Originally intended for use in training aircraft, this engine became far better known as the power-plant of Naval airships. It was designed, in 1915, for an output of 75 h.p. at 1,350 r.p.m., but ratings soon increased. In February 1916 it was giving 91 h.p.; in October 1918, 94 h.p.; and later in the same month, at 1,500 r.p.m., 105 h.p.
In the R.N.A.S. volume dealing with the S.S. Zero airship one reads: "The Rolls-Royce Hawk type engine of 75 h.p. was adopted as possessing a degree of reliability and other properties of particular suitability higher than other makes available."
Of 55,700 hours flown by British airships, 36,000 were put in by Hawk-engined craft. In August 1918 a flight of 50 hr 55 min duration was recorded, and patrols of 23-30 hr were not uncommon. Heavicr-than-air machines fitted with the Hawk included the Sage trainer Type III, a few B.E.2Es, and Avro 50417s.
A six-cylinder, ungeared, vertical water-cooled unit, the Hawk had one magneto and one carburettor. Oil consumption was half a gallon an hour. An official Rolls-Royce description ran: "It is usual, when starting this small engine, to turn it by means of the propeller for filling the cylinders after the induction pipes have been primed. The operation of the hand magneto (supplied by Messrs. Rolls-Royce) then starts the engine. The Rolls-Royce Patented Device is supplied for priming. This is a light and simple apparatus, embodying a hand pump, which can be fixed in any convenient position near the pilot's seat, or as desired. One priming device may serve two or more engines with the use of a change-over cock. When required, a starting handle can be supplied, arranged in line with the crankshaft at the timing gearease end of engine, and connected thereto by a reduction gear. This apparatus is specially suitable for airship installations."
Data for the Hawk with a nominal output of 100 h.p. at 1,500 r.p.m. were: max. permissible r.p.m., 1,700; fuel consumption at normal power, 6.5 gal/hr; weight, 405 lb.