Minesweeping is a form of defensive active MCM and can involve mechanical sweeping (towing wires and cutters to sever moored mines from their sinkers or chain sweeps dragged across the seabed) or influence sweeping (using magnetic, acoustic and pressure sources to actuate or jam mine sensors). The following paragraphs relate the history of these two forms of mine countermeasures.
Minesweeping is a form of defensive active MCM and can involve mechanical sweeping (towing wires and cutters to sever moored mines from their sinkers or chain sweeps dragged across the seabed) or influence sweeping (using magnetic, acoustic and pressure sources to actuate or jam mine sensors). Most sweeping techniques in use today involve sweep devices towed behind a surface ship or helicopter or incorporated in remote-controlled drones. Minehunting is also a form of defensive active MCM but was developed much later than minesweeping. The following paragraphs relate the history of these two forms of mine countermeasures.
During the American Civil War, the first known ship mine-protection device is thought to have been employed. This involved the use of a wood and bamboo ‘cow catcher’ projecting 20 to 30 feet from the bow of a ship. This cow-catcher or ‘torpedo rake’ was attached to USS Saugus and used during the James River Operation. The US Navy also experimented with nets extending from all sides of a ship to protect it from contact mines.
In 1870 the US Navy established the ‘Torpedo Corps’ and experimented with both defensive and offensive naval mines. Organized by Admiral David Porter, this group developed a system of mine countermeasures.
In 1881 Captain Thomas Selfridge became head of the US Navy Torpedo Corps. His own ship USS Cairo had been sunk by a mine during the Civil War. He started work on a system for ship defence against both mines and self propelled torpedoes. Captain Selfridge is credited with the first attempt to develop an effective mine countermeasures unit within the US Navy.
At the turn of the century, the Russians were credited with the first attempt to sweep a mine field. Two ships steaming parallel courses towed a 2-inch thick cable between them to drag mines clear of a fairway. The cable was weighed down to keep it at a fixed depth.
In December 1907, two Grimsby trawlers were hired by the Admiralty with their fishing crews and sent to Portland for experiments. The first trials, carried out against a field of dummy moored mines with a sweep not unlike an ordinary fishing trawl towed between the two vessels. The sweep was adopted for use and the first of 6 ex-torpedo gunboat minesweepers was commissioned in 1908. A senior officer was appointed to the Admiralty to conduct further experiments and to develop the system more fully.
In 1908 Lt (later Capt) C L Lambe RN proposed the trial of an experimental cow-catcher fitted to a battleship. This comprised a number of booms carrying a transverse horizontal steel bar projecting ahead of the ship at keel depth with a net fitted above the bar. The trial was not approved because the design was considered too heavy and complicated. Also, unsatisfactory results had been experienced with similar gear in the Dardanelles in 1878. In 1911 the British fitted cowcatchers to torpedo gunboats and trawlers used as minesweepers. In 1913 a trial was conducted using a false bow, representing a torpedo gunboat, fitted to an old vessel called Mastiff. A guncotton charge of 501 lbs was exploded beneath the gear and it was subsequently approved for torpedo gunboats.
WORLD WAR I
At the beginning of WW I, British regular minesweeping forces comprised 10 ex-torpedo gunboats fitted with the Actaeon or ‘A’ sweep in 1908/95. These were HM Ships Speedy, Circe, Hebe, Jason and Leda of the Alarm Class and HM Ships Gossamer, Seagull, Skipjack, Spanker and Speedwell of the Sharpshooter Class (called the Gossamer Type after conversion). HMS Circe had been built in 1892. The first British mining casualty occurred on the second day of the war when the new British 3,500 ton light cruiser HMS Amphion was sunk by a mine laid in the Thames Estuary by the German auxiliary minelayer Konigin Luise.
