Metal Detector Mk47

[Mack Mosely]

Possiblythe first water mine in the world with a target-fired trigger (as opposed to earlier ones which had simple, slow-burning fuzes) was invented by David Bushnell, the man who is most famous for inventing a one-man submersible, the Turtle. Like the Turtle, these sea mines were unsuccessful when they were used against the British fleet in Philadelphia during January 1778, although the resulting "Battle of the Kegs" did cause a few casualties and inspired a well-known Revolutionary War poem - Scroll down to Note III.

For most of the 19th century the US Army was responsible for the development and use of mines, as mines were considered to be defensive weapons that were useful for protecting harbors and coastal waters. The Confederate Navy used mines, or torpedoes, as they were then called, quite extensively during the American Civil War, and sank approximately 27 Federal vessels and damaged many more. By comparison, only nine Federal vessels were sunk by gunfire. Many different kinds of mines were used by the Confederates, with one of the most effective being the frame torpedo, which was a large nose-fuzed artillery shell that was mounted on a wooden frame and located where a ship might strike it.

Not until late in the 19th century did the Navy start to take any interest in these weapons, but none were used during the Spanish-American War of 1898 and it was not until the Russo-Japanese War of 1905 that this interest became serious. During that war, both Imperial Russia and Japan had some success with their naval mines, which prompted the US Navy to ask Congress for funds for a mine depot ship. The old cruiser USS San Francisco (C-5) was converted in 1912 and she became the first mine warfare ship in the US Navy.

The first mines procured by the Navy were originally called "Naval Defense Mines" and were either purchased from European powers or were copies of European designs. Progress on mines was considered satisfactory prior to the start of World War I, but with the US entry into the war it was discovered that British mines of the same general types had proved unreliable. Bureau of Ordnance reports of 1917 declared that the status of mining in the USN with these Naval Defense Mines was "very unsatisfactory" and recommended development of new types, which led to the Mark 5 contact mine and the Mark 6 antenna mine.

These Naval Defense Mines were all declared obsolete in 1930 and removed from inventory. None of these designs were very successful, so perhaps it is surprising that it took so long to take them out of service.

The first USA designed mine, the Mark 5, was of the "Horned" type. Horns were made of soft metal such as lead and held a glass ampoule containing battery acid, usually potassium-bichromate. The lower end of the horn contained an electric battery minus the electrolyte. Contact with the horn broke open the acid container, energizing the battery which then heated a platinum wire in a mercury fulminate detonator, thus exploding the mine. By definition, this was a weapon with limited range and fields needed to be densely packed in order for it to be effective against shipping. However, such close-laid fields ran the risk of one mine setting off adjacent mines as fraternal kills.

The "K-pistol" of the Mark 6 used a copper antenna which extended upwards to just below the surface. This was connected by a relay to a copper plate on the outside of the mine. Seawater acted as the electrolyte of a battery which would be formed when a ship with a steel hull approached and touched the antenna. The current running down the antenna operated the relay and exploded the mine. This method allowed each mine to cover a wider area, meaning that fewer mines could be used to cover a given area than with the horn type. In modern terms, the "K" device exploited the Underwater Electric Potential (UEP) effect.

Magnetic triggers were originally only used on ground (bottom) mines. This is because, if they were moored, the changing of the magnetic field as they rose and fell with the tide would set them off. Near the end of World War II, a trigger that measured the total field around the mine was developed. This device added up the fields in such a way that the tides did not affect it.

Pressure detector fuzes measure the pressure wave created by a ship moving through the water. These were simultaneously developed by both Germany and the USA during World War II, but both held off deploying them for fear that the technology would be captured by the other side. They were first used in combat off the Normandy beaches and were heavily used against the Japanese home islands near the end of the war.

There were also some ex-mercantiles and former Army "mine-planters" listed, but no mine outfit numbers are given for these ships. However, USS Salem (CM-11) is recorded as having laid 202 mines off Casablanca on 27 and 28 December 1942. USS Argonaut (SS-166) was converted to a transport submarine immediately following the attack on Pearl Harbor and apparently never laid a mine under combat conditions.

The widely used and long-lived (1917 to ca. 1985) Mark 6 "K-pistol" mine was relatively easy to deploy and did not require specialized minelayers. See the picture below of the tug USS Ute (ATF-6) being used to launch one of these mines. This ease of use by non-specialized mine ships is one of the reasons that there were so few dedicated mine-layers in the USN and why many of the smaller coastal minelayers were converted for other duties.

