A naval mine is a self-contained explosive device placed in water to destroy ships or submarines. Unlike depth charges, mines are deposited and left to wait until they are triggered by the approach of or contact with an enemy ship. Naval mines can be used offensively, to hamper enemy ships or lock them into a harbour; or defensively, to protect friendly ships and create "safe" zones.
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 a thousand United States dollars (USD), although more sophisticated mines can cost millions of USD, be equipped with several kinds of sensors, and deliver a warhead by rocket or torpedo.
Their flexibility and cost-effectiveness make mines attractive weapons to the less powerful belligerent in asymmetric warfare. The cost of producing and laying a mine is usually anywhere from 0.5% to 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 of these mines might remain dangerous for hundreds of years.
There are three main uses of mines: offensive, defensive, and psychological. Offensive mines are placed in enemy waters, outside harbours and in important shipping routes to sink civilian and military ships. Defensive minefields protect a coast from enemy ships and submarines and force them into areas that are easier to defend. Minefields designed for psychological effect are usually placed in trade routes and are used to stop shipping to an enemy nation. They are also spread out thinly, to create a feeling of random minefields in large areas. A single mine along a shipping route can stop shipping for days until the entire area is swept.
International law requires nations to declare when they mine an area, in order to make it easier for civil shipping to avoid the mines. The warnings do not have to be specific; during World War II, Britain declared simply that it had mined the English Channel, North Sea, and French coast.
The precursor to naval mines was first described by the early Ming Dynasty Chinese artillery officer Jiao Yu, in his 14th century military treatise known as the Huolongjing. Chinese records tell of naval explosives in the 16th century, used to fight against Japanese pirates (Wokou). This kind of naval mine was loaded in a wooden box, sealed by putty. General Qi Jiguang made several timed, drifting explosives to harass Japanese pirate ships. However, in the Tiangong Kaiwu ('The Exploitation of the Works of Nature') treatise, written by Song Yingxing in 1637 AD, it describes naval mines with a rip cord pulled by a hidden ambusher located on the nearby shore which 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 with naval mines, Jiao Yu had described their use for land mines back in the 14th century.
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. The Dutch inventor Cornelius Drebbel was employed in the Office of Ordnance by King Charles I of England to make weapons, including a "floating petard" which proved a failure. Weapons of this type were apparently tried by the English at the Siege of La Rochelle in 1627.
American David Bushnell invented the first practical 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.
In 1812 Russian engineer Pavel Shilling exploded an underwater mine using an electrical circuit. In 1854, during the unsuccessful attempt of the Anglo-French fleet to seize Kronshtadt fortress, British steamships HMS Merlin (9 June 1855, the first successful mining in history), HMS Vulture and HMS Firefly were damaged by underwater explosions of Russian naval mines. More than 1500 naval mines, or infernal machines, designed by Moritz von Jacobi and Alfred Nobel were set by Russian naval specialists in the Gulf of Finland during the Crimean War. The mining of Vulcan led to the worlds first minesweeping operation during the next 72 hours, 33 mines were swept.
The American Civil War also saw the successful use of mines. The first ship sunk by a mine was the USS Cairo in 1862 in the Yazoo River. Rear Admiral David Farragut's famous statement, "Damn the torpedoes, full speed ahead!" refers to a minefield laid at Mobile, Alabama.
In the 19th century, mines were called torpedoes, a name probably conferred by Dennis Fletcher 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. The H. L. Hunley used one to sink the USS Housatonic on February 17, 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" attached to ships or propelled themselves. One such weapon, called the Whitehead torpedo after its inventor, caused the word "torpedo" to be used for self-propelled underwater missiles rather than static devices.
The next major use of mines was in the Russo-Japanese War in 1904, sinking the Russian battleship Petropavlovsk, killing the fleet commander, Admiral Makaroff, and most of the crew. In the course of this conflict the Japanese Navy lost two battleships, four cruisers, two destroyers and a torpedo-boat to offensively laid mines. In May 1905 the Russian minelayer Amur laid a minefield off Port Arthur and sank Japan's battleship Hatsuse.
Many early mines were fragile and dangerous to handle, with glass containers filled with nitroglycerin or mechanical devices that activated them upon tipping. Several mine-laying ships were destroyed when their cargo exploded.
During World War I, mines were used extensively to defend coasts, coastal travel, ports and naval bases. 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 Northern Barrage during a period of five months from June 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.
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 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 hole ships’ hulls. By the beginning of World War II, most nations had developed mines that could be dropped from aircraft and floated on the surface, making it possible to lay them in enemy harbors. The use of dredging and nets was effective against this type of mine, but this consumed time and resources, and required harbors 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 magnetically and detonating at a distance, causing damage with the shock wave of the explosion. Ships that had successfully run the gauntlet of the Atlantic crossing were sometimes destroyed entering freshly cleared British harbors. More shipping was being lost than could be replaced, and Churchill ordered the intact recovery of one of these new mines was of the highest priority.
