In the past decade, global warming has resulted in an acceleration of the melting of the polar ice caps, and scientists have predicted that the Greenland Ice Sheet could completely melt within the next 200 years. Enormous amounts of water are locked up in the ice sheets, and if they are to melt completely, this will raise sea levels by a catastrophic 100 feet.

In the deep ocean, unimaginable life goes on, unknown even to our best scuba divers. In recent months they have been troubled by a new enemy: the “swimming pillbug.”

In the late 19th century, the German navy set up a series of underwater telegraph cables linking the North Sea and the English Channel. For the sake of secrecy, the cables were buried just below the surface of the sea, only visible when the water was disturbed. These cables were used for communication for a long time until the submarine advent of the mid 20th century. By then, the British Admiralty already had a network of cables running across the North Sea. The German navy still didn’t understand what the Brits were up to and there was no way to spy on their underwater cables either.

When Germany laid magnetic mines early in the war, the British responded with aircraft that could be detonated by simulating the magnetic signal of a ship.

The 3rd. September 1939, two days after the German invasion of Poland, German submarines U-13, U-14 and U-17 begin laying three magnetic minefields on the seabed off the British east coast. Within days, four ships with a total tonnage of 14,575 gross tons were sunk and another 10,391 damaged. Although mines were suspected, minesweepers sent to the area detected none, so most Royal Navy officials believed the casualties were due to torpedo attacks by submarines, although survivors did not report seeing torpedoes. The mystery remained until the mine was discovered and successfully removed on November 21, 1939. HMS Vernon, the Royal Navy’s Coastal Technology Research Centre in Portsmouth, began to investigate the mine’s detonation mechanism and recommend effective countermeasures.

Steel warships create a magnetic trail as they pass through shipping lanes and cross the Earth’s magnetic field. German magnetic mines were designed to detonate at great depth when this signal was detected.

The Wellington DWI Mark II prepares for mine clearance in Egypt. (IWM CM5312)

Ingenious Solution to a Hidden Undersea Menace

Britain sought to rapidly develop equipment to degauss ships and neutralize or remove ships’ magnetic signatures. The Royal Navy has also introduced magnetic drag nets and minesweeping tactics on board in record time, but full implementation is still months away. Moreover, building and manning the large number of minesweepers required to cover all British ports and coastal waters would take months that Britain does not have.

By the end of the year, Germany had laid 470 magnetic mines, destroying 79 ships with a displacement of 162,697 tons. With so much coastline and water to protect, it was imperative for Britain to develop a fast-acting magnetic countermeasure system. The solution was to develop an aircraft that would copy the ship’s magnetic signature so that the mines could be detonated at a safe distance by flying over them.

Based on this, the Royal Air Force Coastal Command asked Vickers to modify the Wellington bomber for use as an airborne minehunter. It was a revolutionary idea. At the time, few naval leaders were aware of the existence of magnetic landmines. Mine towing therefore consisted of towing devices that cut the mooring ropes of traditional contact mines, so that they floated to the surface and could be destroyed.

Royal Navy officers in HMS Vernon’s mine-fighting division expected Germany to place magnetic mines. In fact, Britain developed and laid magnetic mines off the German coast in 1918 and off the coast of Estonia in 1919 during its peripheral involvement in the Russian Civil War. The management of HMS Vernon rightly believed that the Soviets had recovered some of these mines and handed them over to the Germans in the 1920s. The task was to determine the specific parameters of the German detonation system – the detonation threshold and the detonation time. Without this knowledge, the effectiveness of countermeasures cannot be guaranteed. The British received it in December 1939 and quickly saw the need for countermeasures.

This DWI from Wellington was one of six sent to the 202nd Airborne Division. Group in Egypt were sent to clear the Suez Canal and the Mediterranean coast. (IWM CM5313)

Ingenious Solution to a Hidden Undersea Menace

Wellington was the natural choice for a lift. It was already in production, had a good range and, with a large number of crews experienced in maritime operations, was a fast and cheap platform, provided possible aerodynamic problems were solved. This was considered the main problem, so Vickers first installed a 48-foot diameter balsa wood ring on the outside of the aircraft and attached it to the underside of the fuselage and wings. The ring contained coils of aluminum tape that emitted magnetic impulses when charged with an electric current. Aluminum was used to reduce weight and cost, as copper wire was heavier and scarcer. The first flight tests showed that the ring had surprisingly little effect on the flight performance and controllability of the aircraft.

Vickers engineers then removed the bomb racks, bomb aimers, cannons and all unnecessary equipment to save weight and make way for a Ford V8 car engine that powered a 35 kilowatt Mawdsley electric generator. The old gun emplacements were redesigned to simplify the hull. Since the magnetic coil made conventional compasses obsolete, the Wellington was also equipped with a gyrocompass.

