The M3 arrived at H.M. Dockyard, Chatham in June 1927 and the conversion from Monitor to Minelayer was completed in one year. The 12 inch gun and 3 inch HA were removed to obtain the space required for the mines and equipment.
The mine compartment consisted of a huge free-flooding casing arranged on top of the pressure hull. This extended from the bulkhead between the torpedo stowage and the crew's mess to the stern. Enclosed within the casing were two sets of mine rails and the space to carry 100 mines and their trolleys. The mines were moved along the rails and dropped over the stern.
The conversion also gave the boat an additional stowage of fuel and decreased the displacement by 150 ton.
4 double armature motors 1600 bhp.
9.5 kts sm
(8 torpedoes carried).
4 x 0.303 inch Lewis MG.
100 'S' Mines.
80 miles at 10 kts sm.