Naval Mine Warfare, WWII


EMB Mine being laid from an S-Boote. Photograph from Suddentscher Verlag.


EMC Contact Mines aboard a Leberecht Maas class destroyer in Autumn 1940. Note the trolley rails.

Hidden, unseen, and inflicting their damage below the waterline, mines were one of the most feared naval weapons of World War II. Worldwide, mines sank 534 ships, accounting for some 1.4 million tons. Only torpedoes sank more ships than mines. The same had held true in World War I, and yet most of the great naval powers entered World War II poorly prepared to deal with mines. Only Germany and the Soviet Union included strong mine countermeasures forces in their fleets, no doubt because of their mutually bad experiences with each others’ mines in World War I. They also had the largest mine inventories at war’s start.

More significantly, Germany exploited British magnetic mine technology from World War I to produce a magnetic mine of its own. It was a technical surprise that cost the British dearly in the war’s early months, but fortunately the Germans had few in stock and their production had a low priority. Moreover, the German Navy commander, Grand Admiral Erich Raeder, wasted the opportunity by using them in small numbers instead of concentrating them for a decisive effect.

The British implemented countermeasures by mid-1940 and began to develop advanced mines of their own. Thus began a technological war between the Western Allies’ mine and mine-countermeasure experts and their German counterparts. It was a war the Allies eventually won, but not without difficulty or pitfalls.

Although seemingly unglamorous and unexciting, mine warfare was a critical element of naval operations in World War II. By 1943, for example, mine-countermeasure forces constituted nearly 60 percent of the German Navy, while their Allied counterparts had grown to more than 1,100 ships and boats from a force of only two dozen at war’s start.

German minefields were the deciding factor in Russo-German naval operations in the Baltic Sea, and a major hazard to Allied shipping. Axis minefields affected Allied planning for every amphibious operation in the war except Operation TORCH in Northwest Africa. The Western Allies, on the other hand, used mine warfare to choke German Baltic coastal and river commerce during the war’s final years, while Soviet minefields severely inhibited Axis naval operations in Soviet coastal waters. In each instance, the country employing mines found them to be a cost-effective weapon, particularly in circumstances and areas where the opposition had naval supremacy.

Mines can be used either offensively or defensively, with the latter being the most common. Laying defensive minefields around key ports and coastal areas was one of the first naval actions undertaken by nations entering the war. This activity also highlights where a country either felt most vulnerable or most expected an enemy attack. These minefields served to restrict maritime movements within the mined areas to “transit lanes”—areas within the minefields, or between them, in which no mines were present.

Since mines, particularly moored mines, often drift with the tide and current and are indiscriminate in what they destroy, transit lanes had to be maintained by constant clearing. Hiding the locations of the transit lanes from the enemy was a major concern as well, since he might use the lane to penetrate the minefield or lay mines within it.

Mines are classified by how they are detonated (contact or influence) or deployed (moored, bottom, or free-floating). Contact mines explode when the target makes contact with the mine. Influence mines detonate as a result of the target’s influence on the local environment—either due to the noise it makes (acoustic mines), its effect on local water pressure (pressure mines), or magnetic attraction (magnetic mines). Moored mines are secured by cable to a casing that lies on the bottom. As the name implies, bottom mines rest on the bottom, while free-floating mines float just below the surface. There are three types of free-floating mines: drifting mines drift in the surface current; creeping mines drift at a fixed depth in the subsurface current; and oscillating mines drift at varying depths within the subsurface current.

Each type of mine has its own strengths and weaknesses. All bottom mines, for example, are influence mines and must be laid in waters less than 100 fathoms deep. Otherwise, the targets may not pass close enough to detonate the mines, or they may be too far away for the blast to be effective.

Influence mines were the most difficult to detect and counter. They also drifted less than the other types of mines, and therefore were easier to “reseed” (that is, lay additional mines in the field) and sustain. Moored mines could be either contact or influence and could be laid almost without depth restrictions. They were the easiest to detect and remove, however, and had a tendency to drift with wind and current over time. This made moored minefields more difficult to maintain.

All free-floating mines of World War II were contact mines. Almost totally random weapons, they were rarely laid in fields but generally were employed near enemy harbors or staging areas, where current and tides would preclude their becoming a threat to friendly forces. They are the most difficult mines to defeat and they are such a random hazard that international law requires free-floating mines to sink within eight hours of being laid.

The mine’s primary effectiveness is in its psychological impact. Mines can be laid by any platform (ship, plane, or submarine), encountered anywhere, and they are virtually undetectable. Thus, prudent mariners avoid known or suspected minefields. More significantly, mines require more effort to clear than they do to deploy. The best minefields include a mixture of moored and bottom, contact and influence mines, but such fields are exceptionally difficult and dangerous to lay. Whatever that difficulty, however, it is little compared to what is required to clear such a minefield.

Minesweeping was the only available method of clearing mines in World War II. So-called because the original mine-clearing equipment employed a steel cable towed behind the sweeping ship to “sweep” away the mines’ mooring cables so they would float to the surface for destruction, minesweeping was a tedious and dangerous task. The ships that carried the gear were called minesweepers.