Tests conducted in December 1939 against a defused German magnetic mine confirmed the validity of the concept. The success of the prototype led to three more modifications of the Wellington on the assembly line, bringing the total to four by January 1940. Vickers built 11 other aircraft on production lines at other plants. These 15 aircraft were designated Mark Ia DWIs (Directional Wireless Installations) and assigned to General Reconnaissance Unit 1 (GRU 1) to conceal their true mission. Operating from RAF Manston, GRU 1 was responsible for clearing the Thames Estuary of magnetic mines.

(IWM MH31337)

Ingenious Solution to a Hidden Undersea Menace

Once the modified Wellingtons were commissioned, the next task was to determine the altitude and throughput required for the influence scan that simulated the ship’s magnetic signature. The planes had to fly low enough to detonate the mines on the sea floor. Speed was also an issue. If you fly too fast, the mine sensors can’t reach the detonation threshold. Flying too slowly or too low exposes the aircraft to the risk of mine detonation. Tests have shown that 35 and 60 feet are the minimum and maximum heights respectively. The speed of the aircraft must not exceed 130 mph on takeoff. These tight flight parameters have made aerial mine detection a tense and dangerous business.

GRU-1 achieved its first success on the 9th. of January 1940 with the sure detonation of a mine. The second success came five days later, but the crew learned a painful lesson when a mine exploded under their plane, nearly causing it to crash. They flew at less than 35 feet and detonated the mine about three tenths of a second too early. The explosion lifted the Wellington about 40 feet into the air, the hatches were blown away, and the accelerometer registered a force of 10 G on the airframe. The fact that there was no structural damage other than the loss of the hatches is an indication of the strength of the bomber.

A bomb disposal crew clears a German magnetic mine on the British coast. (AWM P05468.013)

Ingenious Solution to a Hidden Undersea Menace

In addition to the British waters, three Wellington GRU-1s skimmed the waters around HMS Hereward when it evacuated the Dutch royal family to Britain in May 1940. Fighter planes escort unarmed minesweepers on missions in dangerous waters, but neither the RAF nor the Luftwaffe report that any of them have been attacked.

Early in 1940 Vickers designers made several improvements. The resulting Mark II DWI uses a lighter, more powerful de Havilland Gipsy Six engine to power a 96 kilowatt generator, reducing weight by more than 1,000 pounds. By increasing the generation capacity, the diameter of the coil ring could also be reduced. Gipsy engines generate more heat, so the designers were forced to install an air duct to improve engine cooling and a smaller duct to route air to the coil to prevent overheating.

The gyrocompass proved to be unreliable and had to be replaced. Vickers engineers discovered that by mounting a conventional compass on the back, it was insulated from the magnetic influence of the coil. Placing the compass display on the dashboard eliminated the need for a gyroscope, which saved weight and improved navigation. In August 1941 all Wellington DWI aircraft were upgraded to Mark II standard.

In April 1940, the Royal Air Force formed a second airborne minesweeping unit as GRU 1 and equipped it with two DWI Mark Ia and the first DWI Mark II. Operations along the British coast were mostly successful, with the Wellingtons used mainly as a rapid reaction force to clear suspected minefields or to clear ports critical to ongoing operations.

Concerned about the possible presence of Italian mines in Egyptian ports and in the Suez Canal, Britain sent word on the 20th. May Mark Ia to the Mediterranean, as well as technicians and equipment to convert five later GRU 1 Wellingtons to Mark II standard. The 202. The six aircraft, assigned to Middle East Command Group 2, searched for mines in the Suez Canal, off the coasts of Egypt and North Africa, and on the approach to Malta. Ironically, with the Allied advance into North Africa in 1943, airborne minehunters focused on removing the Allied mines originally laid to seal off North African ports from the Axis so that these ports could be reopened.

Although not as well known as German magnetic developments, British developments in German waters also involved magnetic mines. One of these mines was discovered by the Kriegsmarine off the coast of Jutland at the end of September 1939. Although German losses from mines were not as high as those in Britain, the potential threat they posed to German naval training areas in the Baltic and North Sea required a quick solution. Like the RAF, the Luftwaffe chose an existing airborne platform, the Junkers Ju-52/3m transport, as its test bed.

The prototype used a 51 hp diesel engine to power the coil, which drove a 35 kilowatt generator borrowed from a searchlight unit, but otherwise the program was similar to the British program. A 14 m long balsa ring with a spool of aluminium was attached to the wings of the Ju-52 by plywood struts. The first flight took place in mid-October 1939. Two weeks later a successful test followed in Flushing, where the Ju-52 detonated several mines between 10 and 20 metres during the flight.

