Magnetic Mine

A naval mine is a self-contained explosive device placed in water to damage or destroy surface ships or submarines. Unlike depth charges, mines are deposited and left to wait until they are triggered by the approach of, or contact with, any vessel or a particular vessel type, akin to anti-infantry vs. anti-vehicle mines. Naval mines can be used offensively, to hamper enemy shipping movements or lock vessels into a harbour; or defensively, to protect friendly vessels and create "safe" zones. Mines allow the minelaying force commander to concentrate warships or defensive assets in mine-free areas giving the adversary three choices: undertake an expensive and time-consuming minesweeping effort, accept the casualties of challenging the minefield, or use the unmined waters where the greatest concentration of enemy firepower will be encountered.

Although international law requires signatory nations to declare mined areas, precise locations remain secret; and non-complying individuals may not disclose minelaying. While mines threaten only those who choose to traverse waters that may be mined, the possibility of activating a mine is a powerful disincentive to shipping. In the absence of effective measures to limit each mine's lifespan, the hazard to shipping can remain long after the war in which the mines were laid is over. Unless detonated by a parallel time fuze at the end of their useful life, naval mines need to be found and dismantled after the end of hostilities; an often prolonged, costly, and hazardous task.

Modern mines containing high explosives detonated by complex electronic fuze mechanisms are much more effective than early gunpowder mines requiring physical ignition. Mines may be placed by aircraft, ships, submarines, or individual swimmers and boatmen. Minesweeping is the practice of the removal of explosive naval mines, usually by a specially designed ship called a minesweeper using various measures to either capture or detonate the mines, but sometimes also with an aircraft made for that purpose. There are also mines that release a homing torpedo rather than explode themselves.

Description

Mines can be laid in many ways: by purpose-built minelayers, refitted ships, submarines, or aircraft—and even by dropping them into a harbour by hand. They can be inexpensive: some variants can cost as little as US$2,000, though more sophisticated mines can cost millions of dollars, be equipped with several kinds of sensors, and deliver a warhead by rocket or torpedo.

Their flexibility and cost-effectiveness make mines attractive to the less powerful belligerent in asymmetric warfare. The cost of producing and laying a mine is usually between 0.5% and 10% of the cost of removing it, and it can take up to 200 times as long to clear a minefield as to lay it. Parts of some World War II naval minefields still exist because they are too extensive and expensive to clear. Some 1940s-era mines may remain dangerous for many years.

Mines have been employed as offensive or defensive weapons in rivers, lakes, estuaries, seas, and oceans, but they can also be used as tools of psychological warfare. Offensive mines are placed in enemy waters, outside harbours, and across important shipping routes to sink both merchant and military vessels. Defensive minefields safeguard key stretches of coast from enemy ships and submarines, forcing them into more easily defended areas, or keeping them away from sensitive ones.

Shipowners are reluctant to send their ships through known minefields. Port authorities may attempt to clear a mined area, but those without effective minesweeping equipment may cease using the area. Transit of a mined area will be attempted only when strategic interests outweigh potential losses. The decision-makers perception of the minefield is a critical factor. Minefields designed for psychological effect are usually placed on trade routes to stop ships from reaching an enemy nation. They are often spread thinly, to create an impression of minefields existing across large areas. A single mine inserted strategically on a shipping route can stop maritime movements for days while the entire area is swept. A mine's capability to sink ships makes it a credible threat, but minefields work more on the mind than on ships.

International law, specifically the Eighth Hague Convention of 1907, requires nations to declare when they mine an area, to make it easier for civil shipping to avoid the mines. The warnings do not have to be specific; for example, during World War II, Britain declared simply that it had mined the English Channel, North Sea and French coast.

History

Early use

Precursors to naval mines were first invented by Chinese innovators of Imperial China and were described in thorough detail by the early Ming dynasty artillery officer Jiao Yu, in his 14th-century military treatise known as the ''Huolongjing''.Needham, Volume 5, Part 7, 203–205. Chinese records tell of naval explosives in the 16th century, used to fight against Japanese pirates (''wokou. General Qi Jiguang made several timed, drifting explosives, to harass Japanese pirate ships. The ''Tiangong Kaiwu'' (''The Exploitation of the Works of Nature'') treatise, written by Song Yingxing in 1637, describes naval mines with a ripcord pulled by hidden ambushers located on the nearby shore who rotated a steel wheellock flint mechanism to produce sparks and ignite the fuse of the naval mine. Although this is the rotating steel wheellock's first use in naval mines, Jiao Yu described their use for land mines in the 14th century.Needham, Volume 5, Part 7, 199.

