British Freak Submarines – The Disastrous M-Class

© IWM (Art.IWM ART 3065)
© IWM (Art.IWM ART 3065)

During the last months of the first world war, the Royal Navy built the M-Class submarines. These submarines were diesel-electric and had a feature that was unique, they had a gun (12 inches) mounted on a turret in front of the conning tower. These particular submarines are sometimes known as submarine monitors.

They were designed to be used in bombardment against coasts held by the enemy; however, their use changed well before construction had even begun.

Their new mission was to engage any merchant ships and remain at periscope depth while attacking with their gun. It would also be able to surface to use the gun, instead of wasting torpedoes.

Torpedoes were generally classed as ineffective when used on moving warships that were more than 1000 yards away.

The gun, when fired at a shorter range, would follow a flat trajectory which simplified the aiming process, and they expected not many ships to survive a shot from it.

The guns used were taken from Formidable-Class warships, which were no longer used by the back end of 1919. The elevation of the guns could reach 20 degrees; it could depress by 5 degrees and train up to 15 degrees in any direction.

Sectioned model in the Science Museum, showing the 12 in (300 mm) turret, by Andy Dingley CC BY-SA 3.0 / Wikipedia
Sectioned model in the Science Museum, showing the 12 in (300 mm) turret, by Andy Dingley CC BY-SA 3.0


The gun was generally fired when the submarine was at periscope depth; this required using a simple bead sight at a range of approximately 1000 yards.

A big disadvantage of this was the fact the sub had to surface each time the gun required reloading, and this took 3 minutes each time.

m1 big
M1 Submarine Monitor

The idea didn’t really take off, and they only ended up building 3 of the 4 M-Class suns that were ordered. These were completed in 1917-1918.

M1 Firing its guns
M1 Firing its guns

The M1 & M2 submarines also held four torpedo tubes (18 inches), and the M3 and M4 had 21 inches; this required them to have tubes 3 meters long!
Submarine armament was s limited by the Washington Naval Conference in 1922, and so the M2 and M3 had their guns removed from them.

The M2 model was adapted to carry a small seaplane, and the M3 was converted into a minelayer.

M1 docked

M1 Monitor Submarine

HMS M1 from airport bow

The M1 submarine was the only one of the 3 to enter service whilst World War 1 was still going; however she didn’t see any action. Whilst undergoing sea trials her captain was Commander Max Horton.

She was lost, with all crewmembers, in 1924 (November 12th) while exercising in the English Channel when she collided with a Swedish collier ship (SS Vidar).

The SS Vidar struck the M1 whilst she was submerged, and she sank in 70 meters of water. The force of the collision ripped the gun from her hull and allowed water to flood into the sub.

The crewmembers appeared to have attempted escape by flooding the interior of the sub and opening the escape hatch, but no bodies were found.

The M1 wreckage was rediscovered by Innes McCartney and his diving team in 1999. She was at depths of 73 meters. Later on, in 1999 a BBB documentary crew visited, with Richard Larn, and they released a film in March 2000.

M2 Aircraft Carrier Submarine

British Submarine HMS M2

The M2 was transformed, in 1925, into a seaplane carrier and they replaced the gun turret with a hangar. The M2 was lost in 1932 (January 26th) when off the coast of Chesil Beach. It is suspected that someone opened the doors to the hangar prematurely.

The M2’s entire crew of 60 people was all killed. The M2 was discovered on February 3rd, a total of 6 days after her accident. Ernest Cox was called in to lead a salvage team to work on the M2.

This mission lasted almost a year and took 1,500 dives. Finally, on December 8th, 1932, they managed to raise her to 20 feet below the surface, but a gale appeared and sent her back down below.

M 2 Hanger with the Peto inside
M 2 Hanger with the Peto inside

The M2 now sits in waters of 32 meters depth, and at low tide her conning tower sits only 20 meters below the surface.

Scuba divers find this area a great place to dive; it is not unusual to see 5-6 boats anchored above the wreckage on busy tourist days.

M3 Minelayer Submarine

HMS M3 submarine

The M3 was transformed into a minelayer in 1927 and had storage created that could hold 100 mines. These mines were carried along a conveyor belt along her deck and then taken through a door at her stern.

M3, original monitor configuration
M3, original monitor configuration
M3, minelayer configuration (1928 onwards)
HMS M3 submarine minelayer

M3, minelayer configuration (1928 onwards)

The decision was made to scrap the M3 in 1933, but this ended up happening earlier as she was getting unfavorable reports of her service.

She was scrapped in April 1932.


The M4 was never completed.


Joris Nieuwint

Joris Nieuwint is a battlefield guide for the Operation Market Garden area. His primary focus is on the Allied operations from September 17th, 1944 onwards. Having lived in the Market Garden area for 25 years, he has been studying the events for nearly as long. He has a deep understanding of the history and a passion for sharing the stories of the men who are no longer with us.