Naval mine

A naval mine is an explosive charge placed in the water that detonates when the hull of a warship or merchant ship strikes it. In the First World War both sides mined harbours, straits and shipping lanes, laying an estimated 235,000 mines. Hundreds of warships and merchant ships were lost to them.

How it worked

The most common type was the moored mine. A metal float with the charge and the firing device was held on a steel cable by an anchor lying on the seabed, so that the mine floated just below the surface. The fuze worked when a hull crushed one of the “horns” projecting from the mine. The Hague Conventions of 1907 restricted minelaying in order to protect neutral shipping, but both sides also laid mines on the open sea, far from enemy ports.

Minelayers

On the night of 4–5 August 1914 the converted German excursion steamer Königin Luise laid a minefield. The next day the cruiser Amphion ran onto it, the first Royal Navy ship lost in the war. A field laid by the auxiliary cruiser Berlin off the north of Ireland sank the dreadnought Audacious. From June 1915 small UC-type submarines from the Flanders ports laid mines regularly, and from 1916 larger submarine minelayers that carried several dozen mines each and reached from the White Sea to the west coast of Africa.

The great barrages

The British laid their first minefields off the Belgian coast in the autumn of 1914, and on 2 November declared the whole North Sea a military area, which helped the blockade of Germany. In the spring of 1915 the barrage in the Strait of Dover closed that route to large U-boats, and on 24 April 1916 Admiral Bacon’s ships laid off Zeebrugge a barrage of mines and mine nets nearly 30 kilometres long.

The largest barrage of the war was the North Sea Mine Barrage (Northern Barrage) between Orkney and Norway. It was laid mainly by the US Navy with British help: from June 1918 it came to hold 70,177 mines, of them 56,571 American. Most were new antenna mines, which exploded when a submarine touched a copper wire hanging above them. Four U-boats are believed to have been sunk on it, but when the war ended the barrage still had gaps, and its effectiveness is debated to this day.

Minesweeping

Mines were cleared by minesweepers that towed a wire between them to cut the mooring cables. Minesweepers had been in short supply since the first days of the war, so the Royal Navy mobilised hundreds of excursion steamers and fishing boats. According to Archibald Hurd, by the end of 1916 British minesweepers had destroyed 4,574 German mines, but nearly 400 vessels had been sunk or damaged doing this work.

Losses

Mines wrecked the naval attack in the Dardanelles in 1915. The weak minesweepers could not clear the strait under the fire of the batteries, and on 18 March 1915 the battleships Bouvet, Irresistible and Ocean sank on a line of 20 mines (according to some sources 26) laid in secret by the Turkish minelayer Nusret. The old ships proved helpless against mines and mobile batteries.

In the Baltic the cruiser Friedrich Carl sank on Russian mines in November 1914. On 5 June 1916 the cruiser Hampshire went down off Orkney on mines laid by U-75, and with her died Lord Kitchener.

Mines sank the hospital ships Anglia and Britannic, the largest ship lost in the war, and in July 1918, off New York, the cruiser San Diego. The last victim of the war at sea was the Norwegian steamer Ener, which struck a mine near Fair Isle on 11 November. Mines left behind threatened shipping for years afterwards.