Lord Fisher submitted a proposal to the First Lord, Lord Balfour, in August of 1915 to mount a 60 ton 12 inch gun from a battleship of the Majestic class. The gun was to be situated in front of the conning tower.
The idea was then to search for a target at periscope depth of 30 feet. Once the target was in sight the submarine was to be lined up with the target with the gun elevated at the appropriate angle. With about 6 feet of the gun barrel above the surface, the gun would be fired and the submarine would return to a safe depth.
The idea was eventually approved with orders for four, two at Vickers and two with Armstrong Whitworth using the keels of (See Footnote) K18, K19, K20 and K21. A surface speed of 15 knots was achieved during trials using two Vickers designed 12 cylinder diesels. Submerged they were driven by four double armature motors able to reach a speed of 9 knots.
The boats were reported as being very stable when handling and were able to dive in 30 seconds. The gun proved reliable, even after being underwater for several hours. The firing and submerging operation took 55 seconds. These boats had a diving depth of 200 feet.
The first orders were placed for the first of the class in February 1916 but it was not until April 1918 that M1 was ready.
After the Washington Disarmament Treaty of 1920 which stated that no submarine could have a gun larger than an 8 inch, M2 and M3 had their guns removed.
M2 was refitted with a seaplane hangar replacing the gun and a catapult to launch a small Parnall Peto seaplane.
M3 was converted to an experimental minelayer in 1927, with 100 mines on rails inside a free-flooding casing outside the hull. The mines were laid over the stern by means of a conveyor belt. This method proved to be more than efficient.
These boats (as K's) were originally armed with four 18 inch bow tubes but M3 had these replaced with four 21 inch.
4 double armature motors 400 bhp each.
9 kts sm
M2 110 ton
4 x 21 inch tubes or 4 x 18 inch tubes.
10 miles at 10 kts sm.