Mine Warfare In
'Second World War at Sea'
Part I: A Brief History of Mines
By Steven Ford High and Kristin Ann High
February 2007

Beginning with Bomb Alley, 119694_avalanche Press included rules for laying, sweeping, and running afoul of sea mines in their Second World War At Sea series. Mines, like bombs and torpedoes, are dealt with abstractly — operationally a mine is a mine. This article and those that follow look at mines a little more closely.

Origin and Development

The employment of mines began as early as the American Civil War, when the mine and the torpedo were rather dissimilar to their modern namesakes. The mine was a huge explosive, physically attached to a ship’s hull, and set off by a fuse or similar device, exactly as was a mine upon land. Both the early anti-ship mine and the land-mine drew their name from the use of similar explosives in working ore deposits in rock—commercial or speculative mining.

The torpedo was a more modest explosive, detonated by a trigger that activated upon impact with a ship. That impact could be caused by the ship striking a floating or anchored torpedo, or by the torpedo being run against the target ship by another ship. The latter form of torpedo was generally known as a spar torpedo. This was the weapon of the progenitor of all modern submarines, the CSS Hunley, and was also a favourite weapon of the CS Navy’s ironclad gunboats.

A mine from a previous war: A Confederate contact torpedo.

Torpedoes that were intended to detonate when hit by a ship were generally “floated” in the likely path of a ship’s approach. The “floated” torpedo could also be anchored, the torpedo being attached by a tether to a weight lying on the seabed. This served to both hold the torpedo in place—thus sowing a field of torpedoes—and to give them a certain amount of concealment. At high tide, when enemy ships were most likely to run in to a defended harbour, the torpedoes would be held below the water level by their tethers, making them difficult to spot, and hopefully keeping them at just the right depth to let gunboats and other light ships pass, but still strike the enemy’s big ships below the waterline.

This torpedo (that of U.S. Navy RADM David G. Farragut’s famous exclamation) is what became known in modern times as the sea mine. Its cousin, the heavy explosive physically attached to a ship’s hull, became known by a variety of names — the most common perhaps being the British term, “limpet mine,” though there were numerous others. The development of the self-propelled torpedo defined the point of departure for the modern nomenclature, as torpedo came to mean exclusively a self-propelled, submerged vehicle for delivering a warhead against a ship’s hull. By comparison, a sea mine thus came to mean a static or drifting warhead, intended to explode against a ship by contact or proximity.

Defensive Employment of Mines

The defensive employment of mines is what might be called their traditional use — guarding the access to naval yards, naval bases, ship channels, and coastlines. In this case, mines are carefully sown in a prescribed pattern, with special channels through the minefield for use by friendly forces (who have charts and may be escorted through the channel by minesweepers, patrol boats, or pilot boats familiar with the harbour).

One of the principal operational differences between the defensive employment of mines and the offensive employment of mines, is that secrecy is not as vital to the success of the defensive minefield. The fact that the enemy knows mines have been sown in a particular area must give them pause when contemplating action in that area. Sweeping operations must be undertaken to clear the mines, and that may offer opportunities for a surface action or attack by submarines. Deterrence is as much a positive benefit to defense as is the actual sinking of enemy ships.

Offensive Employment of Mines

Neither as organized nor as effective as defensive minefields, offensive minefields rely on secrecy and surprise to achieve kills, cause confusion, and delay enemy operations in the mined areas. Sowing a minefield in secret requires darkness, and either stealth or speed. Only the submarine provides sufficient stealth, while either aircraft, or fast surface ships, provide the speed.

Limpet mines placed by Royal Navy divers sank the Norwegian transport ship Barenfels in 1944.

Offensive minefields are sown in places where enemy ships must or are very likely to transit in normal operations. Big naval bases that use river estuaries, like Liverpool, England, or the French naval base at Bordeaux, are of this sort of offensive mining target, because the enemy’s fleet must enter and exit using known channels.

Unfortunately for the offensive minelayer, this vulnerability is also obvious to the enemy, which makes it likely that the enemy will aggressively protect against just such an operation. In such cases, the employment of aircraft is the only reliable tactic, though more than once, daring submarine skippers, or small boat commandoes, attacked just such targets.

Another profitable tactic for offensive mining relies on the enemy falling into a pattern, something all navies do far too often. When a prolonged naval battle is being fought in contested waters — such as the British blockades of Germany in both wars, or the battles in the Solomon Islands and New Guinea in WWII — the adversaries tend to fall into routines while in supposedly safe waters. The ships enter and exit their bases along the same route, or always take a certain channel when threading through a chain of islands. A swift run into the “safe” area with a load of mines will not realize the same prolonged confusion and disorder that the sustained and careful mining of a major port does, but it is more likely to sink an active enemy warship, and it will certainly shake up the enemy fleet and impact operations for several days.

