or arrangement of engine controls. Power ratings quoted are usually maximum "military" power. All but the Merlin 131 and 134 engines were "right-hand tractor", i.e. the propeller rotated clockwise when viewed from the rear. In addition to the mark numbers, Merlin engines were allocated experimental numbers by the Ministry of Supply (MoS) – e.g.: RM 8SM for the Merlin 61 and some variants – while under development; these numbers are noted where possible. Merlin engines used in Spitfires, apart from the Merlin 61, used a propeller reduction ratio of .477:1. Merlins used in bombers and other fighters used a ratio of .42:1. Data from Bridgman (Jane's) unless otherwise noted: Merlin II (RM 1S) at 3,000 rpm at using + 6 psi boost (41 kPa gauge; or an absolute pressure of 144 kPa or 1.41 atm); used 100% glycol coolant. First production Merlin II delivered 10 August 1937. Merlin II used in the Boulton Paul Defiant, Hawker Hurricane Mk.I, Supermarine Spitfire Mk.I fighters, and Fairey Battle light bomber. Merlin III (RM 1S) Merlin III fitted with "universal" propeller shaft able to mount either de Havilland or Rotol propellers. From late 1939, using 100-octane fuel and +12 psi boost (83 kPa gauge; or an absolute pressure of 184 kPa or 1.82 atm), the Merlin III developed at 3,000 rpm at ; using 87-octane fuel the power ratings were the same as the Merlin II. Used in the Defiant, Hurricane Mk.I, Spitfire Mk.I fighters, and Battle light bomber. First production Merlin III delivered 1 July 1938. Merlin X (RM 1SM) at 3,000 rpm at ; maximum boost pressure +10 psi; this was the first production Merlin to use a two-speed supercharger; Used in Halifax Mk.I, Wellington Mk.II, and Whitley Mk.V bombers. First production Merlin X, 5 December 1938. Merlin XII (RM 3S) ; fitted with Coffman engine starter; first version to use 70/30% water/glycol coolant rather than 100% glycol. Reinforced construction, able to use constant boost pressure of up to +12 psi using 100-octane fuel; Used in Spitfire Mk.II. First production Merlin XII, 2 September 1939. Merlin XX (RM 3SM) at 3,000 rpm at ; two-speed supercharger; boost pressure of up to +14 psi; Used in Hurricane Mk.II, Beaufighter Mk.II, Halifax Mk.II and Lancaster Mk.I bombers, and in the Spitfire Mk.III prototypes (N3297 & W3237). First production Merlin XX, 4 July 1940. Merlin 32 (RM 5M) at 3,000 rpm at ; a "low