as well as Hut 8 work. A total of 121 Navy bombes were produced. In Alexander's "Cryptographic History of Work on German Naval Enigma", he wrote as follows. The US Army also produced a version of a bombe. It was physically very different from the British and US Navy bombes. A contract was signed with Bell Labs on 30 September 1942. The machine was designed to analyse 3-rotor, not 4-rotor traffic. It did not use drums to represent the Enigma rotors, using instead telephone-type relays. It could, however, handle one problem that the bombes with drums could not. The set of ten bombes consisted of a total of 144 Enigma-equivalents, each mounted on a rack approximately long high and wide. There were 12 control stations which could allocate any of the Enigma-equivalents into the desired configuration by means of plugboards. Rotor order changes did not require the mechanical process of changing drums, but was achieved in about half a minute by means of push buttons. A 3-rotor run took about 10 minutes. German suspicions The German navy was concerned that Enigma could be compromised. Key schedules were printed in water-soluble inks so they could not be salvaged. The navy policed what its operators did and disciplined them when errors that could compromise the cipher were made. The navy minimized its exposure. For example, Enigma machines were not carried by ships that might be captured or run aground. When ships were lost in circumstances where they might be salvaged, the Germans investigated. After investigating some losses in 1940, Germany changed some message indicators. In April 1940, the British sank eight German destroyers in Norway. The Germans concluded that it was unlikely that the British were reading Enigma. In May 1941, the British deciphered some messages that gave the location of some supply ships for the battleship Bismarck and the cruiser Prinz Eugen. As part of the Operation Rheinübung commerce raid, the Germans had assigned five tankers, two supply ships, and two scouts to support the warships. After the Bismarck was sunk, the British directed its forces to sink the supporting ships Belchen, Esso Hamburg, Egerland, and some others. The Admiralty specifically did not target the tanker Gedania and the scout Gonzenheim, figuring that sinking so many ships within one week would indicate to Germany that Britain was reading Enigma. However, by chance, British forces found those two ships and sank them.