The North Sea Mine Barrage, also known as the Northern Barrage, was a large minefield laid by the United States Navy (assisted by the Royal Navy) between Scotland and Norway during World War I. The objective was to inhibit the movement of the German U-boat fleet.
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The idea of mine barrage across the North Sea was first proposed in the summer of 1916 by Admiral Reginald Bacon and was agreed at the Allied Naval Conference on 5 September 1917. The Royal Navy—and in particular Admiral Beatty as Commander in Chief of the Grand Fleet—was sceptical about the value of the operation and did not feel it justified the large logistical and manufacturing commitment required.
The United States was altogether more enthusiastic about the operation as the loss of transatlantic shipping was a major domestic concern and this plan allowed the United States to play an active part in tackling this while playing to their industrial strength and with minimal risk of American casualties.
The objective was to prevent U-boats from operating in the North Atlantic and preying on trans-Atlantic shipping. A similar barrage had already been placed across the English Channel, which had resulted in U-boats diverting north around Scotland. The North Sea Mine Barrage was intended to close this alternative route, and it also made it hard for the U-boats to get supplies.
In October 1917, the U.S. Navy tendered an order for the 100,000 mines necessary to lay a minefield stretching 230 mi (200 nmi; 370 km) and dangerous to a depth of 200 ft (61 m). The mines were a version of "antenna" mine that had only been developed in July 1917. Such was the scale of the operation that 80,000,000 ft (24,000,000 m) of steel wire was required to moor the mines to the seabed. The operation was beset with technical difficulties and delays, with the final mines eventually being laid on 26 October 1918.
The design of the minefield meant there was a theoretical 66% chance of a surfaced U-boat triggering a mine and a 33% chance for a submerged U-boat. In practice the actual odds were assessed at being closer to 20% for a surfaced U-boat and 10% for a submerged one.
As the final mines were laid only a matter of days before the end of World War I, it is impossible to assess the success of the plan. It is known three U-boats were sunk on the barrage and a further three are thought to have done so. This represented a return of one U-boat kill for roughly every $13 million spent. Clearing the barrage after the war took 82 ships and 5 months, working around the clock.[1]