The British experienced a stroke of luck in November 1939. A German mine was dropped from an aircraft onto the mud flats of the Thames estuary during low tide. As if this was not sufficiently good fortune, the land belonged to the army, and a base with men and workshops was at hand. Experts were dispatched from London to investigate the mine. They had some idea that the mines used magnetic sensors, so everyone removed all metal, including their buttons, and made tools of non-magnetic brass. They disarmed the mine and rushed it to labs at Portsmouth, where scientists discovered a new type of arming mechanism. A large ferrous object passing through the Earth's magnetic field will concentrate the field through it; the mine's detector was designed to trigger at the mid-point of a steel-hulled ship passing overhead. The mechanism had an adjustable sensitivity, calibrated in milligauss. (As it turned out, the German firing mechanism was overly sensitive, making sweeping easier.) The U.S. began adding delay counters to their magnetic mines in June 1945.
From these data, methods were developed to clear the 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 induced 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. Wartime Japanese sweep methods, by contrast, never advanced much past 1930s standards, and failed entirely to keep up with new American mines, clearing no more than 15% of all the mines laid in Japan's coastal waters. Moreover, IJN's minesweeping force was derisively small, only 350 ships, numbering 20,000 men.
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 RMS Queen Mary and RMS Queen Elizabeth, which were used as troopships. This was felt to be impracticable for the myriad of smaller warships and merchant vessels, not least due to the amount of copper that would be required. It was found that "wiping" a current-carrying cable up and down a ship' 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. Many of the boats that sailed to Dunkirk were degaussed in a marathon four day effort by degaussing stations.
The Allies deployed acoustic mines, against which even wooden-hulled ships (in particular minesweepers) remained vulnerable. Japan developed sonic generators to sweep these; the gear was not ready by war's end. The primary method Japan used was small air-delivered bombs. This was profligate and ineffectual; used against acoustic mines at Penang, she needed 200 bombs to detonate just 13 mines.
The Germans had also developed a pressure-activated mine and planned to deploy it as well, but they saved it for later use when it became clear the British had defeated the magnetic system. The U.S. also deployed these, adding "counters" which would allow a variable number of ships to pass unharmed before detonating. This made them a great deal harder to sweep. Japan's antiquated sweep methods, lifting mines in nets, accidentally proved useful against these mines; it remained too slow and hazardous to be truly effective, especially in light of the high numbers being laid.
Mining campaigns could have devastating consequences. The U.S. effort against Japan, for instance, closed major ports, such as Hiroshima, for days, and by the end of the Pacific War had cut the amount of freight passing through Kobe–Yokohama by 90%.
Since World War II, mines have damaged 14 United States Navy ships, whereas air and missile attacks have damaged four. During the Korean War, mines laid by North Korean forces damaged 11 U.S. naval vessels.
During the Iran–Iraq War from 1980 to 1988, the belligerents mined several areas of the Persian Gulf and nearby waters. On April 14, 1988, the USS Samuel B. Roberts (FFG-58) struck an Iranian M-08/39 mine in the central Persian Gulf shipping lane, wounding 10 sailors.
In the summer of 1984, magnetic sea mines damaged at least 19 ships in the Red Sea. The US concluded that Libya was probably responsible for the minelaying. In response the US, Britain, and France launched Operation Intense Look, a minesweeping operation, in the Red Sea.
On the orders of the Reagan administration, the CIA mined Nicaragua's Sandino port in 1984 in support of the Contra guerrilla group. A Soviet tanker was among the ships damaged by these mines. In 1986, in the case of Nicaragua v. United States, the International Court of Justice ruled that this mining was a violation of international law.
Naval mines may be classified into two major groups.
The earliest mines were usually of this type. They are still used today, as they are extremely low cost compared to any other anti-ship weapon and are effective, both as a terror weapon and to sink enemy ships. Contact mines need to be touched by the target before they detonate, limiting the damage to the direct effects of the explosion and usually affecting only the single vessel that triggers them. (Read the section on damage below for explanation).
Early mines had mechanical mechanisms to detonate them, but these were superseded in the 1870s by the Hertz Horn (or chemical horn), which was found to work reliably even after the mine had been in the sea for several years. The mine's upper half is studded with hollow lead protuberances, each containing a glass vial filled with sulfuric acid. When a ship's hull crushes the metal horn, it cracks the vial inside it, allowing the acid to run down a tube and into a lead-acid battery which until then contains no acid electrolyte. This energizes the battery, which detonates the explosive.
Earlier forms of the detonator used a vial filled with sulfuric acid, surrounded by a mixture of potassium perchlorate and sugar. When the vial was crushed, the acid ignited the perchlorate-sugar mix, and the resulting flame ignited the gunpowder charge.