The first plan for a sea mine in the West was by Ralph Rabbards, who presented his design to Queen Elizabeth I of England in 1574.Needham, Volume 5, Part 7, 205. The Dutch inventor Cornelius Drebbel was employed in the Office of Ordnance by King Charles I of England to make weapons, including the failed "floating petard". Weapons of this type were apparently tried by the English at the Siege of La Rochelle in 1627.

American David Bushnell developed the first American naval mine, for use against the British in the American War of Independence. It was a watertight keg filled with gunpowder that was floated toward the enemy, detonated by a sparking mechanism if it struck a ship. It was used on the Delaware River as a drift mine, destroying a small boat near its intended target, a British warship.

19th century

The 1804 Raid on Boulogne made extensive use of explosive devices designed by inventor Robert Fulton. The 'torpedo-catamaran' was a coffer-like device balanced on two wooden floats and steered by a man with a paddle. Weighted with lead so as to ride low in the water, the operator was further disguised by wearing dark clothes and a black cap. His task was to approach the French ship, hook the torpedo to the anchor cable and, having activated the device by removing a pin, remove the paddles and escape before the torpedo detonated. Also to be deployed were large numbers of casks filled with gunpowder, ballast and combustible balls. They would float in on the tide and on washing up against an enemy's hull, explode. Also included in the force were several fireships, carrying 40 barrels of gunpowder and rigged to explode by a clockwork mechanism.

In 1812, Russian engineer Pavel Shilling exploded an underwater mine using an electrical circuit. In 1842 Samuel Colt used an electric detonator to destroy a moving vessel to demonstrate an underwater mine of his own design to the United States Navy and President John Tyler. However, opposition from former president John Quincy Adams, scuttled the project as "not fair and honest warfare". In 1854, during the unsuccessful attempt of the Anglo-French fleet to seize the Kronstadt fortress, British steamships HMS ''Merlin'' (9 June 1855, the first successful mining in history), HMS ''Vulture'' and HMS ''Firefly'' suffered damage due to the underwater explosions of Russian naval mines. Russian naval specialists set more than 1,500 naval mines, or ''infernal machines'', designed by Moritz von Jacobi and by Immanuel Nobel, in the Gulf of Finland during the Crimean War of 1853–1856. The mining of ''Vulcan'' led to the world's first minesweeping operation. During the next 72 hours, 33 mines were swept.

The Jacobi mine was designed by German-born, Russian engineer Jacobi, in 1853. The mine was tied to the sea bottom by an anchor. A cable connected it to a galvanic cell which powered it from the shore, the power of its explosive charge was equal to of black powder. In the summer of 1853, the production of the mine was approved by the Committee for Mines of the Ministry of War of the Russian Empire. In 1854, 60 Jacobi mines were laid in the vicinity of the Forts Pavel and Alexander (Kronstadt), to deter the British Baltic Fleet from attacking them. It gradually phased out its direct competitor the Nobel mine on the insistence of Admiral Fyodor Litke. The Nobel mines were bought from Swedish industrialist Immanuel Nobel who had entered into collusion with the Russian head of navy Alexander Sergeyevich Menshikov. Despite their high cost (100 Russian rubles) the Nobel mines proved to be faulty, exploding while being laid, failing to explode or detaching from their wires, and drifting uncontrollably, at least 70 of them were subsequently disarmed by the British. In 1855, 301 more Jacobi mines were laid around Krostadt and Lisy Nos. British ships did not dare to approach them.

In the 19th century, mines were called torpedoes, a name probably conferred by Robert Fulton after the torpedo fish, which gives powerful electric shocks. A spar torpedo was a mine attached to a long pole and detonated when the ship carrying it rammed another one and withdrew a safe distance. The submarine used one to sink on 17 February 1864. A Harvey torpedo was a type of floating mine towed alongside a ship and was briefly in service in the Royal Navy in the 1870s. Other "torpedoes" were attached to ships or propelled themselves. One such weapon called the Whitehead torpedo after its inventor, caused the word "torpedo" to apply to self-propelled underwater missiles as well as to static devices. These mobile devices were also known as "fish torpedoes".

