The “war of nations,” as World War I is called, was remarkable for the novelty of its instrumentation and cruelty. The armies fought in trenches with bayonets, machine guns, tanks, poison gas, and airplanes. The navies fought with big battleships and big minefields—the biggest the world had ever seen. The most menacing instrument of warfare, however, was the German U-boat.
The British decided that the best protection for their harbors was to restrict passage of the U-boats to the Atlantic. With this in mind, they maintained barrier minefields at points of egress from the waters bordering Germany. In the Heligoland area, for instance, they maintained minefields to limit easy passage from January 1915 to November 1918.21 These fields consisted of contact mines equipped with Hertz horns for firing upon impact. They were also fitted with anchors for automatically holding the seamines in place and mooring the case at a predetermined depth. The latter was required to provide devices for seamines that would attack U-boats at whatever depth they might attempt to move. The seamines were deliberately planted in areas where sweeping by German minesweepers could be conducted on a regular schedule. Hence, to maintain the desired threat against U-boats, additional seamines were planted at frequent intervals. About 20,000 were planted during the four-year period.
The Heligoland minefield was vastly larger than the Confederates’ mining effort during the U. S. Civil War. Nevertheless, U-boats continued to escape from the North Sea into the Atlantic to harass traffic and sink ships conveying agricultural and industrial supplies for Britain and France. With the entry of the United States into the war against Germany on 6 April 1917, America proposed to Great Britain that together they build a great minefield to choke the U-boats’ movements. The U-boats had been escaping into the Atlantic through the expanse of water between Scotland and Norway—an area 250 miles long and 50 miles wide.
It remained for the discovery of a new seamine type to make the Northern Barrage minefield possible. Invented by Ralph Cowan Browne, it was officially designated the Mine Mark 6, but was generally referred to as the antenna mine because of its most noteworthy feature.22 The antenna mine accepted components already in use in seamine warfare. One was a British device for automatically mooring the mine case at a predetermined depth. Another adopted characteristic was a provision for systematic sterilization in the event that the case broke loose from its anchor. This safety feature was derived from early American experience, and was required by the 1907 Hague Convention. The mine case itself enclosed 300 pounds of explosive and an additional volume of air to assure buoyancy of all components and the mooring cable.
The distinguishing feature of the antenna mine, however, was a long copper antenna wire hung from a small float above the case and attached to its firing mechanism. When it brushed against the steel hull of a passing submarine, the contact of these two dissimilar metals in the conductive path of seawater produced an elementary battery. The small electric current this cell generated was sufficient to close a relay, exploding the moored mine. Its firing mechanism could be actuated by contact of the submarine and any point of its 75-100-foot antenna; thus several simple contact mines moored at different depths would have been required to replace a single antenna mine over as great a vertical distance.
The British maintained a barrage minefield from Folkestone, England, to Gris Nez, France, to obstruct passage of U-boats from the North Sea to the Atlantic through the English Channel. In American waters, German submarines planted 58 contact mines at seven coastal locations from New England to Cape Hatteras. The armored cruiser USS San Diego (ACR-6) and five merchantmen were sunk from contact with these mines, and the battleship USS Minnesota (BB- 22) was damaged.23
American minelayers began to plant the Northern Barrage on 6 June 1918. The British planted their seamines in the terminal sections near the Orkney Islands and near Norway. The Americans planted antenna mines in the central section, an area about 134 miles long and 50 miles wide. This area was mined vertically from ten feet to 240 feet to attack both surfaced and submerged U-boats. Only two U. S. minelayers were available, so eight commercial ships were equipped for the task. Even though the minelayers could plant the seamines at 17 knots, it took from 6 June to 26 October to complete the task. The terminal armistice with Germany was signed shortly thereafter on 11 November 1918.
U. S. minelayers made 13 round trips to plant 56,611 antenna mines. The British planted 16,300 seamines with four minelayers. The total number in the greatest of all minefields was 72,911.24 For a time, submarines escaped from the North Sea by sneaking through the territorial waters of neutral Norway. Norway later closed the route by planting its own seamines in these waters.
