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.