HMS Amphion sinking
By 8 August 1914, 94 fishing trawlers had been mobilised and converted for minesweeping. By 22 Aug, a further 100 trawlers had been commandeered and fitted out. By the end of the war, British minesweeping forces comprised 726 vessels including 110 regular naval vessels (mostly Acacia Class, Azalea Class, Arabis Class, Hunt Class and Aberdare Class Fleet Sweeping Sloops), 412 trawlers, 142 drifters, 52 hired paddle steamers and 10 Dance Class ‘Tunnel Tug’ shallow draught minesweepers. 214 British minesweepers had been lost in action while sweeping over 30,000 mines.
Early British minesweeping was limited to the towing of a ground chain from two spars set across the stern of a vessel but this resulted in an extremely narrow swept path and the chain was easily snagged by seabed obstructions. The next development was a serrated wire sweep towed between two ships. Otter boards, used by fishermen to keep open the mouths of their nets, were employed to increase the width of the bight of wire in contact with the seabed. This simple design frequently became snagged on rocks and wrecks on the seabed but technicians based at HMS Vernon overcame this problem with the introduction of redesigned otter boards known as kite otters. These were not only used to divert the ends of the sweep laterally but others could be rotated 90 degrees and used to depress the ships’ ends of the sweep wire to a chosen depth. This was the basis for the British Type Actaeon or ‘A’ sweep used for almost all Royal Navy minesweeping operations during WW I. It was effective for depths down to 50 fathoms.
Later in the war, technicians at HMS Vernon developed an improved technique that could be used by one ship alone. This led to the Fleet accepted design in 1919 of a single ship wire mine sweep named the Oropesa Mk 1 after the Fleet minesweeping trawler involved in the trials. This system avoided the need for ships to operate in pairs. The Oropesa sweep was held at the correct depth by a kite otter and taken out on the quarter of the sweeping vessel by another kite on its side or by an otter board supported by a float. A ‘V’ cutter was secured to the end of the sweep wire to sever the mooring cable of any mines that had not already been cut by the serrated sweep wire.
In 1914 trials were proposed for a new type of bow protection gear to be fitted to British merchant ships operating in the North Sea. This comprised four heavy spars heeled against a chain passed around the forefoot and stayed from the forecastle with a net spread between the four forward ends of the spars. However, it was subsequently decided that there was no requirement for bow protection against mines so no trial was conducted. A demand then arose for bow protection for battleships as well as improvements in the gear provided for trawlers and torpedo gunboats. It was decided to fit such protection to as many minesweeping vessels as possible and to test any promising gear. Trials of similar systems were conducted between Oct 1914 and Dec 1916. These included:
- Wilson Gear.
- Ollis Gear.
- Exmouth Gear.
- ‘Campania’ Gear (Director Naval Construction Type).
- SCW Gear (Superintendent of Contracts).
- Skipjack Gear.
- ‘Raglan Castle’ Gear.
In view of the potent German mining threat in the North Sea, Admiral Jellicoe called for minesweepers to proceed ahead of the Grand Fleet but he soon realised this would severely reduce the Fleet’s speed of advance. He therefore urged the introduction of a minesweeping device that could be fitted to Fleet units themselves. Commander Cecil V Usborne RN of the battleship HMS Colossus proposed trials of an apparatus that could deflect any mine not encountered head on. It comprised two sweep wires, each fitted with a ‘hydro-vane’. This was originally conceived by Lt Dennis Burney RN of the destroyer HMS Velox to deflect an experimental anti-submarine device clear of the ship towing it. When ‘hydro-vanes’ were secured to the ends of two sweep wires, they diverted the tail ends out from each side of the ship. Ultimately, this minesweeping device became known as the ‘Paravane’ and was fitted to the bow of all larger Royal Navy ships and many merchant vessels.
In September 1914 the Turkish commander responsible for the defence of the Dardanelles closed the narrow strait with minefields. In August 1915 Vice Admiral Guido von Usedom (a German officer assigned to the Turkish as ‘Inspector General of Coastal Fortifications and Minefields’), was sent to inspect the defences. Von Usedom expanded the Turkish mining effort and created a defensive minefield of over 300 German ‘Carbonit’ moored contact mines in eleven lines protected by the fixed guns and searchlights of forts and 74 mobile artillery pieces.
