Northern Barrage

The Northern Barrage was the name given to an extensive series of defensive minefields laid by the British during World War II in order 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.

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, however 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. Ability of a submarine to submerge under that depth would require several 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 300 feet (91 m) below the surface anchored in water as deep as 6,000 feet (1,800 m). 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 ridgetop is less than 500 metres (1,600 ft) below the surface and significantly shallower east of Iceland and on the Faeroe shelf. Channels are 850 metres (2,790 ft) 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.

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