This was the British naval delivery to the USSR of the Soviet allocation of surrendered U-boats (November 1945).
The object was the transfer of 10 surrendered U-boats from Lisahally in Lough Foyle in Northern Ireland to the Soviet-controlled port of Libau in Latvia. As part of the post-war Tripartite Naval Agreement it had been agreed between the Allies that while ‘The larger part of the German submarine fleet shall be sunk, not more than thirty submarines shall be preserved and divided equally between the USSR, the UK and the USA for experimental and technical purposes.’
Late in August and early in September Soviet officers from the UK Tripartite Naval Board inspected the 136 U-boats moored in Lough Foyle and in Loch Ryan in the south-west of Scotland, and it was decided that the 10 boats to be transferred to the USSR would be U-1057, U-1058, U-1064, U-1231, U-1305, U-2353, U-2529, U-3035, U-3041 and U-3514. Before the transfer, seven of the boats were moored at Lisahally and other three (U-1057, U-1064 and U-2353) in Loch Ryan. However, the last three were moved to Lough Foyle on 31 October.
It was originally intended by the Soviets that the 10 boats should be fully serviceable and should sail to Libau under their own power. The boats were to have British crews, but there was to be one Soviet officer on each as an observer. In the event, only five of the boats were deemed capable of proceeding the whole way under their own power, and it was decided that the other five would be towed.
The undertaking started on 24 November, when all of the boats but U-3515 sailed from Moville at the mouth of Lough Foyle. The planned route was around the north of Scotland, through the Pentland Firth between Scotland and the Orkney Islands, across the North Sea to southern Norway, through the Skagerrak and Kattegat to Copenhagen, and finally past southern Sweden and across the Baltic to Libau. The five boats under power were U-1057, U-1058, U-1064, U-1305 and U-1231, which were escorted by the British warships Garth, Eglinton and Zetland. The four towed boats were U-2353 towed by Riou, U-2529 by Zephyr, U-3035 by Tremadoc Bay and U-3041 by Narborough.
On 23 November, while being readied for departure to Libau, U-3514 collided with another U-boat, damaging its steering and after hydroplanes. It was then discovered that its stern was distorted, and that the boat could not be transferred even under tow. The Soviets this agreed that U-3515 should be transferred in place of U-3514.
There were major technical and weather problems during ‘Cabal’, which was completed only on 2 February 1946.