under water, and went down with it. Further code books were captured by raids on weather ships. was boarded by crew from the Canadian ship on 6 March 1944, and codes were taken from her, but by this time in the war, most of the information was known. The , a Type IXC, was captured by the United States Navy in June 1944. It is now a museum ship in Chicago at the Museum of Science and Industry. U-570 a Type VIIC was captured by the British in mid-1941. Although the crew managed to destroy their Enigma machine, the British captured a quantity of papers that proved extremely useful to the cryptanalysts at Bletchley Park. The Royal Navy were able to tow the sinking submarine to Iceland, where it was salvaged and later commissioned into the Royal Navy as HMS Graph, carrying out three war patrols before being relegated to training duties.