Bombe developed by Alan Turing, Gordon Welchman, and 'Doc' Keen found the rotor settings for a three-rotor machine. It could not find the settings for four rotors. Once again German submarine messages were indecipherable. Admiral Doenitz' U-boats began again to successfully prowl the waters of the Atlantic.

America Joins the Secret Battle

 

The complete cessation of Ultra intelligence concerning U-boats in the Atlantic coincided with the devastating attacks on shipping by U-boats off [U.S.] coasts. In fact, the outlook during the summer of1942 was rather gloomy.(10)

Despite its involvement with the British convoys, the United States took none of the precautions learned by its allies. Merchant ships came up the east coast of the United States without the benefit of convoys or escorts. Sailing alone, there were no other ships to come to their defense when attacked by the U-boats. To make matters worse, ships were in plain sight even at night. The United States did not initially require its citizens to black out their homes and businesses at night as the British did. Hence, ships coming up the coast were silhouetted against the bright lights of the cities they passed. The U-boats had no trouble seeing them and sinking them. Add to this the fact that the German cryptologic service, B-Dienst, could read the Allied Naval Cipher No. 3 used for convoy communications. By reading the convoy messages, the Germans learned of changes and movements in Allied shipping. This enabled them to easily respond and continue to follow and attack the ships. The Germans referred to the spring of 1942 as the 'Happy Time.' Between January and March they sank 216 ships off the East Coast.

The United States didn't begin sending its ships in convoys or require blackouts on the East Coast until May 1942. Unfortunately, even these measures made only a small improvement. The Allies still could not read the U-boat four-rotor machine's messages, so there were few ways of knowing where the U-boats were located.