U-864 was a Type IX U-boat, designed for ocean-going voyages far from home ports with limited re-supply. She was on a long-range, covert mission codenamed Operation Caesar to deliver highly sensitive technology to Germany's wartime ally, the Empire of Japan. This included parts for jet engines and missile guidance systems, and 65 tonnes of mercury. She also carried Tadao Yamoto (a Japanese acoustic torpedo expert) and Toshio Nakai (a Japanese fuel expert). Also included were two Messerschmitt engineers, Rudolf ("Rolf") von Chlingensperg and Riclef Schomerus. On 6 February 1945, U-864 passed through the Fedje area (off the coast of Norway) without being detected. During this voyage a normally quiet engine began making an abnormally loud and rhythmic noise that could be easily detected by any ASW equipment in the area. There were many Allied (primarily British) ships, submarines and aircraft in the area on anti-submarine patrol. U-864's commander, Ralf-Reimar Wolfram, decided to return to the pens at Bergen to repair the engine.
By this stage of the war the German naval code, Enigma, had been broken by the British and was being decrypted by a team at Bletchley Park using a device called the Bombe. The German naval command was unaware that the British were reading communications sent to the U-boat fleet and so aware of Operation Caesar. Wanting to avoid giving the Japanese any advantage that might allow them to extend the duration of the war in the Pacific, Royal Navy submarine command dispatched Venturer to intercept and destroy U-864.
Venturer was now on her eleventh patrol out of the British submarine base at Lerwick in the Shetland Islands. Under the command of 25-year-old Lieutenant Jimmy Launders, Venturer had sunk thirteen German vessels during ten patrols over the previous 12 months, including the destruction of the Type VIIc U-boat U-771 off the Lofoten Islands on 11 November 1944, seven nautical miles (13 km) east of Andenes.
Launders received a brief message from Royal Navy Submarine Command as to the estimated whereabouts of U-864, that she was somewhere near the island of Fedje, off Norway's southwest coast, just north of the submarine pens at Bergen, along with instructions to destroy her. Launders set about the task, making one risky but calculated decision: he decided to switch off Venturer's ASDIC (sonar). They would rely solely on Venturer's hydrophone, a common and long-used, though far less sophisticated underwater acoustic detection device than ASDIC, to try to detect U-864 along the course that the Enigma-encoded traffic suggested.
ASDIC is an early name for sonar, an active echo location system. Sonar can quickly provide enough data on range and course for a target's speed and bearing to be calculated, and a firing solution for a weapon to be arrived at, even against a vessel taking evasive action. However, the ping given off can be heard by the target as well, and at a farther range than allows detection by the sonar. Hydrophones are passive devices, very sensitive underwater directional microphones that an operator can use to tell where a noise is coming from by rotating the microphone and listening to from what direction the sound seems the loudest. Working out distance and exact speed are much more difficult, especially if a vessel takes evasive action (although rough speed can be discerned by how fast the enemy's propeller and engine are spinning, and total volume gives a very rough idea of distance). In using hydrophones, the hunting vessel has to move quite slowly so as not to mask out any external noises by its own sounds. Launders' choice was to hide and see if he could hear the U-boat in the area. His plan was to listen for the German vessel and stalk her rather than risk alerting the enemy craft, although ASDIC would improve his chances of detecting her. He didn't need to worry about the German captain using active sonar, because the Germans didn't have any.
The intelligence derived from the German messages led Launders' commanders to direct him to search for the German U-boat near Fedje but U-864 had already left the area on her mission to Japan. Wolfram's decision to return for repairs at the U-boat pens at Bergen to fix the abnormal engine noise problem brought U-864 back past Fedje and the area where Venturer was located.