British M class submarine
The British Royal Navy
Royal Navy
The Royal Navy is the naval warfare service branch of the British Armed Forces. Founded in the 16th century, it is the oldest service branch and is known as the Senior Service...

 M-class submarines were a small class of diesel
Diesel engine
A diesel engine is an internal combustion engine that uses the heat of compression to initiate ignition to burn the fuel, which is injected into the combustion chamber...

 electric submarine
A submarine is a watercraft capable of independent operation below the surface of the water. It differs from a submersible, which has more limited underwater capability...

 built during World War I
World War I
World War I , which was predominantly called the World War or the Great War from its occurrence until 1939, and the First World War or World War I thereafter, was a major war centred in Europe that began on 28 July 1914 and lasted until 11 November 1918...

. The unique feature of the class was a 12-inch (305 mm) gun mounted in a turret forward of the conning tower
Conning tower
A conning tower is a raised platform on a ship or submarine, often armored, from which an officer can con the vessel; i.e., give directions to the helmsman. It is usually located as high on the ship as practical, to give the conning team good visibility....



They were ordered in place of the last four of the first group of steam propelled K-class submarines
British K class submarine
The K class submarines were a class of steam-propelled submarines of the Royal Navy designed in 1913. Intended as large, fast vessels which had the endurance and speed to operate with the battle fleet, they gained notoriety, and the nickname of Kalamity class, for being involved in many accidents....

, K17-K21, the original orders being cancelled.

They were initially intended as coastal bombardment vessels, submarine monitors
Monitor (warship)
A monitor was a class of relatively small warship which was neither fast nor strongly armoured but carried disproportionately large guns. They were used by some navies from the 1860s until the end of World War II, and saw their final use by the United States Navy during the Vietnam War.The monitors...

, but their role had been changed before detailed design begun. The intention was that merchant ships could be engaged at periscope depth or on the surface using the gun, rather than torpedoes. At that time torpedoes were considered ineffective against moving warships at more than 1000 yards (900 m). Nevertheless it is unlikely that a well-constructed ship would be sunk by a single 12-inch (305 mm) shell hit.


The guns were 12-inch (305 mm) 40 calibre Mark IX guns from spares for the Formidable-class battleships
Formidable class battleship
The Royal Navy's Formidable class battleships was an eight-ship class of pre-dreadnoughts designed by Sir William White and built in the late 1890s...

. The mounting allowed them to elevate by 20 degrees, depress 5 degrees and train 15 degrees in either direction from the centre line. The weapon was normally fired from periscope depth using a simple bead sight on the end of the gun aligned with the target through the periscope at a range of around 1200 metres. The exposure time of the gun above the surface was around 75 seconds. The submarine had to surface to reload the gun, which would take about 3 minutes. In practice the concept was not very successful and only three of the four M-class boats ordered were completed, all between 1917 and 1918. M-class submarines are sometimes called submarine monitors.

M1 and M2 also had four 18-inch (450-mm) torpedo tubes whilst M3 and M4 had 21-inch (533 mm) diameter tubes and were 3 metres longer to accommodate them.


  • M1
    HMS M1
    HMS M1 was a submarine of the British Royal Navy, one of four vessels of her class ordered towards the end of the First World War. She sank with the loss of her entire crew in 1925....

     was the only one to enter service before the end of World War I
    World War I
    World War I , which was predominantly called the World War or the Great War from its occurrence until 1939, and the First World War or World War I thereafter, was a major war centred in Europe that began on 28 July 1914 and lasted until 11 November 1918...

     but did not see action. She was captained during her sea trials by experienced submariner Commander Max Horton
    Max Kennedy Horton
    Admiral Sir Max Kennedy Horton, GCB, DSO and two bars was a British submariner in World War I and commander-in-chief of the Western Approaches in the latter half of World War II, responsible for British participation in the Second World War's Battle of the Atlantic.-First World War:Horton joined...

     after his return from the Baltic, and was later lost with all hands while on exercise in the English Channel
    English Channel
    The English Channel , often referred to simply as the Channel, is an arm of the Atlantic Ocean that separates southern England from northern France, and joins the North Sea to the Atlantic. It is about long and varies in width from at its widest to in the Strait of Dover...

     near Start Point in Devon
    Devon is a large county in southwestern England. The county is sometimes referred to as Devonshire, although the term is rarely used inside the county itself as the county has never been officially "shired", it often indicates a traditional or historical context.The county shares borders with...

     after a collision with a Swedish collier, SS Vidar, on 12 November 1925. The wreck of M1 was discovered by a diving team led by Innes McCartney in 1999 at a depth of 73 metres. Later that year the wreck was visited again by Richard Larn
    Richard Larn
    Richard James Vincent Larn, OBE is a retired Chief Petty Officer in the Royal Navy, a businessman and maritime history writer who is widely regarded as one of Britain's leading historic shipwreck experts.-Career at Sea:...

     and a BBC TV documentary crew, and the resulting film was aired in March 2000.

  • M2
    HMS M2
    HMS M2 was a Royal Navy aircraft-carrying submarine shipwrecked in Lyme Bay, Dorset, Britain, on 26 January 1932. She was one of three M-class boats completed.Four M-class submarines replaced the order for the last four K-class, K17-K21...

     was converted to a seaplane carrier in 1925, a hangar replacing the gun turret. She was lost off Chesil Beach
    Chesil Beach
    Chesil Beach, sometimes called Chesil Bank, in Dorset, southern England is one of three major shingle structures in Britain. Its toponym is derived from the Old English ceosel or cisel, meaning "gravel" or "shingle"....

     on 26 January 1932. It is thought that the hangar door was opened prematurely. M2 lies in much shallower water, 32 metres deep with the top of the conning tower only 20 metres below the surface at low tide. She is a popular attraction for local scuba divers
    Scuba diving
    Scuba diving is a form of underwater diving in which a diver uses a scuba set to breathe underwater....

     with as many as six boats anchored above her on busy days.
  • M3
    HMS M3
    HMS M3, built by Armstrong Whitworth, Newcastle Upon Tyne was an M class submarine of the Royal Navy.M3 was ordered from Armstrong Whitworth on 28 July 1916 and laid down at Elswick in December as an M-class submarine, but was referred to as K20. She was launched on 19 October 1919, and...

     was converted to a minelayer in 1927 with stowage for 100 mines, primarily to test the mine-handling equipment of the Porpoise class
    Grampus class submarine
    The Grampus-class submarines were a group of minelaying submarines built for the Royal Navy in the late 1930s. These boats are sometimes referred to as the Porpoise class from the single prototype, HMS Porpoise built in 1932. Five boats to a modified design were built between 1936 and 1938...

    . The mines were carried on a conveyor belt which ran along her upper deck and covered over by an enlarged casing. The mines were laid through a door at the stern. She was scrapped in 1932 after the trials had been completed.
  • M4
    HMS M4
    HMS M4 was an M-class submarine of the Royal Navy built by Armstrong Whitworth, Newcastle Upon Tyne, and laid down in 1917. M4 was cancelled and sold as an incomplete hulk on 30 November 1921.- References :...

     was broken up before completion.

In 1924 all three completed members of the class were used to test hull camouflage
Camouflage is a method of concealment that allows an otherwise visible animal, military vehicle, or other object to remain unnoticed, by blending with its environment. Examples include a leopard's spotted coat, the battledress of a modern soldier and a leaf-mimic butterfly...

 to reduce the visibility of submarines from aircraft—M1 was painted grey-green, M2 dark grey and M3 was painted dark blue.

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