During World War I the British heavily mined the English Channel and later large areas of the North Sea to prevent German submarines from using it. As the submarine could be at any depth down to the seabed, an American invention, the antenna mine, was widely used. This had a copper wire attached to a buoy that floated above the mine. The top part of the cable connecting the mine to the weight on the seabed was also made of copper. The copper wire was insulated from the steel cable below it. If a submarine's steel hull touched the copper wire, the slight voltage produced because of the contact between two dissimilar metals was amplified and detonated the explosive.
Generally, this mine type is set to float just below the surface of the water or as deep as five meters. A steel cable connecting the mine to an anchor on the seabed prevents it from drifting away. The explosive and detonating mechanism is contained in a buoyant metal or plastic shell. The depth below the surface at which the mine floats can be set so that only deep draft vessels such as aircraft carriers, battleships or large cargo ships are at risk, saving the mine from being used on a less valuable target. In littoral waters it is important to ensure that the mine does not become visible when the sea level falls at low tide, so the preset cable length is adjusted to take account of tides. Even at the time of the Second World War there were mines which could be moored in 300-metre-deep water.
During WWII mine traps were used for blocking port entrances. Two floating mines were anchored some distance apart on either side of a shipping channel, linked by a chain. When a deep draft vessel passed through the trap it would pull the chain along with it, dragging the mines onto the sides of the ship; the resulting double explosion often sank it. This system was not used extensively, but proved effective in blocking ports.
Drifting mines were occasionally used during World War I and World War II. However, they were more feared than effective. Sometimes floating mines break from their moorings and become drifting mines; modern mines are designed to deactivate in this event. After several years at sea, the deactivation mechanism might not function as intended and the mines may remain live. Admiral Jellicoe's British fleet did not pursue and destroy the outnumbered German High Seas Fleet when it turned away at the Battle of Jutland because he thought they were leading him into a trap: he believed it possible that the Germans were either leaving floating mines in their wake, or were drawing him towards submarines, although neither of these was the case.
Churchill promoted "Operation Royal Marine" in 1940 and again in 1944 where floating mines were put into the Rhine in France to float down the river, becoming active after a time calculated to be long enough to reach German territory.
After World War I the drifting contact mine was banned, but was occasionally used during World War II. The drifting mines were much harder to remove than tethered mines after the war, and they caused about the same damage to both sides.
A bottom contact mine is the simplest form of mine. It is merely an explosive charge with some form of fuze fitted lying on the seafloor. They have been used against submarines, as submarines sometimes lie on the seafloor to reduce their acoustic signature. They are also used to prevent landing craft from reaching the shore and were a major obstacle during the D-Day landings. The Germans used antitank mines here with minor modifications to make them more reliable underwater, attaching the mines to the front of many of the obstacles seen in photos of the landing.
These mines usually weighed 2 to 50 kg, including 1 to 40 kg of explosives (TNT or hexatonal).
Frequently used in combination with coastal artillery and hydrophones, controlled mines (or command detonation) mines can be in place in peacetime, which is a huge advantage in blocking important shipping routes. The mines can usually be turned into "normal" mines with a switch (which prevents the enemy from simply capturing the controlling station and deactivating the mines), detonated on a signal or be allowed to detonate on their own. The earliest ones were developed around 1812 by Robert Fulton. The first remotely controlled mines were moored mines used in the American Civil War, detonated electrically from shore. They were considered superior to contact mines because they did not put friendly shipping at risk.
Modern examples usually weigh 200 kg (440 lb), including 80 kg (175 lb) of explosives (TNT or hexatonal).
These mines are triggered by the influence of a ship or submarine, rather than direct contact. Such mines incorporate electronic sensors designed to detect the presence of a vessel and detonate when it comes within the blast range of the warhead. The fuzes on such mines may incorporate one or more of the following sensors: magnetic, passive acoustic or water pressure displacement caused by the proximity of a vessel.
First used during the Second World War, the sophistication of influence mine fuzes has increased considerably over the years as first transistors and then microprocessors have been incorporated into designs. Simple magnetic sensors have been superseded by total-field magnetometers. Whereas early magnetic mine fuzes would respond only to changes in a single component of a target vessel's magnetic field, a total field magnetometer responds to changes in the magnitude of the total background field (thus enabling it to better detect even degaussed ships). Similarly, the original broadband hydrophones of 1940s acoustic mines (which operate on the integrated volume of all frequencies) have been replaced by narrow-band sensors which are much more sensitive and selective. Mines can now be programmed to listen for highly specific acoustic signatures (e.g. a gas turbine powerplant and/or cavitation sounds from a particular design of propellor) and ignore all others. The sophistication of modern electronic mine fuzes incorporating these Digital Signal Processing capabilities makes it much more difficult to detonate the mine with electronic countermeasures because several sensors working together (e.g. magnetic, passive acoustic and water pressure) allow it to ignore signals which are not recognised as being the unique signature of an intended target vessel.
