Dark-class patrol boat

Class overview
Name	Dark class
Builders	Saunders-Roe; Vosper & Company; Taylor, Chertsey; McGruer & Co Ltd, Clynder; Miller, St.Monance; Thornycroft; Morgan Giles, Teignmouth;
Operators	Royal Navy; Myanmar Navy; Finnish Navy; Japan Maritime Self-Defense Force;
Preceded by	Gay class
Succeeded by	Brave class
Built	1954–1958
In commission	1954–1971
Planned	35
Completed	26
Cancelled	9
Active	0
Retired	26
General characteristics
Type	Fast patrol boat
Displacement	50 long tons (51 t) (standard); 64 long tons (65 t) (deep);
Length	71 ft 8 in (21.84 m)
Beam	19 ft 5 in (5.92 m)
Draught	6 ft 1 in (1.85 m)
Propulsion	2 shafts, Napier Deltic 18 cylinder two-stroke opposed-piston diesels
Speed	40 knots (46 mph; 74 km/h)
Complement	15
Armament	Either:; 1 × 40 mm guns; 4 × 21-inch torpedoes; Or:; 1 × 40 mm gun; 1 × 4.5-inch gun;

The Dark class, or Admiralty "Type A", were a class of eighteen fast patrol boats that served with the United Kingdom's Royal Navy starting in 1954. All were named with a prefix of 'Dark'. The class could be fitted as either motor gun boats or motor torpedo boats, depending on the type of armament carried. They were the only diesel engined fast patrol boats in the Royal Navy. The class was fitted (along with the Nasty class) with the Napier Deltic two-stroke diesel engine. This was of unique layout, an opposed-piston engine with a triangular layout of three banks, 18 cylinders in total.