The American Civil War of 1861–1865 also saw the successful use of mines. The first ship sunk by a mine, , foundered in 1862 in the Yazoo River. Rear Admiral David Farragut's famous/apocryphal command during the Battle of Mobile Bay in 1864, "Damn the torpedoes, full speed ahead!" refers to a minefield laid at Mobile, Alabama.

After 1865 the United States adopted the mine as its primary weapon for coastal defense. In the decade following 1868, Major Henry Larcom Abbot carried out a lengthy set of experiments to design and test moored mines that could be exploded on contact or be detonated at will as enemy shipping passed near them. This initial development of mines in the United States took place under the purview of the U.S. Army Corps of Engineers, which trained officers and men in their use at the Engineer School of Application at Willets Point, New York (later named Fort Totten). In 1901 underwater minefields became the responsibility of the US Army's Artillery Corps, and in 1907 this was a founding responsibility of the United States Army Coast Artillery Corps.

The Imperial Russian Navy, a pioneer in mine warfare, successfully deployed mines against the Ottoman Navy during both the Crimean War and the Russo-Turkish War (1877-1878).

During the War of the Pacific (1879-1883), the Peruvian Navy, at a time when the Chilean squadron was blockading the Peruvian ports, formed a brigade of torpedo boats under the command of the frigate captain Leopoldo Sánchez Calderón and the Peruvian engineer Manuel Cuadros, who perfected the naval torpedo or mine system to be electrically activated when the cargo weight was lifted. This is how, on July 3, 1880, in front of the port of Callao, the gunned transport ''Loa'' flies when capturing a sloop mined by the Peruvians. A similar fate occurred with the gunboat schooner ''Covadonga'' in front of the port of Chancay, on September 13, 1880, which having captured and checked a beautiful boat, it exploded when hoisting it on its side.

During the Battle of Tamsui (1884), in the Keelung Campaign of the Sino-French War, Chinese forces in Taiwan under Liu Mingchuan took measures to reinforce Tamsui against the French; they planted nine torpedo mines in the river and blocked the entrance.

Early 20th century

During the Boxer Rebellion, Imperial Chinese forces deployed a command-detonated mine field at the mouth of the Peiho river before the Dagu forts, to prevent the western Allied forces from sending ships to attack.

The next major use of mines was during the Russo-Japanese War of 1904–1905. Two mines blew up when the struck them near Port Arthur, sending the holed vessel to the bottom and killing the fleet commander, Admiral Stepan Makarov, and most of his crew in the process. The toll inflicted by mines was not confined to the Russians, however. The Japanese Navy lost two battleships, four cruisers, two destroyers and a torpedo-boat to offensively laid mines during the war. Most famously, on 15 May 1904, the Russian minelayer ''Amur'' planted a 50-mine minefield off Port Arthur and succeeded in sinking the Japanese battleships and .

Following the end of the Russo-Japanese War, several nations attempted to have mines banned as weapons of war at the Hague Peace Conference (1907).

Many early mines were fragile and dangerous to handle, as they contained glass containers filled with nitroglycerin or mechanical devices that activated a blast upon tipping. Several mine-laying ships were destroyed when their cargo exploded.

Beginning around the start of the 20th century, submarine mines played a major role in the defense of U.S. harbours against enemy attacks as part of the Endicott and Taft Programs. The mines employed were controlled mines, anchored to the bottoms of the harbours, and detonated under control from large mine casemates onshore.

During World War I, mines were used extensively to defend coasts, coastal shipping, ports and naval bases around the globe. The Germans laid mines in shipping lanes to sink merchant and naval vessels serving Britain. The Allies targeted the German U-boats in the Strait of Dover and the Hebrides. In an attempt to seal up the northern exits of the North Sea, the Allies developed the North Sea Mine Barrage. During a period of five months from June 1918, almost 70,000 mines were laid spanning the North Sea's northern exits. The total number of mines laid in the North Sea, the British East Coast, Straits of Dover, and Heligoland Bight is estimated at 190,000 and the total number during the whole of WWI was 235,000 sea mines. Clearing the barrage after the war took 82 ships and five months, working around the clock. It was also during World War I, that the British hospital ship, HMHS ''Britannic'', became the largest vessel ever sunk by a naval mine. The ''Britannic'' was the sister ship of the RMS Titanic, and the RMS Olympic.