Modern influence mines such as the BAE Stonefish are computerised, with all the programmability that this implies e.g. the ability to quickly load new acoustic signatures into fuzes, or program them to detect a single, highly distinctive target signature. In this way, a mine with a passive acoustic fuze can be programmed to ignore all friendly vessels and small enemy vessels, only detonating when a very large enemy target passes over it. Alternatively, the mine can be programmed specifically to ignore all surface vessels regardless of size and exclusively target submarines.
Even as far back as the Second World War it was possible to incorporate a "ship counter" facility into mine fuzes e.g. set the mine to ignore the first two ships to pass over it (which could be mine-sweepers deliberately trying to trigger mines) but detonate when the third ship passes overhead—which could be a high-value target such as an aircraft carrier or oil tanker. Even though modern mines are generally powered by a long life lithium battery, it is important to conserve power because they may need to remain active for months or even years. For this reason, most influence mines are designed to remain in a semi-dormant state until an unpowered (e.g. deflection of a mu-metal needle) or low-powered sensor detects the possible presence of a vessel, at which point the mine fuze powers up fully and the passive acoustic sensors will begin to operate for some minutes. It is possible to program computerised mines to delay activation for days or weeks after being laid; similarly, they can be programmed to self-destruct or render themselves safe after a preset period of time, e.g., 12 days or 12 months. Generally, the more sophisticated the mine design, the more likely it is to have some form of anti-handling device fitted in order to hinder clearance by divers or remotely piloted submersibles.
The moored mine is the backbone of modern mine systems. They are deployed where water is too deep for bottom mines. They can use several kinds of instruments to detect an enemy, usually a combination of acoustic, magnetic and pressure sensors, or more sophisticated optical shadows or electro potential sensors. These cost many times more than contact mines. Moored mines are effective against most kinds of ships. As they are cheaper than other anti-ship weapons they can be deployed in large numbers, making them useful area denial or "channelizing" weapons. Moored mines usually have lifetimes of more than 10 years, and some almost unlimited. These mines usually weigh 200 kg (440 lb), including 80 kg (175 lb) of explosives (hexatonal). In excess of 150 kg (330 lb) of explosives the mine becomes inefficient, as it becomes too large to handle and the extra explosives add little to the mine's effectiveness.
Bottom mines are used when the water is no more than 60 meters (180 ft) deep or when mining for submarines down to around 200 meters (660 ft). They are much harder to detect and sweep, and can carry a much larger warhead than a moored mine. Bottom mines commonly use pressure sensitive exploders, which are less sensitive to sweeping.
These mines usually weigh between 150 and 1,500 kilograms (330 to 3,300 pounds), including between 125 and 1,400 kg (275 to 3,090 pounds) of explosives.
Several specialized mines have been developed for other purposes than the common minefield.
The torpedo mine is a self-propelled variety, able to lie in wait for a target and then pursue it e.g. the CAPTOR mine. Other designs such as the Mk 67 Submarine Launched Mobile Mine (which is based on a Mark 37 torpedo) are capable of swimming as far as 10 miles through or into a channel, harbor, shallow water area and other zones which would normally be inaccessible to craft laying the device. After reaching the target area they sink to the sea bed and act like conventionally laid influence mines. Generally, torpedo mines incorporate computerised acoustic and magnetic fuzes.
During the Cold War a number of tests were conducted with naval mines fitted with tactical nuclear warheads. These weapons were experimental and never went into production.
This comprises two moored, floating contact mines which are tethered together by a length of steel cable or chain. Typically, each mine is situated approximately 60 feet (18 m) away from its neighbour, and each floats a few metres below the surface of the ocean. When the target ship hits the steel cable, the mines on either side are drawn down the side of the ship's hull, exploding on contact. In this manner it is almost impossible for target ships to pass safely between two individually moored mines. Daisy-chained mines are a very simple concept which was used during the Second World War.
Plastic drums filled with sand or concrete are periodically rolled off the side of ships as real mines are laid in large mine-fields. These inexpensive false targets (designed to be of a similar shape and size as genuine mines) are intended to slow down the process of mine clearance: a mine-hunter is forced to investigate each suspicious sonar contact on the sea bed, whether it is real or not.
Historically several methods were used to lay mines. During the First and Second World Wars, the Germans used U-boats to lay mines around the UK. In the Second World War, aircraft came into favour for mine laying with the one of largest such examples being the mining of the Japanese sea routes in Operation Starvation.
Laying a minefield is a relatively fast process with specialized ships, which is still today the most common method. These minelayers can carry several thousand mines and manoeuvre with high precision. The mines are dropped at a predefined interval into the water behind the ship. Each mine is recorded for later clearing, but it is not unusual for these recordings to be lost together with the ships. Therefore many countries demand that all mining operations shall be planned on land and records kept so the mines can later be recovered more easily.
