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Naval Warfare

HMS M1 (M-class)

Diesel-Electric Coastal Attack Submarine [ 1918 ]

HMS M1 led her class of three-strong and featured the largest guns ever fitted to a submarine.

Authored By: Staff Writer | Last Edited: 01/06/2021 | Content ©www.MilitaryFactory.com | The following text is exclusive to this site.

By the time of World War 1 (1914-1918), British diesel-electric submarines carried the requisite torpedo armament for undersea work and small-caliber deck guns for surface work. The latter was primarily used against poorly-defended merchant types and proved much more cost-effective when compared to a single torpedo. The same weapon could, to some extent, be brought to bear against land-based coastal targets if need be which made them a dual-purpose threat. British submarine engineers did not stop there when envisioning all-new attack weapons for their fleet of submarines and drew up plans for a submersible class of boats to be used to attack fortified shoreline defenses with full-fledged naval artillery weapons.

This resulted in the "M-class", a Royal Navy submarine group given the largest artillery weapons ever to be fitted to an undersea boat. The group comprised just three vessels in all, designated HMS M1, HMS M2, and HMS M3 and all three carried primary armament of 1 x 12" (305mm) /40 caliber Mark IX naval guns to go along with the more traditional installations of 1 x 3" (76mm) Mk II HA Anti-Aircraft (AA) gun and 4 x 18" (450mm) torpedoes. The main gun was given 50 projectiles from which to work with while only a single salvo of four torpedoes was carried - showcasing the true battlefield nature of these boats (coastal attack).

HMS M1 measured 295.8 feet and had a beam of 24.7 feet, displacing 1,620 tons when surfaced and nearly 2,000 tons when submerged. Power was from a conventional diesel-electric configuration involving 2 x Vickers 12-cylinder diesel units of 2,400 horsepower for surface propulsion and 4 x Electric motors offering 3,200 horsepower for undersea travel. These were used to drive a pair of 3-bladed shafts featured astern.

Speeds reached up to 15 knots when surfaced and fell to 9 knots when submerged. Range was 2,000 nautical miles (2,300 miles) when running on the surface at 15 knots. This could be increased to 4,500 nm (5,200 miles) when cruising at 10 knots. The hull was tested down to a depth of 200 feet.

Aboard was a typical crew complement of sixty two officers and sailors.

While given a traditional submarine-like appearance, with its tapered bow and stern to go with its sail/conning tower, the boat was immediately identifiable thanks to the massive artillery piece integrated into the hull/sail. The elevation span of the gun allowed the submarine to remain submerged while the barrel's muzzle protruded beyond the waterline. Sighting was through a periscope-and-foresight alignment which gave reasonably good accuracy as submerged, bouncing artillery pieces went. The gun fired a massive 850lb projectile towards the target/target area and its reliability was unquestioned.

As an attack submarine, the M1 was never intended to directly assail enemy submarines or even warships but instead target and destroy German emplacements and targets-of-opportunity along the conquered Belgian coastline.

One of the primary deficiencies in the concept was the requirement that the vessel be surfaced to reload. The reloading process was about as fast as reloading a land-based artillery piece - relatively quick - but the action as a whole was time-consuming and left the boat quite vulnerable to return-fire or air attack.

While three total boats were built to the M-class standard, just HMS M1 was launched and operated during the war years. She left her shipyard on July 9th, 1917 and began operations in 1918, spending some time in the Mediterranean Theater - but the war was soon over with the Armistice of November 1918 and thus she never fired her gun in anger.

In 1923, her barrel suffered firing damage from leaking water. Her end came in English Channel waters when on November 12th, 1925 when the Swedish ship SS Vidar collided with the submerged form of HMS M1. She was lost with all hands aboard. Her wreck was not discovered until 1999 at a depth of 240 feet.


Service Year

United Kingdom national flag graphic
United Kingdom

No Longer in Service.


HMS M1; HMS M2; HMS M3; M4 (not completed)

National flag of the United Kingdom United Kingdom
(OPERATORS list includes past, present, and future operators when applicable)
Submerged Attack
Traveling under the surface to search, track, and / or engage or reconnoiter areas.
Maritime Patrol
Active patroling of vital waterways and maritime areas; can also serve as local deterrence against airborne and seaborne threats.
Fleet Support
Serving in support (either firepower or material) of the main surface fleet in Blue Water environments.

295.8 ft
90.16 m
24.7 ft
7.53 m

Installed Power: 2 x Vickers 12-cylinder diesel engines developing 2,400 horsepower with 4 x Electric motors generating 3,200 horsepower to 2 x Shafts astern.
Surface Speed
15.0 kts
(17.3 mph)
Submerged Speed
9.0 kts
(10.4 mph)
1,999 nm
(2,300 mi | 3,701 km)

kts = knots | mph = miles-per-hour | nm = nautical miles | mi = miles | km = kilometers

1 kts = 1.15 mph | 1 nm = 1.15 mi | 1 nm = 1.85 km
1 x 12" (305mm) /40 caliber Mark IX artillery gun (50 x 305mm projectiles carried).
4 x 18" (450mm) torpedo tubes in the bow (four reloads carried).
1 x 3" (76mm) Mk II HA Anti-Aircraft (AA) deck gun.

Supported Types

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