In December 1914 the British approved an operation to open the Dardanelles using naval forces. They used 21 former North Sea trawlers converted into minesweepers and manned by civilian fishermen. These operated in pairs about 500 yards apart sweeping with a single 2.5 inch wire and a one-ton twelve foot long kite to keep the wire at depth. British minesweeping was ineffective on the first eight nights owing to strong currents and enemy fire; the trawlers repeatedly withdrew under harassing fire from enemy gun batteries and one was mined on 10 March. This led to a daylight operation which again saw the converted minesweepers withdraw under fire. On 18 March 1915, a major Fleet attack was attempted in order to silence the Turkish coastal guns. The minefields caused the loss of the British pre-dreadnoughts HMS Irresistible and HMS Ocean and the French pre-dreadnought Bouvet and inflicted severe damage on the battle-cruiser HMS Inflexible. Two more British capital ships were lost from gun fire. The British thought the mine losses were due to floating mines but the losses were actually caused by a new and undetected line of moored mines laid by the small Turkish steamer Nusret on the night of 7/8 March.
WORLD WAR II
At the outbreak of WWII, the Royal Navy only had about 40 Fleet minesweepers. Half of these were ‘Smokey Joes’ i.e. WW I coal-burning sweepers laid up at Malta and Singapore. By D-Day, there were over 1200 RN minesweepers of various types in operation including Algerine, Halcyon and Bangor Class Fleet minesweepers, British American Minesweepers (BAMS), Motor Minesweepers (MMS or ‘Mickey Mouse’), British Yard Minesweepers (BYMS) as well as numerous converted trawlers and drifters. 50,000 RN personnel were involved in operational Mine Countermeasures.
The British had continued to develop mine sweeping systems between World Wars I and II. When war came in 1939, the British had the most efficient and largest wire mine sweep in the world. The Oropesa Mk 1 was swiftly followed by the Oropesa Mk 2, a smaller single-ship wire sweep capable of being towed by small craft.
Early in the war the Germans laid the ‘Falmouth Field’, the first offensive mine field against the UK. The UK had developed ‘k cutters’ (static cutters with sharp jaws) for wire sweeping but when they swept this minefield they found the Germans had planted sweep obstructors and delayed action mines to thwart their efforts.
The first German laid magnetic mine was recovered by Lt Cdr John Ouvry RN on 23 November 1939. This resulted in the British development of magnetic mine sweeping. In the first development, the ‘M’ sweep carried 34 magnetized bars towed on pendants 40 feet long and spaced 15 feet apart. The M Mk 2 sweep carried seventy 27 inch magnets spaced 10 feet apart across its swept path of 700 feet. Like the Mk 1 sweep, the Mk 2 was designed to sweep in the water column and not on the surface.
The first operational magnetic sweep called the ‘LL’ comprised two cables of different lengths with electrodes towards their tail ends. A petrol-powered 35KW generator set and batteries pulsed at 3000 amps for 5 seconds each minute. This early magnetic sweep was not only used to clear British areas but was also used in cooperation with the French to clear German air-laid magnetic mines from the entrance to Le Havre at the beginning of WW II. This sweep led to the requirement for a buoyant magnetic sweep cable and its development.
During this same period, the UK developed a ‘Portable Pulsing Unit’ designed to be placed on available craft-of-opportunity for clearing continental ports. Also known as the ‘Portable Mine Destructor’, it used the UK developed floating magnetic cable.
In the summer of 1941, German submarines laid moored magnetic mines in water 600 to 960 feet deep. These mines could also be laid in as little as 90 feet of water. Because of these developments, the UK developed a single ship deep sweep. The test ship was HMS Fraserburgh, a Bangor class minesweeper.
In 1943 when the invasion of Europe plan was being planned, attention turned to developing an amphibious assault sweep to the beaches. A shallow ‘A’ sweep was developed but was never used in operation. Additional testing was done on the Oropesa Mk 5 wire sweep and was found to be effective to a depth of as little as 10 feet.
