The fact that the M Class design got further than the conference table is a reflection of the failure by the Admiralty to recognize 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.
