This was a British naval undertaking to lay a substantial anti-submarine minefield in the South-Western Approaches to the UK and the English Channel (9/30 April 1945).
The ships involved were the coastal minelayers Plover and Free Dutch Willem van de Zaan for most of these 'two-line' minefields, and the auxiliary minelayer Nightingale and coastal craft for the rest. In the Western Approaches, at this time work on anti-submarine traps around the north coast of Cornwall was completed and a new minefield was laid across St George’s Channel. In the North-Western Approaches an additional field was laid by the cruiser minelayer Apollo and three 'O' class destroyer minelayers after the laying of planned field off the north coast of the USSR had been cancelled.
The second lay in the North-Western Approaches, started on 5 May, was completed on VE-Day and was the last British minelaying operation in European waters.
The British policy of concentrating the minelaying effort on anti-submarine protection was extended into the Irish Sea after work in the entrance to the South-Western Approaches had been covered. Deep minefields were laid by Apollo off Anglesey and the Isle of Man, and another was laid by the cruiser minelayer Ariadne off Strumbles Head.
In overall terms, greater emphasis was placed on offensive minelaying after 1942 after it had become clear that the defensive minelaying, on which greater emphasis had been placed up to this time, had been an abject failure. With the possible exception of the Dover Barrage, defensive minefields had proved to have little effect on German strategy or deployments, and conversely had resulted in Allied casualties. Equally significantly, the defensive minelaying effort had diverted warships, badly needed for Home Fleet service and convoy defence, to provide escorts for the minelayers at a time of significant shortages. The Northern Barrage in particular was described by the commander-in-chief of the Home Fleet as 'the least profitable voluntary major undertaking of the war'.
Anti-submarine minelaying in the Heligoland Bight, which the Germans recognised as an area vital to their naval activities, had been too infrequent and, when carried out, had been insufficient in quantity. Defensive minefields laid after 1940 were thus a wholly unnecessary drain on materials and manpower. In order to be most effective, Deep Trap minefields should have been placed in focal points through which convoys with adequate anti-submarine escorts transited. The St George�s Channel Field, reinforced in 1942/43, had been laid in an area not being used by convoys, and indeed presented many dangers to British naval forces.
Eight main areas were targeted for minelaying by surface warships. The East Coast Barrage defensive field of 38,045 mines claimed only the torpedo boat T 6; the North Coast Barrage defensive field of 92,083 mines claimed only one probable victim (a U-boat); the Western Approaches Deep Trap field of 30,154 mines claimed two U-boats; the Home Waters offensive fields of 8,898 mines claimed 15 vessels and craft; the Norway offensive field of 841 mines claimed four vessels and craft; the Heligoland Bight offensive field of 2,240 mines claimed four vessels and craft as well as two destroyers by accident; the Mediterranean offensive fields of 2,038 mines claimed 28 vessels and craft; and the Fast East field of just 18 mines claimed no victim.
In overall terms, offensive minelaying, particularly that carried out by the motor launches and motor torpedo boats of the coastal forces, as well as by aircraft, proved more effective. Coastal forces craft were easily converted for this duty and could lay accurately in restricted waters, and the 6,642 mines laid by these craft claimed 134 ships and craft. In the Mediterranean, despite the limited choice of suitable areas, this type of mining was well rewarded, especially that carried out by submarines. Experience showed that submarines by their very nature could observe shipping movements with comparative safety and select suitable areas based on this knowledge.
Aircraft minelaying was the most successful of all methods used, for the 55,646 mines thus laid claimed 1,666 vessels and craft.
Minelaying by fast minelayers and destroyers was generally unsuccessful, especially and in the final analysis can be seen to have been of little value, especially if used for the laying of defensive minefields or anti-submarine traps in unsuitable or unnecessary locations. Geographic factors played an important part in determining the outcome of a mining campaign, and when carried out in shallow coastal waters with a high density of traffic, enabled the achievement of a high ratio of mines laid to shipping losses. Another important aspect of the minelaying effort was that any sustained minelaying in such areas, quite apart from its disruptive effects on shipping, placed a continual drain on mine countermeasures operations with high risk of casualties being incurred.
In 1939 although the only types of British mine available for immediate use were of the moored contact type, though research and development had been carried out into the future use of influence mines. These latter could be detonated by changes in the local magnetic field resulting from the passage of a ship or submarine. Influence mines detonated by acoustic means had not been developed by the British at the outbreak of war. Surface minelaying thus used the existing stock of Mk XVH moored contact mines. In order to reduce the number needed to cover any given area a new Mk XX variant was introduced in 1941 for use in the Northern Barrage: this replaced the 'Hertz Horn' antennae in the mine casing with a floating antenna, contact with which would cause detonation. The Mk XX mine was unsatisfactory, and was replaced late in 1941 by the more reliable Mk XXII mine, which had an upper and lower antenna to increased its lethal range.
The M.Mk 1 moored magnetic mine, in production by September 1939, was first laid in February 1941. The A.Mk 1 magnetic ground mine, developed as an air-dropped weapon, became available early in 1940, and a variant suitable for laying by motor launches and motor torpedo boats was first used in September 1941.
Moored acoustic mines were developed after the outbreak of war, and were laid by motor launches from October 1942. Combinations of magnetic and acoustic influence ground mines were in general use by 1945 and greatly added to the problems faced by the German defence forces. The addition of time delay mechanisms, the intermixing of moored and ground mines in the same field, and use of obstructors to compound minesweeping difficulties all served the further to extend Germany’s mine countermeasures resources.
What cannot be disputed, however, is the fact that the Germans inevitably found a solution to these complications before the situation became irrecoverable. Apart from the actual losses due to mining, considerable disruption was caused by the threat of minefields to German seaborne traffic, and damage resulting from mine detonations imposed a great burden on German and, at an earlier time, Italian repair facilities.
British losses included the cruiser minelayer Welshman and the minelaying destroyers Esk and Ivanhoe, as well as larger numbers of coastal craft and submarines. Ships which suffered severe damage were the cruiser minelayers Abdiel and Manxman, the minelayer Adventure, and the destroyer minelayer Express. The total number of naval personnel casualties were 461 killed, 49 wounded and 108 taken prisoner.