In spite of prewar financial limitations and errors in judgment, Great Britain entered the war with an adequate stock of mines—both technically and quantitatively. These included some 1917 H.II Herz horn type moored contact mines, but the standard moored contact mine was the switch horn type Mark XVII. It could be configured to take a charge of 320 or 500 lbs and could be laid with a depth setting of up to 303 ft. In 1939, Great Britain had both moored (M Mark I) and air-laid ground magnetic mines (A Mark I) available for deployment. Both were based on the magnetic induction principle (the horizontal magnetic field of a ship induced an electric current in a coiled rod) and proved so satisfactory that they remained in service throughout the war. The airborne ground mine was subsequently modified, in order to conserve scarce materials (A Mark II, III, and IV), to improve compatibility with bomb gears (A Mark V, 1940), and to include acoustic actuation (A Mark VI, 1944).
The majority of the mines laid by Great Britain were dropped by aircraft; hence, the British effort for a ship-laid ground mine was confined to the magnetic M Mark III, a cylindrical device that was introduced in April 1941. The charge weight was increased from 1,500 lbs to 1,600 lbs of amatol (or 1,750 lbs of minol) in 1942, and in 1943, acoustic mechanisms were added. An upgraded model with improved sensitivity and a pressure firing unit was canceled in 1944. Owing to the wartime shortages of TNT and RD (cyclonite), most British mines had 50/50 ammonium nitrate/TNT (amatol) explosive charges. This low-quality explosive was later improved by the addition of around 20 percent aluminum powder (minol).
Of all belligerents, Germany displayed the greatest creativity and activity in the field of mine technology. It developed a large number of sophisticated mines before and during World War II, with a focus on influence mines. The intense interservice rivalry between the German navy and the air force severely hampered the development of airborne mines, although ultimately, the project was successful. For moored contact mines, the German navy relied on the eminently successful EM designs of World War I, the most common being the EMA (331 lb charge), the EMB (497 lb charge), and the EMC (661 lb charge). The Germans also perfected the art of sweep obstruction. Many of the EM-type mines were fitted with chains (to resist wirecutters) and Kontaktauslösung (KA) gear, comprising a 98 ft Tombac tube over the upper part of the mooring wire, which, when subjected to the upward drag in sweeping, closed a switch and fired the mine.
The best-known German influence mine was probably the LMA airborne parachute magnetic ground mine; when one was retrieved on 22 November 1939, the British were alerted to the fact that the Germans, too, had a capability in noncontact mines. Unlike the British magnetic mines, the German mines were actuated by a change in the vertical component of the magnetic field (the dip needle principle). The LMA had a 661 lb charge and a practical depth limit of 130 ft. It was superseded by the larger (1,554 lb charge) LMB with magnetic (1939), acoustic (1940), combined magnetic/acoustic and magnetic/pressure (1944) firing. The development of the BM 1000 (1,500 lb charge) greatly improved the efficacy of German aerial mining. That mine was dropped without parachute from altitudes of up to 19,700 ft in water depths up to 115 ft, and it was regularly equipped with a variety of antisweeping and antilifting devices. For minelaying from torpedo tubes of submarines and E-boats, the Germans produced the TMA (moored magnetic) and TMB (ground magnetic and acoustic) mines. Both weapons suffered from serious construction defects that limited their endurance on the seabed to 6 to 12 months.
The construction of the Atlantic Wall, Germany's seaward defense against the Allied invasion, inspired the KMA anti-invasion mine. This was a shallow-water ground mine with a 165 lb charge set in a rectangular concrete block anchor and surmounted by a steel tripod frame with a single Herz horn on top. The total height of the mine was 8 ft, 10 inches, and it could be fitted with a snagline for depths of 16 to 33 ft. German charge weights in general were conspicuously heavy by international standards in order to compensate for the generally poor quality of the explosive fillings, usually made up of a mixture of ammonium nitrate, sodium nitrate, potassium nitrate, cyclonite, ethyline diamine dinitrate, and aluminum—TNT being an exception.
Many of the mines employed by Italy during the war were left over from World War I, some of them from the Austro-Hungarian Empire service. Some Italian mines were specifically built for deployment in warm-water conditions, such as off Libya. The P200 appeared in 1936. It weighed about 2,244 lbs and had an explosive charge of 441 lbs.
The Soviet navy relied mainly on material developed before 1917, such as the M06, M08, and M12. Conditions in the Soviet Union between 1917 and 1941 had stifled mine development, yet the Soviet navy put to use a handful of mines developed in the interwar period.
Notably, Italy and the United States distinguished themselves as substantial mine producers, the latter with an emphasis on submarine and air-laid mines such as the Mk 12/3 and the Mk 12/4 (1,200 lb Torpex charges). Japan, as during World War I, neglected mine development and relied solely on a handful of obsolescent moored contact mines. Dirk Steffen
Campbell, John. Naval Weapons of World War Two. London: Conway Maritime Press, 1985.; Cowie, J. S. Mines, Minelayers and Minelaying. London: Oxford University Press, 1949.; Ledebur, Gerhard Freiherr von. Die Seemine. Munich, Germany: J. F. Lehmanns Verlag, 1977.; Lott, Arnold S. Most Dangerous Sea: A History of Mine Warfare and an Account of U.S. Navy Mine Warfare Operations in World War II and Korea. Annapolis, MD: Naval Institute Press, 1959.