Other methods to lay minefields include:
In some cases, mines are automatically activated upon contact with the water. In others, a safety lanyard is pulled (e.g. one end attached to the rail of a ship, aircraft or torpedo tube) which starts an automatic timer countdown before the arming process is complete. Typically, the automatic safety-arming process takes some minutes to complete. This is in order to give the people laying the mines sufficient time to move out of its activation/blast zone.
In the 1930s, Germany had experimented with the laying of mines by aircraft; it became a crucial element in their overall mining strategy. Aircraft had the advantage of speed, and they would never get caught in their own minefields. German mines held a large 1,000 lb. (450 kg) explosive charge. From April to June 1940, the Luftwaffe laid 1,000 mines in British waters. Soviet ports were mined, as was the Arctic convoy route to Murmansk. The Heinkel He 115 could carry two medium or one large mine while the Heinkel He 59, Dornier Do 18, Junkers Ju 88 and Heinkel He 111 could carry more.
The USSR was relatively ineffective in its use of naval mines in WWII in comparison with its record in previous wars. Small mines were developed for use in rivers and lakes, and special mines for shallow water. A very large chemical mine was designed to sink through ice with the aid of a melting compound. Special aerial mine designs finally arrived in 1943–1944, the AMD-500 and AMD-1000. Various Soviet Naval Aviation torpedo bombers were pressed into the role of aerial mining in the Baltic Sea and the Black Sea, including Ilyushin DB-3s, Il-4s and Lend Lease Douglas Boston IIIs.
In September 1939, the UK announced the placement of extensive defensive minefields in waters surrounding the Home Islands. Offensive aerial mining operations began in April 1940 when 38 mines were laid at each of these locations: the Elbe River, the port of Lübeck and the German naval base at Kiel. In the next 20 months, mines delivered by aircraft sank or damaged 164 Axis ships with the loss of 94 aircraft. By comparison, direct aerial attacks on Axis shipping had sunk or damaged 105 vessels at a cost of 373 aircraft lost. The advantage of aerial mining became clear. The United Kingdom geared up for mining; a total of 48,000 aerial mines were laid by the Royal Air Force (RAF) in the European Theatre during World War II.
The United States's early aerial mining efforts used smaller aircraft that are unable to carry many mines. Using TBF Avenger torpedo bombers, the US Navy mounted a direct aerial mining attack on enemy shipping in Palau on 30 March 1944 in concert with simultaneous conventional bombing and strafing attacks. The dropping of 78 mines stopped 32 Japanese ships from escaping Koror harbor; the combined operation sank or damaged 36 ships. Two Avengers were lost; their crews were recovered. The mines brought port usage to a halt for 20 days; further mine-laying in the area contributed to the Japanese abandoning Palau as a base.
As early as 1942, American mining experts such as Naval Ordnance Laboratory scientist Dr. Ellis A. Johnson, Commander, Naval Reserve, suggested massive aerial mining operations against Japan's "outer zone" (Korea and northern China) as well as the "inner zone", their home islands. First, aerial mines would have to be developed further and manufactured in large numbers. Second, laying the mines would require a sizable air group. The US Army Air Force had the carrying capacity but considered mining to be the Navy's job. The US Navy lacked suitable aircraft. Johnson set about convincing General Curtis LeMay of the efficacy of very heavy bombers laying aerial mines.
In the meantime, B-24 Liberator, PBY Catalina and other available bomber aircraft took part in localized mining operations in the Southwest Pacific and the China Burma India (CBI) Theaters, beginning with a very successful attack on the Yangon River in February 1943. Aerial minelaying operations involved a coalition of British, Australian and American aircrews, with the RAF and the Royal Australian Air Force (RAAF) carrying out 60% of the sorties and the USAAF and US Navy covering 40%. Both British and American mines were used. Japanese merchant shipping suffered tremendous losses, while Japanese mine sweeping forces were spread too thin attending to far-flung ports and extensive coastlines. Admiral Thomas C. Kinkaid, who directed nearly all RAAF mining operations in CBI, heartily endorsed aerial mining, writing in July 1944 that "aerial mining operations were of the order of 100 times as destructive to the enemy as an equal number of bombing missions against land targets."
Finally, in March 1945, Operation Starvation began in earnest, using 160 of LeMay's B-29 Superfortress bombers to attack Japan's inner zone. Almost half of the mines were the US-built Mark 25 model, carrying 1250 lbs of explosives and weighing about 2,000 lbs. Other mines used included the smaller 1,000 lb Mark 26. 15 B-29s were lost while 293 enemy merchant ships were sunk or damaged. 12,000 aerial mines were laid, a significant barrier to Japan's access to outside resources. Prince Fumimaro Konoe said after the war that the aerial mining by B-29s had been "equally as effective as the B-29 attacks on Japanese industry at the closing stages of the war when all food supplies and critical material were prevented from reaching the Japanese home islands." The United States Strategic Bombing Survey (Pacific War) concluded that it would have been more efficient to combine the United States's effective anti-shipping submarine effort with land- and carrier-based air power to strike harder against merchant shipping and begin a more extensive aerial mining campaign earlier in the war. Survey analysts projected that this would have starved Japan, forcing an earlier end to the war. After the war, Dr. Johnson looked at the Japan inner zone shipping results, comparing the total economic cost of submarine-delivered mines versus air-dropped mines and found that, though 1 in 12 submarine mines connected with the enemy as opposed to 1 in 21 for aircraft mines, the aerial mining operation was about ten times less expensive per enemy ton sunk.