German Junkers Ju-52/3m MS minesweeper goes up in flames after being hit by an RAF Hawker Typhoon aircraft off Lorient, France. (IWM C4095)

Ingenious Solution to a Hidden Undersea Menace

Production was slow, as equipment for units involved in the 1940 Western Campaign was given a higher priority. The first production Ju-52/3m MS minesweepers were delivered in June 1940, and in September the first of six minesweeper squadrons, the Sonderkommando Mausi, was formed. The Ju-52/3m MS aircraft were modified on the assembly line by installing a 150 kilowatt diesel or gasoline powered generator in the cargo area and connecting it to an aluminum coil. Since the British laid both acoustic and magnetic mines, about half of the German Ju-52/3m MS aircraft were equipped with a KK (Knallkörpergerät) device to destroy acoustic mines. The KK device consisted of a container containing 30 10-kilogram explosive charges designed to neutralize acoustic mines by destroying their hydrophones. The first MS aircraft were equipped with a 15 mm machine gun and two 7.92 mm beam cannons for self-defense.

Three Ju-52/3m MS minesweepers patrol a part of the lake. (Federal Archives photo 101I-643-4755-30A Photo: Ohmyer)

Ingenious Solution to a Hidden Undersea Menace

The tactics of the German minesweeper differed somewhat from the British tactics. The speed is almost the same, 125-135 mph, but the altitude is determined by the depth of the water. German magnetic levitation aircraft flew at an altitude of 40 meters above the sea floor, with most flights requiring an altitude of 10 to 20 meters. In addition, the Germans used two MS aircraft with solenoids placed one behind the other at a distance of 30-40 meters, followed by a KK aircraft at about 40 meters behind them. In general, the mines were activated about 5-10 meters behind the magnetic tracks, which made for some exciting moments for the KK-Gerät pilots. In addition, German airmine fighters encountered resistance in most of their areas of operation, and the Luftwaffe did not provide escort fighters. As losses mount, defensive armaments are increased. In October 1943, MS aircraft were equipped with 20 mm guns in the dorsal position and 13 mm machine guns in the jet position, but losses continued.

The Sonderkommando Mausi was renamed Minensuch Gruppe 1 in October 1942 and became the administrative command authority of the MS squadrons. Like Britain, Germany used its air minesweepers as a rapid reaction force and to clear sea lanes. As a result, he sent his MS squadrons to almost every theatre of war, from the Baltic and North Seas to the Mediterranean. The northern coast of France was the squadron’s most critical and dangerous area of action, where RAF and then US fighters attacked aircraft attempting to clear French coastal waters vital to the Allies. Despite losses and reduced fuel supplies, they remained in service until the end of the war, and in 1946 they helped the Allies trawl the Baltic and North Seas.

Blohm and Voss Bv-138MS aircraft aboard a seaplane. The modified Bv-138 and the similarly equipped Ju-52/3m were the two main German airborne minesweepers. (Historynet Archives)

Ingenious Solution to a Hidden Undersea Menace

As Allied mine clearance operations increased after 1942, the Kriegsmarine modified several of its seaplanes for aerial mine clearance. The four three-engine Blohm and Voss Bv-138C seaplanes were stripped of all armament and a diesel engine powering a 53-kilowatt generator was installed in the bow. They used the same magnetic ring as the Ju-52/3m, but it was mounted above the nose and attached with aluminium struts. They were designated Bv-138MS, but the crews called them Mausi. Blohm and Voss also modified two four-engine Ha-139 seaplanes for minesweeping by adding a magnetic loop to the nose and wingtips. A shortage of spare parts limited the usefulness of the aircraft, and it was withdrawn from service in early 1943. Unarmed and flying alone, the mine-clearing seaplanes were used from June 1942 to August 1944 to clear canals, rivers and estuaries.

The naval mine war played a key role in the Atlantic and in Europe. It sank more than a million tons of Allied ships and damaged nearly twice as many. Five percent of British and German losses to warships were caused by mines. All belligerents made extensive use of mines, and as the war progressed they became increasingly sophisticated, increasing the importance and complexity of mine action.

The introduction and widespread use of landmines added a new dimension to the threat, which needed to be addressed quickly. Aerial demining was the only solution for an immediate response. These aircraft were efficient, relatively inexpensive, and could quickly reach remote areas and fly over large bodies of water. Although their activities are not very well known, aerial minehunters played a key role in keeping waterways and harbours open during World War II and must be considered the forerunners of modern helicopter units for mine action.

Carl O. Schuster retired from the U.S. Navy in June 1999 with the rank of captain. He is a professor of military history and international relations at Hawaii Pacific University in Honolulu. Read more: History of the Wellington bomber, Martin W. Bowman; The Hidden Threat, Maurice Griffiths; and Junkers Ju 52 : Planes and Legends, Heinz Novarra.

This article was published in the May 2023 issue of Aviation History magazine. To subscribe, click here !In the late 19th century, a young Englishman named William Beebe was one of the first people to dive deeper than the deepest known point on Earth. Beebe’s deepest dive, to the Marianas Trench, is the second deepest human-occupied place in the world, with a depth of 10,911 meters. His feat has been called “a major contribution to the future of deep sea exploration”.. Read more about sweep mine game and let us know what you think.

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