This mass scuttling was given the operational codename of Deadlight. Deadlight came out the talks between the Allies about how to deal with Germany following the war.

The British had, from 1944, been expecting that they would receive the north-western part of Germany, and administration of Germany's naval bases. To this end, they planned to bring all surrendered U-boats to the anchorages of Lisahally in Ireland, and Loch Ryan, Gare Loch and Faslane in Scotland, in what was called Operation Pledge. In the real event, the latter two ports were not used - about half the U-boats the British expected to surrender had been scuttled by their crews. On the 8th May 1945, the German government surrendered. On the same day, the British Admiralty instructed all U-boats to surrender to Allied naval forces. Those at sea or in Norway were instructed to head for a number of naval bases, with the most important being Loch Eriboll and Scapa Flow. At these ports, they were disarmed, before being taken to Loch Alsh where their crews were taken off. They were then moved to Lisahally or Loch Ryan. In total, 156 U-boats surrendered. Ten were in European ports, in unseaworthy conditions, and could not be moved. Another nine were in ports in Canada, Argentina or the US, and could not be moved across the Atlantic. This left 138 U-boats in the two British ports awaiting disposal.

These U-boats were a major source of discussions during the Potsdam Conference of July-August 1945. Churchill and Truman did not want the Russians to benefit significantly from the U-boats, either technologically, or by adding to their submarine fleet. This meant that sharing them out equally amongst the Allied powers was impossible. Any solution that favoured Britain, America or both over the Soviet Union would be vetoed by Stalin. As such, it was agreed that most of the submarines needed to be scuttled. The only survivors would be thirty boats, to be shared out evenly between Britain, America and the Soviet Union for experimental and technical uses only. A Tripartite Naval Commission was also set up, in order to inspect the scuttling, and to decide which boats should be kept. The Commission began its work on the 15th August, inspecting the boats at Lisahally, Loch Ryan and in the Americas. On the 10th October, the Commission published its list of the boats that each power was to receive. While each power was to receive ten boats, the British transferred two of their ten to the French, who had not been included in the initial distribution. On the 29th October, the decision was made as to what would happen to the remaining U-boats. These were to be sunk at a depth of no less than a hundred meters, by no later than the 15th February 1946. Of the 135 boats (one had been sent to Holland and two to the USA) at Lisahally and Loch Ryan on the 29th, one was to go to the US, eight were to go to Britain, and ten to the USSR. This left 116 boats to be scuttled.

The RN immediately began planning for Operation Deadlight, with the first plans being made immediately following the Commission's decision. On the 31st, the Commander in Chief at Rosyth was ordered to make the detailed decisions necessary for the disposal of the submarines. On the 5th, a planning meeting was held in the Fife town of Pitraevie. Nine days later, the operations order for Deadlight was produced. This called for the scuttling of 86 boats from Loch Ryan and 24 from Lisahally. The remaining six boats were under dispute by the Commission, with it not having been decided whether or not these boats were to be sent to the Allies or not - however, they would ultimately be scuttled. The operations order called for the U-boats to be towed to a position north-west of Loch Foyle. There, they would be scuttled; 36 were to be sunk by Fleet Air Arm and RAF aircraft, a number more were to be sunk by British submarines, but the main method of sinking was to be demolition charges (with gunfire as a reserve method). The first submarines left Loch Ryan on the 25th November, and had reached the scuttling location by the 27th, when they were sunk. The boats from Loch Ryan were scuttled between the 27th and the 30th. Twenty-eight of those from Lisahally were scuttled between 29th December and 9th January, while the remaining two were sunk on the 10th and 12th of February. Four more U-boats (two in the USA, and two in the American-administered port of Bremen) were scuttled by the US Navy. Another four, which were serving under the Japanese flag in the Pacific were scuttled at Singapore by the RN in February 1946 (another three, surrendered to the USN in Japan, managed to remain afloat until April 1946 thanks to legal manoeuvring by the USA).

Deadlight was a success in terms of scuttling the U-boats, but it really didn't go to plan. The British plan had assumed good weather, but the North Atlantic did not cooperate. Just under half of the U-boats foundered before they reached the scuttling location. Some of these were sunk by the weather; others were sunk by the RN as it was too dangerous to tow them thanks to the weather. The majority of the remainder were sunk by British gunfire, as the boats could not be boarded to lay demolition charges. Just two were sunk by the charges. Another seven were sunk by British subs, and thirteen more by aircraft.

Sources:

The U-Boats that Surrendered: Operation Deadlight, Derek Waller, uboat.net, 2010

Black Flag: The Surrender of Germany's U-Boat Forces on Land and at Sea, Lawrence Paterson, Seaforth, 2009