M Class submarines

Inspired by the news of German U-cruisers with 5.9-inch guns, the Committee on Submarine Development decided to construct submarine monitors - the M Class - with 12-inch guns.

Although their 12-inch guns were ideally suited for bombarding coastal defences, their method of attack at sea was rather primitive.The attack procedure was to cruise at periscope depth until the target was lined up'. The submarine was then brought-up until about six feet of the gun barrel protruded from the water.
A round was fired and the submarine would then make a rapid dive - unable to fire again as the gun could not be reloaded under water.

m1
At the end of the war, the question of future employment arose as 'there were no targets and the enemy had not initiated anything bold in submarine policy'. Consequently, during the 1920s, the three M Class submarines led contrasting lives.
M1
M2
M3

On October 25th, 1925, M1 was rammed by the SS Vidal off Start Point and was lost without survivors.

m1

M2 and M3 had their large 12-inch guns removed in the late 1920s to conform with the Washington Disarmament Treaty, which stated that no submarine should have larger than 8-inch calibre guns. M2 was refitted with a seaplane hangar forward of the conning tower and a catapult to launch a small Parnall Peto seaplane.

M2

Parnall Peto seaplane
Specially designed for M2s hanger, the Parnell Peto Seaplane had a 135 hp engine and an endurance of about two hours at 70 knots

This conversion was a success, and M2 could surface from periscope depth, open the hangar door, catapult the plane, close the door and dive again within five minutes. She was subsequently lost during exercises in the English Channel in 1932 when her hangar door was left open.

In 1927, M3 was converted to an experimental minelayer, stowing her 100 mines on rails inside a large free-flooding casing outside the hull. The mines were laid over her stern by means of a chain-conveyor belt. M3 was finally taken out of service in April 1932, and scrapped in 1933.

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From: http://www.submariners.co.uk/Boats/Barrowbuilt/M_Class/index.htm