Between 600,000 and 1,000,000 naval mines of all types were laid in World War II. Advancing military forces worked to clear mines from newly-taken areas, but extensive minefields remained in place after the war. Air-dropped mines had an additional problem for mine sweeping operations: they weren't meticulously charted. In Japan, much of the B-29 mine-laying work had been performed at high altitude, with the drifting on the wind of mines carried by parachute adding a randomizing factor to their placement. Generalized danger areas were identified, with only the quantity of mines given in detail. Mines used in Operation Starvation were supposed to be self-sterilizing, but the circuit did not always work. Clearing the mines took so many years that the task was eventually given to the Japan Maritime Self-Defense Force.
For the purpose of clearing all types of naval mines, the Royal Navy employed German crews and minesweepers from June 1945 to January 1948, organised in the German Mine Sweeping Administration, the GMSA, which consisted of 27,000 members of the former Kriegsmarine and 300 vessels. Mine clearing wasn't always successful: a number of ships were damaged or sunk by mines after the war. Two such examples were the Liberty ships Pierre Gibault which was scrapped after hitting a mine in a previously-cleared area off the Greek island of Kythira in June 1945, and Nathaniel Bacon which hit a minefield off Civitavecchia, Italy in December 1945, caught fire, was beached, and broke in two.
The damage that may be caused by a mine depends on the "shock factor value", a combination of the initial strength of the explosion and of the distance between the target and the detonation. When taken in reference to ship hull plating, the term "Hull Shock Factor" (HSF) is used, while keel damage is termed "Keel Shock Factor" (KSF). If the explosion is directly underneath the keel, then HSF is equal to KSF, but explosions that are not directly underneath the ship will have a lower value of KSF.
Usually only created by contact mines, direct damage is a hole blown in the ship. Among the crew, shrapnel wounds are the most common form of damage. This rarely sinks the ship, but might flood one or two compartments—usually in the bow.
The bubble jet effect occurs when a mine detonates in the water some distance away from the ship. The explosion creates a "hole" in the water, and due to the difference in pressure, this sphere will collapse from the bottom. This creates a "pillar" of water that can go over a hundred meters into the air. The damage to the ship is heavy. The water breaks a meter wide hole straight through the ship, flooding one or more compartments, and might break the ship apart. The crew in the areas hit by the pillar are usually killed instantly. Other damage is usually limited.
If the mine detonates at a distance from the ship, the change in water pressure causes the ship to resonate. This is frequently the most deadly type of explosion, if it is strong enough. The whole ship is dangerously shaken and everything onboard is tossed around. Engines rip from their beds, cables from their holders, etc. A badly shaken ship usually sinks quickly, with hundreds, or even thousands of small leaks all over the ship and no way to power the pumps. The crew fare no better, as the violent shaking tosses them around.
Weapons are frequently a few steps ahead of countermeasures, and mines are no exception. In this field the British, with their large seagoing navy, have had the bulk of world experience, and most anti-mine developments, such as de-gaussing and the double-L sweep were British inventions. When on operational missions, such as the recent invasion of Iraq, the US still rely on British and Canadian minesweeping services. The US have worked on some innovative mine hunting countermeasures, such as the use of military dolphins to detect and flag mines. However, they are of questionable effectiveness.
Ships can be designed to be difficult for mines to detect, to avoid detonating them. This is especially true for minesweepers and mine hunters that work in minefields, where a minimal signature outweighs the need for armour and speed. These ships have hulls of glass fibre or wood instead of steel to avoid magnetic signatures, they use special propulsion systems, such as Voith-Schneider propellers, to limit the acoustic signature. They are built with hulls that produce a minimal pressure signature. These measures create other problems. They are expensive, slow, and vulnerable to enemy fire. Therefore, they need protection. Many modern ships have a mine-warning sonar—a simple sonar looking forward and warning the crew if it detects possible mines ahead. It is only effective when the ship is moving slowly.
(See Also SQQ-32 Mine-hunting sonar)
A steel-hulled ship can be degaussed (more correctly, de-oerstedted or depermed) using a special degaussing station that contains many large coils and induces a magnetic field in the hull with alternating current to demagnetize the hull. This is a rather problematic solution, as magnetic compasses need recalibration and all metal objects must be kept in exactly the same place. Ships slowly regain their magnetic field as they travel through the Earth's magnetic field, so the process has to be repeated every six months.
