1916 - 1932: M Class
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.
Four K Class submarines, K18 to K21, had been ordered in February 1916 but, when Vickers received an order to build a boat to the new design, K18 was remodelled and became M1. M2 (ex K19) was ordered from Vickers in May 1916 and two more, M3 (ex K20) and M4 (ex K21), from Armstrong Whitworth in August 1916.
Of the three M Class submarines built (M4 was scrapped before completion), only M1 was completed before the end of the war, though she was never used in action because it was thought that if she was copied by the enemy Britain was likely to suffer more from the use of the 12-inch gun than Germany. Why this was never thought of before construction remains a mystery. M2 and M3 were completed in 1920.
The fact that the M Class design got further than the conference table is a reflection of the failure by the Admiralty to recognise the proper nature of submarine operations. 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. Known as the 'dip-chick' method, this attack procedure took about 30 seconds to complete.
The M Class were partial double-hulled submarines with the double-hull extending for about 65% of the length of the boat. A surface speed of 15 knots was attained from two Vickers 12-cylinder diesel engines, each of 1200 bhp.
Under water, four double armature-type main motors, generating 400 bhp each, gave a submerged speed of 9 knots. Power for submerged operations was supplied from three battery tanks containing a total of 336 Exide cells.
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.
On October 25th, 1925, M1 was rammed by the SS Vidal off Start Point and was lost without survivors.
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.
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.
Although the M Class may be considered unsuccessful because their big guns were never used for the purpose intended, they were popular with their crews and were claimed to be handy under water, quick to dive and easy to handle.