Conventional submarines were also used as minelayers. In 1943, they could carry eleven Mark 10 or Mark 12 mines in place of three torpedoes and many mini-minefields were laid as a result, some quite effectively. But the most common mine-layers during World War II were aircraft.

After World War II, aircraft have been the predominant US minelayers, as shown in the tables below. There are a few submarine-launched mines in the inventory, with at least one new one, ISLMM recently placed in service.

United States ships planted 56,611 mines and the British laid an additional 16,300 as part of the North Sea Mine Barrage. These sank at least six U-boats and damaged another half-dozen seriously enough that they were forced to return to base. It has been suggested that the cost / benefit ratio of the Barrage was grossly in favor of the Germans.

Avenger and Ventura aircraft could carry a single mine and in 1944 Avengers closed Palau harbor by mining the entrances. They then sank all 32 ships in the harbor with conventional bombs and torpedoes. A total of approximately 100 ships were sunk or badly damaged in the Pacific during the war by mines laid by Navy aircraft.

By 1945, the Army Air Force was devoting considerable resources to the mining role, with 80 to 100 B-29s based at Tinian being used to mine the home waters around Japan. These B-29s could carry seven 2,000 lbs. (907 kg). or twelve 1,000 lbs. (454 kg) mines. "Operation Starvation" started in March 1945 and continued until early August with 4,900 magnetic, 3,500 acoustic, 2,900 pressure and 700 low-frequency mines being laid. These mines sank 294 ships outright, damaged another 137 beyond repair and damaged a further 239 that could be repaired. In cargo tonnage, the total was 1.4 million tons lost or damaged which was about 75% of the shipping available in March 1945. This long-term mission had five phases: Phase I was aimed at the narrow Shimonoseki Strait between Kyushu and Honshu which was considered one of the most vulnerable points of Japanese inland shipping. This phase also targeted the naval shipping bases of Kure, Sasebo and Hiroshima. Starting on the night of March 27, seven missions by the 313th Bomb Wing laid 2,030 mines, closing the Shimonoseki Strait for two weeks and incidentally forcing the Yamato battle group to exit the Inland Sea through the easily monitored Bungo Strait.

Phase II was intended to block shipping around the Inland Sea and attacked the Shimonoseki Strait again along with the major harbors of Tokyo, Nagoya, Kobe-Osaka and other points inside the Inland Sea. In just two missions, on May 3 and May 5, B-29s laid a total of 1,422 mines, mostly Mark 25 magnetic types and some of the new Mark 25 pressure type which were considered unsweepable.

Phase III was aimed at blocking traffic between the Asian Mainland and Japan by mining the area between the Shimonoseki Strait to Kyushu and Northwest Honshu. 1,313 mines were laid beginning on May 13. By this point, mines were sinking more ships than were submarines with 113 ships sunk in the Shimonoseki Strait alone.Phase IV saw strengthening of the fields on the western coast of Japan and replenishing existing fields in the Shimonoseki Strait and the Inland Sea. This started on June 7 and went until July 8 with 3,542 mines being laid.Phase V was intended to initiate a "total blockade" of Japan. This began on July 9 and lasted until the end of the war. 3,746 mines were laid to replenish existing minefields and to extend them to Korean harbors. During all phases, a total of 1,529 missions were flown by the B-29s with only 15 aircraft being lost to all causes, an attrition rate of less than one percent.Between January and March 1945, B-29s also closed the approaches to Singapore, Saigon and Camranh Bay harbors by magnetic mining.

British Vickers design. Spherical mine about 30 inches (76 cm) in diameter with a 120 lbs. (54 kg) TNT burster. Used a protruding float made of cork, 3 feet (0.91 m) long, for inertia ignition - contact with a ship made the mine rotate relative to the float. Launched from trolley rails. Production started around 1915 and by 1917 the Portsmouth Navy Yard was manufacturing 140 of these per week with plans to ramp up production to 500 per week.

Spherical antenna type using a K-type pistol, 34 inches (87 cm) in diameter. This mine was designed specifically for the North Sea Mine Barrage of World War I. However, as shown above, it was still being used operationally as late as 1978. On 17 October 1917, the Secretary of the Navy authorized the construction of 100,000 mines of this type at a cost of $40,000,000 (40 million dollars). By the early summer, these were being produced at a rate of 1,000 a day with a peak of 1,500 being produced in one 24 hour period. In order to support this rate of manufacture, the Navy built its own TNT factory at St. Julien's Creek, Virginia, capable of producing 300,000 lbs. (136,000 kg) of TNT per day.

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