Sinking of U-864

The Sinking of "U-864" occurred during the action of 9 February 1945, when HMS "Venturer" – a submarine of the Royal Navy, which was patrolling the waters around Fedje, a Norwegian island in the North Sea – attacked and sank the German U-boat .
The sinking is the only incident where one submarine sank another in combat while both were at periscope depth.
Background.
"U-864" was a Type IX U-boat on a covert mission, Operation Caesar, to the Empire of Japan. On 6 February 1945, "U-864" passed through the Fedje area (off the coast of Norway) without being detected. During this transit, a normally quiet engine began making an abnormally loud and rhythmic noise that could be easily detected by any anti-submarine (A/S) detection gear 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 machine cypher, Enigma, had been broken by the British and was being decrypted, and the Royal Navy were concerned the secret cargo might enable the Japanese to extend the duration of the war in the Pacific. They dispatched "Venturer" to intercept and destroy "U-864".
"Venturer", under the command of Lieutenant Jimmy Launders, received a brief message from the Admiralty (probably Rear Admiral Submarines) as to the estimated whereabouts of "U-864", along with instructions to destroy her. Launders set about the task, making one risky decision: he decided to switch off "Venturer"s ASDIC (active sonar) and rely solely on hydrophones, to try to detect "U-864" without being detected.
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.
The action.
As "Venturer" continued her patrol of the waters around Fedje, her hydrophone operator noticed a strange sound which he could not identify. He thought that the noise sounded as though some local fisherman had started up a boat's diesel engine. Launders decided to track the strange noise. Then the officer of the watch on "Venturer's" periscope noticed what they thought was another periscope above the surface of the water. It is highly likely he had, in fact, spotted the U-boats snorkel. The snorkel was still a new device at that time and probably unknown to Launders and his crew.
The snorkel limited the U-boat's speed and depth. For Launders' hydrophone operator to hear diesel noises from a submerged U-boat, the snorkel would have had to be in operation. In addition, the noise of the diesel engines made the U-boat's own hydrophones much less effective and it is doubtful "U-864" would have heard "Venturer" running slowly on her electric motors.
Combined with the hydrophone reports of the strange noise, which he determined to be coming from a submerged vessel, Launders surmised they had found "U-864". He tracked the U-boat by hydrophone, hoping she would surface and allow a clear shot. "U-864" remained at snorkel depth, and as the hydrophone plot emerged, she was seen to be zigzagging. This made the German submarine quite safe according to the assumptions of the time.
It is very likely Launders tracked the U-boat's snorkel by periscope as well, which would greatly improve the accuracy of his fire control solution.
Launders tracked the U-boat for several hours, and it became obvious she was not going to surface, but he needed to attack her anyway – his batteries had only a limited life. It was theoretically possible to compute a firing solution in all four dimensions – time, distance, bearing and target depth – but this had never been attempted in practice because it was assumed that performing the complex calculations would be impossible, plus there were unknown factors that had to be approximated.
In most torpedo attacks, the target would have been visually acquired; the target's angle relative to the attacker and its bearing would be observed, then a rangefinder in the periscope used to establish the distance to the target; from this speed could be derived, and a basic mechanical computer would offset the aiming point for the torpedo. In addition, any torpedo depth had to be set based on target identification. Too deep and the torpedo would pass under the target, too shallow (in this instance) it would miss above. Launders could only estimate the depth of his target. In terms of a challenge, this was far outside what they had trained for, as they tried to manoeuvre into a firing position without giving their own position away by creating excessive noise, or exhausting their own batteries.
Nevertheless, Launders made the necessary calculations, made assumptions about "U-864"'s defensive manoeuvres, and ordered the firing of all four of his bow torpedo tubes. The torpedoes were fired with a 17.5 second delay between each pair, and at variable depths. "U-864" attempted to evade once it heard the torpedoes coming, but was not agile when diving or turning; additionally, time would have been needed to retract the snorkel, disengage the diesel, and start the electric motors. The fourth torpedo hit. "U-864" sank with the loss of all hands.
Aftermath.
"U-864" sank from the relative safety of the U-boat pens in Bergen. Launders was awarded a bar to his DSO for this action, while several members of "Venturer"s crew were decorated by the Royal Navy. Launders' career in the Navy continued well after the war. The action was the first and so far only battle ever to have been fought entirely under water.