North Sea Mine Barrage - Laying The Minefield

Laying The Minefield

The mine barrage was within a belt 230 mi (200 nmi; 370 km) long and 15 mi (13 nmi; 24 km) to 35 mi (30 nmi; 56 km) wide divided into area B off the east coast of Orkney, area C near the Norwegian coast between Utsira and Bergen, and the longest central area A connecting the two coastal areas between 0° 50′ West and 3° 10′ East. The Royal Navy laid mines in areas B and C while the United States Navy mined area A. The Royal Navy left a 10 mi (8.7 nmi; 16 km) channel open for navigation adjacent to Orkney; and no mines were laid within Norwegian territorial waters. The United States North Sea Mine Force was commanded by Rear Admiral Joseph Strauss aboard the Atlantic Fleet Mine Force flagship USS Black Hawk. Strauss was an ordnance specialist and had been chief of the Bureau of Ordnance from 1913 to 1916. 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.

• USS San Francisco (old protected cruiser converted in 1911 to carry 170 mines) (flagship)
• USS Baltimore (old protected cruiser converted in 1915 carry 180 mines)
• USS Aroostook (former Eastern steamship Bunker Hill carried 320 mines on one deck)
• USS Shawmut (former Eastern steamship Massachusetts carried 320 mines on one deck)
• USS Canandaigua (former Southern Pacific freighter El Siglo carried 830 mines on 3 decks)
• USS Roanoke (former Southern Pacific freighter El Dia carried 830 mines on 3 decks)
• USS Canonicus (former Southern Pacific freighter El Cid carried 830 mines on 3 decks)
• USS Housatonic (former Southern Pacific freighter El Rio carried 830 mines on 3 decks)
• USS Saranac (former Old Dominion steamship Hamilton carried 612 mines on 2 decks)
• USS Quinnebaug (former Old Dominion steamship Jefferson carried 612 mines on 2 decks)

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 (73 m). Since Utsira is slightly north of Orkney, alignment of minefields within the central area A was skewed east-northeasterly from Orkney. 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.

Three to five percent of the new mines dropped into the North Sea detonated as soon as the salt pellets dissolved; and hydrophones detected premature detonations continuing for a week after minelaying. These premature detonations were initially attributed to activation of the horn fuze detonation circuits by seawater leaking into the mines; and mine spacing was increased from 250 feet (76 m) on the first minelaying excursion to 300 feet (91 m) on subsequent excursions to minimize leakage caused by detonation of nearby mines. About one percent of the mines deployed during the first excursion broke free of their mooring cables and washed ashore in Norway within a month. Mines used for the last eleven excursions had springs installed at the mine mooring cable attachment points to buffer wave loading during storms. Premature detonations increased to 14 percent for the fourth minelaying excursion; because some mines had been assembled with the more sensitive antenna fuze relay settings made by the Bureau of Ordnance. The fifth minelaying excursion was halted when 19 percent of the mines detonated prematurely. San Francisco identified relay armature sensitivity as a major cause of premature detonations during a comparative field test minelaying excursion on August 12. Subsequent investigations revealed copper sulfate deposits caused by antenna corrosion created a weak battery increasing the probability of relay activation by shock wave accelerations when nearby mines detonated. Premature detonations dropped to four to six percent when sensitivity was adjusted to 30 to 45 millivolts for mines deployed by the last five minelaying excursions.