The first prototype Fulmar, which acted as a "flying mock-up", was powered by a single Rolls Royce Merlin III engine, which was capable of generating up to 1,080 hp (810 kW). Flight testing revealed the prototype's performance to be relatively poor, the highest speed it could attain being 230 mph (370 km/h). Following the adoption of the more powerful Merlin VIII engine – a variant unique to the Fulmar and with supercharging optimised for low-level flight, as well as various aerodynamic improvements made to the airframe, the prototype's speed was increased to 265 mph (426 km/h) when flown at an altitude of 7500 ft (2286m). Due to the desperate requirement for more modern fighters to equip Britain's carrier fleet, the Fulmar's performance was considered adequate.