Dial ‘M’ for Muddle…
The original concept was a knee-jerk reaction to reports of German U-cruisers; submarines armed with 5.9-inch guns which would surface to sink merchant ships with gunfire. So, in 1916 the Admiralty decided to introduce a submarine armed with one 12-inch gun of the type carried by Formidable class battleships. As a result they were big vessels, 300 feet long and weighing nearly 1,600 tonnes.
Initially intended for use as monitors, to bombard targets on shore, their role then changed to sea combat because the huge gun would have a greater effective range than a torpedo. They would rise to periscope depth, fire one round (the gun could not be reloaded while the boat was submerged) and dive out of harm’s way. Since most battleships had eight guns which they would fire repeatedly, it is not clear how a single gun fired once could have any significant effect.
Although launched in 1917, HMS M1 never saw action. The navy feared that the Germans might retaliate by arming U-boats with equally large guns, which rather calls into question the logic of building her in the first place. Two more M-class boats were built and launched after the war.
While on exercise off the coast of Devon in 1925, HMS M1 was accidentally rammed by a merchant vessel, the SS Vidar, while submerged. Her massive gun turret was sheared off and she sank with all hands. Incredibly, the collision was so light that the captain of the Vidar was unaware that he had hit another vessel, a fact that highlights what a weak point the gun must have been.
Following the Washington disarmament treaty, which limited submarine guns to a maximum of 8-inch calibre, both HMS M2 and M3 had their 12-inch guns removed and M2 had her conning tower converted to a hangar, from which a seaplane could be catapulted. This could be retrieved by crane once its reconnaissance mission was completed.
In 1932, during trials in Lyme Bay, HMS M2 sank with all hands. All we know for certain is that the hangar doors were open and the boat was flooded. Whether they were not fully closed before diving or opened too soon on surfacing remains a mystery.
HMS M3, meanwhile, had been converted into an experimental mine-layer in 1927. She was able to lay 100 mines using a conveyor belt system, a task which she seems to have conducted successfully. However, she was taken out of service in April 1932, three months after her sister ship sank. Fortunately, by the 1930s, the Admiralty had developed a much clearer strategy for submarine operations and future classes, such as the S, T, and U, proved successful throughout the Second World War and continued in service for decades.