The Northern Barrage was the name given to minefields laid by the British during World War II to restrict German access to the Atlantic Ocean. The barrage stretched from the Orkney to the Faroe Islands and on toward Iceland. Mines were also laid in the Denmark Strait, north of Iceland.
Concept
The objective of a defensive minefield is to restrict movement of enemy ships into areas used by friendly shipping. The assumed presence of a minefield may have a morale effect of assumed risk in addition to actually damaging ships attempting to cross the field. In July 1939, before World War II had begun, the possibility of a Northern Barrage between the Orkney Islands and Norway (similar to the North Sea Mine Barrage of World War I) had been considered. Other alternatives were investigated after the occupation of Norway by the Germans in April 1940.
Conventional mines of the era employed a contact-fuzed explosive charge within a buoyant shell suspended over an anchor attached by a wire rope. Use of such mines against surface ships required the mine to be suspended at a depth no greater than the ship's draft. The ability of a submarine to submerge under that depth would require layers of contact-fuzed mines at differing depths to afford equal probability of destruction. The World War I mine barrage had reduced the number of mines required by using a copper wire antenna with a float to extend it above the mine. A ship's steel hull touching the copper antenna would form a battery and seawater acted as an electrolyte completing a circuit with an insulated copper plate on the mine surface to actuate a detonating relay within the mine.
American Mk 6 mines had been used for the deeper water portions of the World War I mine barrage. The Royal Navy standard Mk XVII mine inventory available for use at the outbreak of World War II lacked an antenna fuze. The Mk XVII mine could be suspended at depths as great as below the surface anchored in water as deep as . Those depth capabilities suggested the possibility of laying mines along the Iceland–Faeroes Ridge, a submerged mountain range between Iceland and the Faeroe Islands which separates the deeper portions of the Norwegian Sea to the north and the Atlantic Ocean to the south. The ridge top is less than below the surface and significantly shallower east of Iceland and on the Faeroe shelf. Channels are deep between the Faeroe shelf and Scotland and in the Denmark Strait between Greenland and Iceland. Mk XVII mines would be laid in the shallower areas while the antenna-fuzed Mk XX mine was under development. Gaps were planned to allow passage of the fleet and convoys.
Navigation
Maintaining precise records of minefield locations is an important element of defensive minefields, since these minefields are often close to waters traditionally used by friendly shipping. Most defensive minefields had historically been within visual distance of navigational aids establishing precise locations. Laying the Northern Barrage in waters so far from land was a challenging navigational problem before Global Positioning System information was available. Much of the preparatory survey work was completed by the survey vessel . Scott later operated with the minelayers placing the deep water minefields from the spring of 1942 until minelaying was discontinued in the autumn of 1943. Scott helped establish the boundaries of previously laid adjacent minefields so the minelayers could minimize un-mined gaps without accidentally straying into previously mined areas.
Laying the barrage
The first Minelaying Squadron, based at Kyle of Lochalsh (code-named port ZA for secrecy), was formed in June 1940. Five fast and modern cargo liners were requisitioned from civilian service, but the conversion to auxiliary minelayers , , , and was not completed until October. Lack of escorts and support problems also restricted operations until Town-class destroyers from the Destroyers for Bases Agreement became available. The first minelaying mission left port ZA on 18 October 1940, and 10,300 conventional contact mines were laid at the southern end of the Iceland–Faeroes Ridge before the end of the year.