North Sea Mine Barrage

The North Sea Mine Barrage, also known as the Northern Barrage, was a large minefield laid easterly from the Orkney Islands to Norway by the United States Navy (assisted by the Royal Navy) during World War I. The objective was to inhibit the movement of U-boats from bases in Germany to the Atlantic shipping lanes bringing supplies to the British Isles.

The idea of a 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 skeptical 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 trans-Atlantic 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. Assistant Secretary of the Navy Franklin D. Roosevelt appealed directly to President Woodrow Wilson to overcome opposition to the project from Vice Admiral William Sims, who commanded all United States naval forces in Europe.

The U.S. Navy tendered an order for the Mk 6 mines in October 1917 with 80,000,000 ft (24,000,000 m) of steel wire rope required to moor the mines to the seabed. Project spending of $40 million was shared among 140 manufacturing contractors and over 400 sub-contractors. All mine components other than wire rope, explosives, and detonating circuitry were manufactured by Detroit automobile firms.

Eight civilian steamships were converted to minelayers; and another 24 mine carrying freighters, sailing at a rate of two or three per week, were required to transport manufactured mine components to assembly depots in Scotland.

Mine Squadron One, under the command of Captain Reginald R. Belknap, assembled at Inverness, Scotland in June 1918. Over the following five months, these ships planted 56,571 of the 70,177 mines laid to form the North Sea mine barrage.[3]

USS Shawmut, shown laying the North Sea mine barrage (she sank 23 years later during the attack on Pearl Harbor after being renamed Oglala.)

The mine barrage consisted of 18 rows of mines laid in an east-west direction. Ten rows of mines were laid at a depth of 80 ft (24 m) to be detonated by ships traveling on the surface. Submerged submarines were targeted by four rows of mines at 160 ft (49 m), and another four rows at 240 ft.
Where possible, longitude was determined from a calibrated taut-wire anchored near a landmark and unreeled from a 140 miles (230 km) spool of piano wire aboard one of the cruisers acting as the minelaying formation guide. Latitude was checked from the elevation of the sun when atmospheric conditions permitted.

The mine barrage required multiple missions, called “excursions”, laying parallel rows of mines partway across the North Sea between Norway and Orkney. Mine Squadron One made thirteen two-day minelaying excursions laying parallel rows of mines while steaming in columns 500 yards (460 m) apart with the last ship in each column dropping mines at 100 yards (91 m) intervals. As a minelayer exhausted its supply of mines, another minelayer in that column would drop back to the last position to continue the minelaying sequence.

The minelayers were preceded by Royal Navy destroyers sweeping for enemy mines and submarines.

A covering force of battleships with Royal Navy cruiser squadrons maneuvered nearby to defend the minelaying formation, but no German surface warships attempted engagement.

Buoys were dropped temporarily marking the end point of a mining excursion to avoid leaving an unmined gap when the next excursion started. These buoys were subject to potential movement by storms or enemy action.

Supply problems and technical difficulties caused some delays. Planned additional minelaying excursions to complete the barrage were cancelled when the approaching end of hostilities was recognized upon completion of the thirteenth minelaying excursion 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. On the basis of the number of effective mines observed while sweeping the barrage, 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.

The official statistics on lost German submarines compiled on 1 March 1919 credited the North Sea mine barrage with the certain destruction of four U-boats, presumed destruction of two more, and possible destruction of another two.

Eight more boats were known to have been damaged by the mines, and some Admiralty personnel assumed the field might be responsible for five more U-boats which disappeared without explanation.

The Cleanup Operation

United States participation in the minesweeping effort was overseen by Rear Admiral Strauss aboard the repair ship Blackhawk from which he had commanded the minelaying operation. Tugs Patapsco and Patuxent towed Admiralty wooden sailing smacks Red Rose and Red Fern out to conduct the first trial sweep in December.

Sweeping was accomplished by suspending a serrated wire between two ships on a parallel course. While held underwater by planing devices called “kites”, the wire would foul the cables suspending the buoyant mines above their anchors. If the serrated wire parted the mine mooring cable, the mine would bob to the surface to be destroyed by gunfire. The smacks swept and destroyed six mines before winter weather halted further work at sea.

Royal Navy minesweeping efforts involved 421 vessels manned by 600 officers and 15,000 men from 1 April to 30 November 1919.

Common difficulties with the sweeping procedure involved mine cables becoming entangled in the kites attached to the sweeping wires. Sweeping gear was often lost if the mine detonated and cut the sweeping cables.

Approximately one third of the ships were damaged by exploding mines, most were declared a total loss. Several men were killed in separate incidents.

Doubts about effectiveness of the minesweeping effort persisted into the 21st century.

As 1919 drew to a close, the onset of winter forced the suspension of sweeping for moored buoyant mines, but the Royal Navy resumed minesweeping operations the following spring, continuing to clear sunken mines from fishing grounds, and maintaining a destroyer patrol to track down mines that had broken free of their moorings and gone adrift.

Losses of civilian ships to North Sea mines continued; the origin of the mine in these cases was often difficult to determine. In 1919, twenty crewmen drowned when the Swedish steamship Hollander sank, minutes after striking a mine in October; and the steamer Kerwood struck a mine and sank on 1 December 1919.