The only available mine-countermeasures equipment at war’s start was the Oropesa sweep from World War I. Essentially a “wire sweep” that trailed behind the minesweeper, the Oropesa sweep cut cables out to about eighty meters from the sweeper. Each sweeper could conduct two sweeps, one to a side, per sweep run. The lead ship in a sweep formation had to literally “lead” the unit through the minefield. The position was normally rotated since losses among lead ships exceeded 10 percent. The trail ships also faced danger because they had to avoid, as well as destroy, the mines the lead ships released. Unfortunately, the Oropesa gear could only be used against moored mines. The introduction of German bottom magnetic mines in 1939 came as a total surprise and ultimately led to the development of influence sweep gear and degaussing equipment.

The introduction of influence sweep gear in mid-1940 marked the beginning of the technology race in the mine war. The British LL magnetic sweep used alternating electric current, pulsed through a cable towed behind the minesweeper, to detonate magnetic mines at a safe distance by simulating the passage of a ship. The Germans and the British also modified aircraft, the Ju-52 and Wellington bomber respectively, to conduct influence sweeping. Carrying huge electric coils in rings attached to their fuselage and wings, these aircraft cleared suspected minefields by flying over them at altitudes of less than forty meters. Such aircraft swept large areas of influence mines but proved vulnerable to enemy fighter aircraft.

One other countermeasure to magnetic mines that all sides used after 1940 was the elimination of ships’ magnetic signatures. Since all metal ships acquire the magnetic signature of the area in which they are built, they need a major deperming or signature removal effort to reduce their magnetic vulnerability. Maintaining that magnetically neutral signature requires the installation of electric cables along the ship’s hull. Passing electric current through those cables “degaussed” the ship (that is, prevented its developing a local magnetic signature). Degaussing equipment is a major design feature of all warships to this day.

The Germans got the mine warfare lead again later in 1940 when they introduced acoustic mines. However, Raeder employed them before they were available in large numbers. The British recovered one in August 1940, and one month later put a mechanical acoustic sweep into service that could defeat the German acoustic mine. Unfortunately, sound dampening the equipment on British ships proved too expensive to implement during the war, although they did do it in minesweeping units. The Germans countered by producing a combined acoustic/magnetic mine, but the British soon developed minesweeping tactics to counter it as well.

The Allies were not idle in mine development, Great Britain developed magnetic mines in World War I and employed them again beginning in April 1940. The Germans countered by developing their own magnetic influence sweeps. They also built a specific class of magnetic sweep ships, called Sperrbrecher. Equipped with huge electric coils in their bows to project a strong magnetic field ahead of them, these specially reinforced ships had shock-mounted equipment and other damage reducing features to survive mine detonations. The Germans used these units to lead coastal convoys through suspected and likely enemy minefields. They became increasingly important in the war’s final two years, as the Western Allies, in particular, laid more and larger minefields.

The Germans began to employ mines with arming delay mechanisms in late 1941. The mines could also be set to arm from six hours to twelve days after they were laid. Hence, a field thought safe after multiple sweeps could suddenly become active. The Germans also introduced refinements in their acoustic mine sensors, lowering the frequencies monitored and targeting specific ship equipment, making it more difficult for Allied sweep gear to simulate. They also began to employ multiple polarity magnetic mines and finally, ship counters to complicate sweep efforts—that is, the mines required a variety of magnetic phenomena to detonate and allowed a preset number of ships to pass by safely before they would detonate. Thus, an area was not truly clear until the entire field had been swept to the maximum “number” that an enemy mine could be set. Moreover, the sweep gear had to simulate a wider variety of magnetic signatures to detonate the mine.

Both the Germans and Western Allies had combined acoustic/magnetic mines with ship counters and variable arming delays in service by late 1943, when the Germans introduced their latest technical innovation, the bottom pressure or “oyster” mine. These mines were detonated by the pressure wave a ship generated as it moved through the water. No minesweeper could simulate that wave since each pressure wave was unique to the size and speed of a ship.

Determined not to repeat Raeder’s mistakes from earlier in the war, Grand Admiral Karl Dönitz waited until the Allied Normandy landings to employ oyster mines. By then, however, he lacked the means to lay the mines in the invasion area. Only a handful could be deployed. Had they been laid in the invasion fleet’s assembly areas, they would have had a devastating effect. As it was, after some nasty surprises, the Allies easily avoided the few areas where these mines were laid. A few were recovered and the technology incorporated into Allied mines used against the Germans in the war’s closing months.

Although the Germans developed the most technologically advanced mines of the European theater, they did not employ mines to their maximum effect. Missing several opportunities to exploit their advantages both early and late in the war, the German mine warfare effort was further weakened because it supported only naval operations. Allied mine warfare operations were more opportunistic and better integrated into their overall war effort. Thus, they employed mines effectively to cut German commerce on the Danube and other river systems critical to the German economy as part of the overall Allied strategy of attacking Germany’s infrastructure.

Allied mining was also geared to support the Allied ground campaign in Italy by interdicting German coastal convoys supporting German ground forces. A similar German mining effort directed at the Soviet river and coastal navigation system would have paid huge dividends for the German war effort on that most critical front. As it was, naval mine warfare represents yet another area in which the Germans wasted their initial lead and regained it too late to affect the war.

Additional Reading

Bekker, Cajus, Hitler’s Naval War (1974).

Campbell, John, Naval Weapons of World War II (1985).

Hartmann, Gregory K., Weapons That Wait (1979).

Hough, Richard, The Longest Battle (1986).

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