The Northern Barrage was responsible for sinking not more than a dozen submarines and damaging a few others. This is another instance, however, in which the value of a minefield barrier should not be judged solely by the resulting ship casualties. The cost of each antenna mine was only about $400, and the total number required was much fewer than would have been necessary if another type of seamine was used. Moreover, the U-boats were forced to attempt to reach the Atlantic via longer routes, which required more fuel and time, and resulted in shorter patrols on station. Thus, fewer U-boats were in the Atlantic at any given time, and consequently there were fewer sinkings and less damage. After the war, many of the moored mines in the Northern Barrage remained to be swept. The area was not safe for commerce between the United States and Europe until September 1919.
World War II
Almost immediately after Hitler’s tanks and troops crossed into Poland on 1 September 1939, the German Luftwaffe dropped small groups of seamines in approaches to English Channel ports, in accord with their mine warfare guidebook.25 After the British Empire as a whole joined the hostilities, a German minelayer arrived in the Indian Ocean with 64 seamines and instructions to plant them in eight locations. One of these was Australia’s Bass Strait, where the American ship SS City of Rayville sank on 8 November 1940, a full year before Pearl Harbor.26 Eighteen months later, German U-boats began planting seamines in American ports. In 1942-44, 338 seamines were planted at 13 sites from Newfoundland to Trinidad. Eleven ship casualties were inflicted by these mines and eight ports were closed for a total of 39 days.27 Germany did not adopt the Allied policy of planting hundreds or thousands of seamines in a single minefield.
When the Luftwaffe engulfed the English coast with seamines, large parachutes were used to prevent damage to the weapons. One of the many seamines planted in this manner fell into shallow water, where it could be observed, disarmed, and recovered. This seamine introduced the rest of the world to the Germans’ magnetic needle mine, truly the first influence-actuated seamine of World War II. The antenna mine thus became obsolete. The magnetic needle mine did not require contact with a ship to detonate. The inventors of seamine mechanisms and countermeasures were no longer zealous individuals like Bushnell, Fulton, Colt, and Browne; they were scientists and engineers from universities, service laboratories, and related professional fields of endeavor.
During World War II, large defensive minefields were laid in the Caribbean, from the Dragon’s Mouth to the Serpent’s Mouth, and in the Pacific theater. In addition, seamines were laid for offensive purposes. These seamines consisted of electric circuits with electronic components, specialized magnetic materials, sensitive relays, and acoustic components. The circuits had to be able to distinguish between big ships, small ships, deep water, shallow water, rapid movements, slow movements, and repetitive signals such as those generated by magnetic minesweepers and acoustic hammerboxes. Most important, the circuits had to be adjustable for the variable circumstances and demands of service use and had to be able to provide settings that would assure sterilization or self-destruction of the seamine when the minefield had accomplished its objective.
The U. S. Naval Ordnance Laboratory was the principal developer of these circuits. The laboratory organized a technical school and trained reserve officers in the efficient use of this new ordnance, and assigned technical personnel skilled in electrical and mechanical engineering and operations research to appropriate combat area commands. For example, in the central Pacific area, a mine modification unit composed of officers, professional civilians, and naval complement studied intelligence information concerning the minefields planted by U. S. forces, modified mine firing mechanisms for improved damage capability and improved resistance to countermeasures, conducted full-scale tests of modified seamines, corrected mine component material deficiencies, and provided technical analysis for estimating the operational effectiveness of proposed mechanism modifications.
About 44,000 seamines were laid by the U. S. Army and Allied air forces, and by U. S. Navy aircraft, surface vessels, and submarines. Defensive seamines were planted at locations along the Atlantic coast, such as the Chesapeake Capes, Cape Hatteras, Key West, and Trinidad, and at other locations such as Casablanca, Alaska, Kwajalein, the Philippines, and French Frigate Shoals. In addition, control mines for harbor defense were developed and planted by the U. S. Army at locations such as Boston, New York, Delaware Bay, Norfolk, Columbia River, San Francisco, Panama, and Corregidor.