A simpler variation of this technique, called wiping, was developed by Charles F. Goodeve which saved time and resources.
Between 1941 and 1943 the US Naval Gun factory (a division of the Naval Ordinance Laboratory) in Washington D.C. built physical models of all US Naval ships. Three kinds of steel were used in shipbuilding: mild steel for bulkheads, a mixture of mild steel and high tensile steel for the hull, and special treatment steel for armor plate. The models were placed within coils which could simulate the Earth's magnetic field at any location. The magnetic signatures were measured with degaussing coils. The objective was to reduce the vertical component of the combination of the Earth's field and the ship's field at the usual depth of German mines. From the measurements, coils were placed and coil currents determined to minimize the chance of detonation for any ship at any heading at any latitude.
Some ships are built with magnetic inductors, large coils placed along the ship to counter the ship's magnetic field. Using magnetic probes in strategic parts of the ship, the strength of the current in the coils can be adjusted to minimize the total magnetic field. This is a heavy and clumsy solution, suited only to small-to-medium sized ships. Boats typically lack the generators and space for the solution, while the amount of power needed to overcome the magnetic field of a large ship is impractical.
Active countermeasures are ways to clear a path through a minefield or remove it completely. This is one of the most important tasks of any mine warfare flotilla.
A sweep is either a contact sweep, a wire dragged through the water by one or two ships to cut the mooring wire of floating mines, or a distance sweep that mimics a ship to detonate the mines. The sweeps are dragged by minesweepers, either purpose-built military ships or converted trawlers. Each run covers between one and two hundred meters, and the ships must move slowly in a straight line, making them vulnerable to enemy fire. This was exploited by the Turkish army in the Battle of Gallipoli in 1915, when mobile howitzer batteries prevented the British and French from clearing a way through minefields.
If a contact sweep hits a mine, the wire of the sweep rubs against the mooring wire until it is cut. Sometimes "cutters", explosive devices to cut the mine's wire, are used to lessen the strain on the sweeping wire. Mines cut free are recorded and collected for research or shot with a deck gun.
Minesweepers protect themselves with an oropesa or paravane instead of a second minesweeper. These are torpedo-shaped towed bodies, similar in shape to a Harvey Torpedo, that are streamed from the sweeping vessel thus keeping the sweep at a determined depth and position. Some large warships were routinely equipped with paravane sweeps near the bows in case they inadvertently sailed into minefields—the mine would be deflected towards the paravane by the wire instead of towards the ship by its wake. More recently, heavy-lift helicopters have dragged minesweeping sleds, as in the 1991 Persian Gulf War.
The distance sweep mimics the sound and magnetism of a ship and is pulled behind the sweeper. It has floating coils and large underwater drums. It is the only sweep effective against bottom mines.
Modern influence mines are designed to discriminate against false inputs and are therefore much harder to sweep. They often contain inherent anti-sweeping mechanisms. For example, they may be programmed to respond to the unique noise of a particular ship-type, its associated magnetic signature and the typical pressure displacement of such a vessel. As a result, a mine-sweeper must accurately guess and mimic the required target signature in order to trigger detonation. The task is complicated by the fact that an influence mine may have one or more of a hundred different potential target signatures programmed into it.
Another anti-sweeping mechanism is a ship-counter in the mine fuze. When enabled, this allows detonation only after the mine fuze has been triggered a pre-set number of times. To further complicate matters, influence mines may be programmed to arm themselves (or disarm automatically—known as self-sterilization) after a pre-set time. During the pre-set arming delay (which could last days or even weeks) the mine would remain dormant and ignore any target stimulus, whether genuine or faked.
When influence mines are laid in an ocean minefield, they may have various combinations of fuze settings configured. For example, some mines (with the acoustic sensor enabled) may become active within three hours of being laid, others (with the acoustic and magnetic sensors enabled) may become active after two weeks but have the ship-counter mechanism set to ignore the first two trigger events, and still others in the same minefield (with the magnetic and pressure sensors enabled) may not become armed until three weeks have passed. Groups of mines within this mine-field may have different target signatures which may or may not overlap. The fuzes on influence mines allow many different permutations, which complicates the clearance process.
Mines with ship-counters, arming delays and highly specific target signatures in mine fuzes can falsely convince a belligerent that a particular area is clear of mines or has been swept effectively because a succession of vessels have already passed through safely.
Mine hunting is very different from sweeping, although some minehunters can do both tasks. Mines are hunted using sonar, then inspected and destroyed either by divers or ROVs (remote controlled unmanned mini submarines). It is slow, but also the most reliable way to remove mines. Mine hunting started during the Second World War, but it was only after the war that it became truly effective.