World War II

During World War II, the U-boat fleet, which dominated much of the battle of the Atlantic, was small at the beginning of the war and much of the early action by German forces involved mining convoy routes and ports around Britain. German submarines also operated in the Mediterranean Sea, in the Caribbean Sea, and along the U.S. coast.

Initially, contact mines (requiring a ship to physically strike a mine to detonate it) were employed, usually tethered at the end of a cable just below the surface of the water. Contact mines usually blew a hole in ships' hulls. By the beginning of World War II, most nations had developed mines that could be dropped from aircraft, some of which floated on the surface, making it possible to lay them in enemy harbours. The use of dredging and nets was effective against this type of mine, but this consumed valuable time and resources and required harbours to be closed.

Later, some ships survived mine blasts, limping into port with buckled plates and broken backs. This appeared to be due to a new type of mine, detecting ships by their proximity to the mine (an influence mine) and detonating at a distance, causing damage with the shock wave of the explosion. Ships that had successfully run the gantlet of the Atlantic crossing were sometimes destroyed entering freshly cleared British harbours. More shipping was being lost than could be replaced, and Churchill ordered the intact recovery of one of these new mines to be of the highest priority.

The British experienced a stroke of luck in November 1939, when a German mine was dropped from an aircraft onto the mudflats off Shoeburyness during low tide. Additionally, the land belonged to the army and a base with men and workshops was at hand. Experts were dispatched from to investigate the mine. The Royal Navy knew that mines could use magnetic sensors, Britain having developed magnetic mines in World War I, so everyone removed all metal, including their buttons, and made tools of non-magnetic brass. They disarmed the mine and rushed it to the labs at HMS Vernon, where scientists discovered that the mine had a magnetic arming mechanism. A large ferrous object passing through the Earth's magnetic field will concentrate the field through it, due to its magnetic permeability; the mine's detector was designed to trigger as a ship passed over when the Earth's magnetic field was concentrated in the ship and away from the mine. The mine detected this loss of the magnetic field which caused it to detonate. The mechanism had an adjustable sensitivity, calibrated in milligauss.

From this data, known methods were used to clear these mines. Early methods included the use of large electromagnets dragged behind ships or below low-flying aircraft (a number of older bombers like the Vickers Wellington were used for this). Both of these methods had the disadvantage of "sweeping" only a small strip. A better solution was found in the "Double-L Sweep" using electrical cables dragged behind ships that passed large pulses of current through the seawater. This created a large magnetic field and swept the entire area between the two ships. The older methods continued to be used in smaller areas. The Suez Canal continued to be swept by aircraft, for instance.

While these methods were useful for clearing mines from local ports, they were of little or no use for enemy-controlled areas. These were typically visited by warships, and the majority of the fleet then underwent a massive degaussing process, where their hulls had a slight "south" bias induced into them which offset the concentration-effect almost to zero.

Initially, major warships and large troopships had a copper ''degaussing coil'' fitted around the perimeter of the hull, energized by the ship's electrical system whenever in suspected magnetic-mined waters. Some of the first to be so fitted were the carrier HMS ''Ark Royal'' and the liners and . It was a photo of one of these liners in New York harbour, showing the degaussing coil, which revealed to German Naval Intelligence the fact that the British were using degaussing methods to combat their magnetic mines. This was felt to be impractical for smaller warships and merchant vessels, mainly because the ships lacked the generating capacity to energise such a coil. It was found that "wiping" a current-carrying cable up and down a ship's hull temporarily canceled the ships' magnetic signature sufficiently to nullify the threat. This started in late 1939, and by 1940 merchant vessels and the smaller British warships were largely immune for a few months at a time until they once again built up a field.

The cruiser is just one example of a ship that was struck by a magnetic mine during this time. On 21 November 1939, a mine broke her keel, which damaged her engine and boiler rooms, as well as injuring 46 men with one man later dying from his injuries. She was towed to Rosyth for repairs. Incidents like this resulted in many of the boats that sailed to Dunkirk

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