M15-class monitor

	HMS M21
Class overview
Name	M15 class
Builders	William Gray (10); Sir Raylton Dixon (4);
Operators	Royal Navy
Preceded by	Gorgon class
Succeeded by	M29 class
In service	1915–1959
Completed	14
Lost	3
General characteristics
Type	Monitor
Displacement	540 tons
Length	177 ft 3 in (54.03 m)
Beam	31 ft (9.4 m)
Draught	6 ft 9 in (2.06 m)
Propulsion	2 shafts; Triple-expansion; 800 hp (600 kW); 4 shaft; Bolinder 4-cylinder semi-diesel; 640 hp (480 kW); 4 shafts; Campbell 4-cylinder paraffin engines; 560 bhp (420 kW);
Speed	11 knots (20 km/h; 13 mph)
Complement	69
Armament	1 × BL 9.2-inch (233.7 mm) Mk X gun (M15-M18) or BL 9.2-inch Mk VI gun; 1 × 12-pdr (76mm) QF Mk 1 gun; 1 v 6-pdr (57mm) QF MK 1 AA gun;

The M15 class comprised fourteen monitors of the Royal Navy, all built and launched during 1915.

Class overview
HMS M21
Name	M15 class
Builders	William Gray (10) Sir Raylton Dixon (4)
Operators	Royal Navy
Preceded by	Gorgon class
Succeeded by	M29 class
In service	1915–1959
Completed	14
Lost	3
General characteristics
Type	Monitor
Displacement	540 tons
Length	177 ft 3 in (54.03 m)
Beam	31 ft (9.4 m)
Draught	6 ft 9 in (2.06 m)
Propulsion	2 shafts Triple-expansion 800 hp (600 kW) 4 shaft Bolinder 4-cylinder semi-diesel 640 hp (480 kW) 4 shafts Campbell 4-cylinder paraffin engines 560 bhp (420 kW)
Speed	11 knots (20 km/h; 13 mph)
Complement	69
Armament	1 × BL 9.2-inch (233.7 mm) Mk X gun (M15-M18) or BL 9.2-inch Mk VI gun 1 × 12-pdr (76mm) QF Mk 1 gun 1 v 6-pdr (57mm) QF MK 1 AA gun