During the latter part of World War II, the British addressed the shallow water problem. Wooden motor mine sweepers were not safe against the more sensitive magnetic mines in less than 40 feet of water. The earliest solutions were purely tactical and accomplished by first placing the tail before the mine sweeper using a small unmanned boat to tow the array ahead of the mine sweeper. Secondly they started at the edge of the channel using a programmed sequence. The ‘A’ sweep was used as a precursor and one or more mine sweepers following with a higher setting and a lower setting to improve clearance percentages.
The first known use of an acoustic mine occurred in 1940. However work on an acoustic countermeasure had already been initiated by HMS Vernon and the Sweeping Division of its Mine Design Department. This resulted in the SA acoustic hammer box in the bow compartment of a vessel (usually a converted trawler) but later bow-mounted about 12 feet below the water. By 1942, the bow mounted hammer box was one of the most common acoustic countermeasures and was being streamed abeam, usually in combination with the LL magnetic sweep. The hammer boxes contained a pneumatic or electric driven riveting hammer mounted to strike against a 7/16 inch thick 19 inch diameter steel diaphragm. The pneumatic hammers proved more reliable than the electric type and were incorporated into the standard hammer box designated ‘A Mark 1’ which was either suspended over the bow of a minesweeper or towed in combination with the magnetic sweep.
While Britain and Germany were active between the World Wars, their emphasis was different. Britain worked on mine development and mine countermeasures about equally. Germany spent much more of its energy on mines and mining but did not ignore mine countermeasures.
In 1935 the German company S.V.K. developed the magnetic mine and an acoustic version was laid in 1940. Before WW II it was a cardinal rule in Germany that methods of sweeping were to be successfully achieved before a mine was issued for operational use. After the war started, the Luftwaffe began development of air dropped mines and this control was no longer possible. The German Navy divided its sweeping into the following methods:
- Searching sweep gear
- Combined light sweeps
- Single vessel sweeps.
German sweeping gear involved using two vessels together as well as single vessel sweeps. The Germans also used single vessel sweeps when employing convoy sweeping gear. Their bow-protection gear included otterboard and paravane protection gear and bow cutter gear. One of the faults with their early sweep gear was that it could only capture one mine at a time. The gear then had to be recovered and re-streamed.
Germany had recognized the inherent disadvantages of the two ship sweep before the war but it took some time before single ships sweeps were sufficiently developed by the Kreigsmarine. The first single ship German sweep was known as O.R.G. (OtterRaumGerat). It was originally developed for use on the old larger mine sweepers (M boats and Fischdamphfers). While it was replaced by newer sweeps, the O.R.G. was used by the German Navy until the end of the war.
During the war Germany also used two types of cutters for arming sweep wires. Their mechanical cutters included the RaumGreifer (R.G.r). Introduced in 1936, it required a speed of 12 knots and seemed to be effective against all mooring cables then in use. The DoppelGreifer (Double cutter) was half the weight of the R.G.r. and easy to handle. Another cutter, the Danube Cutter, was used as a special river sweep.
The Germans used three classes of explosive cutters. The first were explosive claws such as the Type ‘B’ SprengGreifer (SP.GR). It was designed before the war for slow speed where mechanical cutters were not effective. The Type ‘D’ SprengGreifer was similar to the Type ‘B’ but larger and contained built-in safety devices. It was introduced about 1940. The EinzelschussigerGreifer was a single explosive cutter developed from the French F.L.R.G. sweep and used by the Germans from 1940. A double explosive cutter was the DoppelSchussigerGriefer. This sweep was also developed from the French F.L.R.G sweep. Both were effective against steel wire cable up to one inch thick. The third class was a special explosive claw known as the Type ‘E’ SprengGrieffer which was based on a Russian design developed for arming bow protection gear against the chain and claw obstructor used by British mine moorings.
The moored minesweeping gear used by the Imperial Japanese Navy was generally inferior. Most of their sweeps were of the two-ship team type which exposed the sweepers to considerable risk. Also no cutters were used except mechanical types on the paravanes of the high speed single ship sweep. Serrated wire was used to some extent but apparently only on bow-deployed paravanes.