Beginning in October 1942, about 25,000 of the Navy seamines were used offensively against Japan; 1,075 ship casualties were inflicted—one ship casualty for every 23 seamines laid. The ship casualties totaled more than 2,250,000 tons, about one quarter of the pre-war strength of the Japanese merchant marine.
At Palau on 30-31 March 1944, the initial U. S. carrier aircraft strikes laid 78 seamines in the ship passages to the open sea. This tactical use prevented the exit of the 32 Japanese ships located there, and all these ships were later sunk or damaged by U. S. Navy carrier-based aircraft. About one-half of the seamines used offensively against Japan were planted in Japanese home waters by U. S. Army B-29 aircraft. This principally strategic effort, known as Operation Starvation, represented only about 6% of the capacity of the XXI Bomber Command. Of 1,528 B-29s used to lay 12,052 seamines in inner-zone target areas, only 15 aircraft were lost. All U. S. seamines were prepared under the supervision of U. S. Navy technical personnel, and Navy mine warfare officers directed or collaborated in the planning and execution of these operations. Operation Starvation began on 27 March 1945 and lasted for four-and-a-half months, until the end of the war. About 670 Japanese ships, totaling 1,140,000 tons, were sunk or damaged. These losses interrupted the entire internal commerce of Japan, and soon could have resulted in defeat without the use of atomic bombs at Hiroshima and Nagasaki.28
Korea and Vietnam
The Soviet Union supplied approximately 5,000 seamines to protect North Korea from invasion. Late in the war, North Koreans used small numbers of seamines locally in an attempt to protect the fishing industry on which they depended—the third largest in the world. A few seamines from World War II were provided to North Vietnam by the Soviet Union and China, some captured from Chiang Kai-shek. Late in the war, at Haiphong, the United States planted a large minefield that included thousands of the newly developed destructor mines. The Vietnam War however, should be noted for the introduction and effective use of seamines of local materials and native design. For instance, in more than 1,000 such seamining incidents attributed to the Viet Cong in South Vietnam, about 40% were successful, a notable accomplishment.
The significance of seamine warfare cannot be denoted solely by the number of seamines involved, or merely by the areas of the waters in which they are planted, or even by the ship casualties that result from their use. Other considerations include: the threat the seamines present, the delays imposed on shipping, the water routes denied, the extent of countermeasures encountered, the costs of damage inflicted, the costs of the seamines themselves, the total human and engineering effort required, and the various ways in which manpower and human lives are affected.
Although seamine warfare continues to experience brief intervals of support followed by extended periods of indifference, discrete applications of seamines remain destructive and deadly threats in modem conflict. Most devices and skills that have been developed for seamine warfare—boat mines, limpet mines, drifting mines, control mines, moored mines, antenna mines, influence mines, and their countermeasures—are still at hand, for use by the United States or against it, or both.
21. Capt. J. S. Cowie, RN, Mines, Minelayers, anti Minelaying (London: Oxford, 1949), chs. 4,5.
22. Who Was Who in America, Vol. Ill 1951-1960 (A. N. Marquis Co, 1960), p. 113.
23. USN Hydrographic Chari, WW I summary of enemy mining activities on U. S. Atlantic coast showing positions of all mines accounted for front beginning of activities until coast was declared clear of mines on 17 February 1919; the information was received from the German government after the armistice was signed.
24. Robert C. Duncan, America's Use of Sea Mines (Washington, DC: U. S. Government Printing Office, 1962), ch. 9.
25. Gcrman Naval Warfare Guide Number 8, Berlin. Germany, 1930.
26. Radm. John B. Heffeman, USN (Ret.), United States Naval Chronology, World War II (Washington, DC: U. S. Government Printing Office, 1955). p. 6.
27. L. E. Hoisington, '“Evaluation of Submarine Minelaying,” Mine Warfare Operational Research Report No. 84, Deputy Chief of Naval Operations, 28 April 1947, pp. 3,5.
28. Ellis A. Johnson and David A. Katcher, Mines Against Japan (Washington, DC: Government Printing Office, 1973), ch. 3.