Submarines vs. Mines

The advent of effective submarines with the outbreak of the Great War brought a new threat to installations traditionally protected by minefields. The mines of the era were almost exclusively contact mines, most often being anchored to the bottom by a heavy weight attached by a tether of steel cable or chain. These mines were quite effective against surface ships, but there was considerable worry that a submarine could proceed submerged through a minefield sown in a deep channel and avoid contacting the mines, thus coming unscathed into the very heart of the fleet’s anchorage and wreaking havoc.

But manoeuvering a submarine through a minefield while submerged turned out to be rather more difficult than imagined. Blockships, tidal variations, and strong currents made navigation difficult, with the submarine barely able to maintain headway while submerged. Tethers could and did foul the submarine’s diving planes, rudder, propeller, and other fittings, and if undetected by the submarines crew, might well be drawn down by the submarine’s own forward motion until it struck the hull or conning tower, almost certainly exploding and destroying the submarine and its crew.

What quickly emerged as the submarine’s true advantage with respect to defensive minefields was its ability to lay near the targeted channel for some time undetected. This allowed the skipper to observe the sailing of the enemy’s ships in and out of the channel, and thereby plot a safe course into the target area. German U-boats penetrated British Fleet bases in the Firth of Clyde and at Scapa Flow by these tactics during the early part of the Great War.

In response to these incursions, fleet bases developed improvements to minefield defenses. The primary improvement was to employ shore control stations linked to defensive minefields by electrical cables that could remotely detonate a mine or series of mines in the channel simply by pressing a firing button. The shore station operators were equipped with hydrophones to detect the passage of submarines and guide them in detonating mines to kill it — sometimes mines laying on the bottom, right in the “safe” channel. Important naval bases and ship channels usually had a whole series of hydrophone listening posts, each post having direct control over a section of the defensive minefield protecting the base or ship channel.

Italian P200 (P5) mine and anchor, 46 inches across.

Once these countermeasures were recognized, an aggressive submarine skipper could defeat them by waiting for a ship that made a routine, scheduled passage through the channel — such as a minesweeper or patrol boat — and follow it in as closely as possible. This tactic, called shadowing, masked the submarine’s noise with that of the surface ship’s, and relied on the boredom of routine to cover any anomaly a shore operator might detect . A particularly skilled hydrophone operator could differentiate between the passage of a single ship, and the additional sounds that indicated a submarine was shadowing it through the channel, but few hydrophone operators achieved this level of expertise.

A more effective counter to shadowing was devised in the form of electrical cables layed across the channel, tied back to the hydrophone stations and wired into a control panel. This panel would light up as a ship progressed along the channel, and unless the submarine’s crew was superb, there would be a lag between the ship and the submarine, indicating an “unknown vessel” was in the channel. A visual and radio check would assure the operators that there were no other friendly ships in the channel, and then the operators would set off mines close to the submarine and sink it.

Mines were among the greatest killers of submarines (on both sides) in the Great War. The British established a dense minefield sown across the Straights of Dover, between Dover, England and Cape Gris Nez, France, almost from the outset of the war. The Dover minefield was intended to block U-boat access to the Atlantic through the English Channel. Because British mines were defective, the Dover Barrage was initially more of a hindrance than a danger. Not until the British copied the German contact mine did British minefields start to pose a positive threat to German U-boats or surface ships. Once the older, defective mines had been cleared away and replaced by the new “German copies”, the Dover minefield killed a number of U-boats, and U-boats soon ceased to be routed through it. The minefield forced the U-boats to go north about Scotland, adding nearly seven days to their sailing time, and thereby reducing their patrol time. Thus, though its presence was known, the Dover-Cape Gris Nez minefield provided a positive benefit to the U-boat war.

Later in the war, the British and Americans deployed the secret American magnetic mine in the Northern Barrage — an ambitious attempt to sow a dense minefield from the Orkneys to Norway, “sealing” the U-boats off from the British west coast completely. The project proved too ambitious, and worse, the new American magnetic mines turned out to be as defective as the early British contact mines. Crippled by both inadequate coverage and defective technology, the Northern Barrage never amounted to more than a hindrance to U-boat operations.

By the outbreak of the Second World War, both mines and submarines had become more sophisticated, but striking a mine still meant death for a submarine and her crew. The British and French promptly reestablished the Dover-Cape Gris Nez minefield to bar U-boats from transiting the English Channel en route to the Atlantic. In the early months of the war, the Germans made several attempts to “break out” through this minefield, as their Great War counterparts had done early in that war. The German commander of the submarine force, Admiral Dönitz, believed — mistakenly — the British had left a gap in the new Dover-Cape Gris Nez minefield, and all of U-boats sent through it were lost to mines. This once again reduced the U-boats’ patrol times by forcing them to go north about Scotland.

Tune in next time for more detail on the ships involved in mine warfare!

See mines at work in Second World War at Sea: Bomb Alley.