Japanese sweep gear recovered during and after the war was found to contain many short lengths of wire, shackles, swivels and floats. When deployed, this made towing and recovery slow and difficult. Testing showed that control of sweep depth was “quite uncertain” and with some types, the variation in depth at towed speed was excessive. Most types of sweep gear were handed manually making deployment and recovery very difficult. Later testing revealed some of the equipment lacked positive buoyancy so that the floats could crush if allowed to sink during recovery. Overall, the Japanese moored sweep gear was considered crude, clumsy, inefficient and slow.
Japanese magnetic sweep gear comprised double catenaries towing magnets above the seabed or rigged to drag along the seabed. The only acoustic sweep used by the Japanese Navy at the end of the war was the sound bomb (Hatsuondan). The doctrine for its use varied. In general, groups of 3 to 5 were dropped with 2 minute intervals between bombs and spaced about 200 metres apart, as the sweep ship travelled at 4 knots. After the war the Japanese admitted that, at best, the sound bombs only swept about 50% of American acoustic mines.
Prior to World War II, US Navy mine countermeasures languished. Before the establishment of the US Naval Mine Warfare Test Station at Solomons, Maryland, all mine and mine countermeasures work was done directly by the Bureau of Ships at various mine sweeping bases along the Atlantic Coast. The Solomons Facility consisted of one officer and one enlisted man but later increased to three officers and 5 enlisted men. The ships used in testing provided the bulk of required labour.
Initially the major portion of the MCM work consisted of testing the development of explosive cutters. At the beginning of the war, the only cutter in the US Navy’s inventory for cutting chain moored mines was the ‘T Mark II Cutter’. This cutter was prone to misfire and was never considered to be satisfactory.
In 1943, the Safety Appliance Company began development of the ‘Y’ or Mark 12 cutter and testing was done at the new test station at Solomons, MD. After testing production began in 1944. However, further field testing showed the cutter was not acceptable. Changes were ordered that greatly improved its capabilities. The Mark 12 was then considered superior to any explosive cutter then in use.
Magnetic sweeping development was undertaken by two groups. The Timer Laboratory worked on electrical and mechanical problems. A special 540KW diesel generator was installed on an YMS class vessel. At Timer Lab, a duplicate power and generator set was installed. This group did all the original cable splicing for the countermeasures department in 1943. A second group, organized in 1944, was directly concerned with the development and testing of magnetic mine sweeping gear. Also, during late 1942 and early 1943, personnel from the Bureau of Ships tested two magnetic spar sweeps, the M Mark 1(a) and the M Mark 1(b). Both were designed for sweeping in restricted and shallow water areas. The Bureau of Ships also developed a two ship catenary sweep, the 00 M Mark 4(a). Later the M Mark 5 became the standard Navy magnetic sweep. The advent of sensitive horizontal induction mines required the development of even newer magnetic sweeps.
After the M Mark 5, the US Navy tested the ‘J’ sweep or M Mark 6(a). The ‘J’ sweep was formed by diverting the long leg of a standard sweep tail with minesweeping gear. This configuration produced an asymmetrical field with desirable characteristics.
The need for a shallow water sweep led to the use of a 50 foot motor launch which was later replaced with a 36 foot landing craft. A non-buoyant 300 metre MCM cable was used with floats. A small craft M Mark 6(a) type sweep was developed using size 5 or size 4 ‘O’ gear for cable diversion. A small craft M Mark 4(d) sweep was also developed. At the same time degaussing experiments were conducted on a 36 foot landing craft. A technique was developed for degaussing these landing craft modified into shallow water minesweepers with a signature of 4 milligauss at 8 feet.
In 1945 experiments were started on a closed loop modification of the M Mark 6(a). This was done by removing the electrodes and completing the circuit with a length of insulated 300 MCM cable.
Two groups developed acoustic countermeasures, Gear Development and Laboratory Measurements. Emphasis was placed on hammer boxes and towed pipe gear since these were the types of acoustic gear used on minesweepers.