Sea mammals (mainly the Bottlenose Dolphin) have been trained to hunt and mark mines, most famously by the U.S. Navy Marine Mammal Program. Mine-clearance dolphins were deployed in the Persian Gulf during the Iraq War in 2003. The Navy claims that these dolphins were effective in helping to clear more than 100 antiship mines and underwater booby traps from Umm Qasr Port.
French naval officer Jacques Yves Cousteau's Undersea Research Group was once involved in mine-hunting operations: They removed or detonated a variety of German mines, but one particularly defusion-resistant batch—equipped with acutely sensitive pressure, magnetic, and acoustic sensors and wired together so that one explosion would trigger the rest—was simply left undisturbed for years until corrosion would (hopefully) disable the mines.
A more drastic method is simply to load a cargo ship with cargo that makes her less vulnerable to sinking (wood for example) and drive her through the minefield, letting the ship to be protected follow the same path. This method was employed by the German Kriegsmarine during WWII, using converted ships known as Sperrbrecher ("barrage breaker"). Alternatively, a shallow draught vessel can be steamed through the minefield at high speed to generate a pressure wave sufficient to trigger mines, with the minesweeper moving fast enough to be sufficiently clear of the pressure wave so that triggered mines do not destroy the ship itself. These techniques are the only way to sweep pressure mines. The technique can be simply countered by use of a ship-counter, set to allow a certain number of passes before the mine is actually triggered. Modern doctrine calls for ground mines to be hunted rather than swept.
An updated form of mine breaking is the use of small unmanned ROVs that simulate the acoustic and magnetic signatures of larger ships and are built to survive exploding mines. Repeated sweeps would be required in case one or more of the mines had its "ship counter" facility enabled i.e. were programmed to ignore the first 2, 3, or even 6 target activations.
The United States Navy MK56 ASW mine (the oldest still in use by the US) was developed in 1966. More advanced mines include the MK60 CAPTOR (short for "encapsulated torpedo"), the MK62 and MK63 Quickstrike and the MK67 SLMM (Submarine Launched Mobile Mine). Today, most U.S. naval mines are delivered by aircraft.
MK67 SLMM Submarine Launched Mobile Mine
The SLMM was developed by the United States as a submarine deployed mine for use in areas inaccessible for other mine deployment techniques or for covert mining of hostile environments. The SLMM is a shallow-water mine and is basically a modified Mark 37 torpedo.
The Quickstrike is a family of shallow-water aircraft-laid mines used by the United States, primarily against surface craft. The MK65 is a 2,000-lb (900 kg) dedicated, purpose-built mine. However, other Quickstrike versions (MK62, MK63, and MK64) are converted general-purpose bombs. These latter three mines are actually a single type of electronic fuze fitted to Mk82, Mk83 and Mk84 air-dropped bombs. Because this latter type of Quickstrike fuze only takes up a small amount of storage space compared to a dedicated sea mine, the air-dropped bomb casings have dual purpose i.e. can be fitted with conventional contact fuzes and dropped on land targets, or have a Quickstrike fuze fitted which converts them into sea mines.
According to a statement made to the UK Parliament in 2002:
However, a British company (BAE Systems) does manufacture the Stonefish influence mine for export to friendly countries such as Australia, which has both war stock and training versions of Stonefish, in addition to stocks of smaller Italian MN103 Manta mines. The computerised fuze on a Stonefish mine contains acoustic, magnetic and water pressure displacement target detection sensors. Stonefish can be deployed by fixed-wing aircraft, helicopters, surface vessels and submarines. An optional kit is available to allow Stonefish to be air-dropped, comprising an aerodynamic tail-fin section and parachute pack to retard the weapon's descent. The operating depth of Stonefish ranges between 30 and 200 metres. The mine weighs 990 kilograms and contains a 600 kilogram aluminised PBX explosive warhead. The shelf life of a Stonefish mine is 20 years, and it has an operational lifetime of 700 days after being deployed on the seabed. Stonefish incorporates arming delays, ship counting and self-sterilisation features which can be configured by the user.
, are part of the detonation mechanism.]] A naval mine is a bomb placed in water to destroy ships or submarines. Unlike depth charges, mines are put in the water and left to wait until they make contact with a ship. Naval mines can be used offensively, to make it harder for enemy ships to sail easily, or trap them into a harbour. They can also be used defensively, to protect friendly ships and create "safe" zones.
Mines can be laid in many ways: by specially built minelayers, refitted ships, submarines, aircraft, or even by dropping them into a harbour by hand. They are a cheap weapon with a basic mine costing as little as US$1000. There are very advanced mines which can cost millions and have several kinds of sensors, and deliver a warhead by rocket or torpedo.
International law says that nations have to say when they put mines in an area. This is to make it easier for civil shipping to avoid the mines. The warnings do not have to be specific; during World War II, Britain said simply that it had mined the English Channel, North Sea, and French coast.