M Class

Minelaying Submarine

L Class Minelaying Submarine


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.

1 Converted

Reconstructed by H.M. Dockyard, Chatham.

Technical Information

1,450 sf
1,950 sm

Length (ft):
303 (oa)

Breadth (ft):

Draught/Height (ft):

2 x 12 cylinder Vickers Diesel engines 2,400 bhp.
4 double armature motors 1600 bhp.
Twin Screws

Speed (kts):
15.5 kts sf
9.5 kts sm

Oil Supply (tons):

4 x 21 inch bow tubes.
(8 torpedoes carried).
4 x 0.303 inch Lewis MG.
100 'S' Mines.

5700 nm at 16 kts sf.
80 miles at 10 kts sm.

67 officers and ratings.