During the winter of 1942-1943, the US Bureau of Ships was assigned the task of improving the British spring hammer. The first improved model was ready by July of 1943, known as ‘X-31’. Testing began and, after several modifications, was ready. The X-31 was designed to be towed rather than mounted on a boom. In early 1944 testing started to develop a simple towing arrangement. Use of a small fin and depressor attached to the box with a suitable bridle gave acceptable results at speeds of 10 to 12 knots. Later a streamlined housing was developed that allowed tow speeds of up to 15 knots. This streamlined version of the spring hammer was designated A Mark 4(v).
By November of 1944 tests began on low speed gear designated A Mk 2. Towed using hinged strikers, this gear met the requirements for acoustic output but only had am operational life of 4 to 14 hours when towed at 7 knots. Further modifications increased its life and later modifications followed. A medium speed sweep of 7 to 10 knots and a high speed sweep 15 knots were examined in the late 40’s.
Project AG-39 was established by the US Bureau of Ships on 17 December 1944 to test the production units of the A Mark 6(b) gear. The first units were tested to determine the acoustic output characteristics. This data was to include measurements of output vs. frequency, output vs. motor current, and output variation of line voltage. Towing tests of the A Mark 6(b) gear were first conducted in early March 1945.
Both the British and Germans experimented with explosive sweep gear. Generally an explosive sweep consisted of a series of timed small underwater explosions. Because of undesirable features in the British explosive gear, the US Navy designed a modified explosive sweep with several improvements. This ‘grenade sweep’ was designated the A MK 5(b). Testing against acoustic mines showed that the size of the grenades and the time intervals between explosions proved critical against each mine tested.
The British tested a hydraulic siren as a acoustic device early in the Second World War but the results were less than promising. At the same time, the Germans had produced a towed hydraulic siren. A US Navy project was started in 1945 to test a siren against acoustic mines. This version proved successful. It was compact and used a portable fire pump to push water out a tubular stator broken up by the rotation of a cup turned with compressed air. Because of the use of water and air it became know as ‘water air powered underwater siren’ or WAPUS for short. Interestingly, later improvements made it possible to eliminate the compressed air supply but the name remained.
In the spring of 1944 the US Bureau of Ships requested a sweep for the German ‘Oyster’ pressure mine. The discovery of pressure mines during the Normandy invasion made this a very urgent project. A towed canvas shape with a large hole in the front and a small hole in the rear was suggested. It was soon referred to as the Loch Ness Monster’. The original design was about 180 feet long and 24 feet in diameter at its largest circumference and was calculated to displace about 2200 tons of water when full. The canvas did not last long but ‘Nylon’, a new material first thought to be too light weight, was found to work well. With the advent of the nylon units, testing began in the autumn of 1944. Test data revealed the results were not acceptable. A larger unit was made (over 200 feet long and 32 feet in diameter) and tested at a maximum speed of 4 knots. While the signature was acceptable the swept path was not very wide.
In 1943 the Sweep Test Division C-4 was organized. The original complement consisted of 12 officers and nine enlisted men, all of whom were inexperienced in mine countermeasures. Only one officer had attended the Mine Warfare School then located at Yorktown, VA. By early 1944 the US Navy began to look at, and record, the acoustic output of its own ships. A series of hydrophones were set up and ships run over the course in both directions.
At the end of World War II, the US Navy used eight sweeping formations which were later adopted by NATO:
- ECHO – an unprotected two ship team mechanical bottom sweep
- FOXTROT – an unprotected two ship team mechanical column sweep
- JULIETT – an unprotected multi-ship mechanical team sweep against antenna mines
- GOLF – a protected echelon formation employing one sided moored sweeps
- INDIA – an unprotected echelon formation employing two-sided moored sweeps
- KILO – an unprotected formation modified to the improved deep moored sweep
- PAPA – an unprotected column formation using combination influence sweeps
- QUEBEC – an unprotected echelon formation employing combination influence sweeps or moored sweeps.