Sea Lion III was entered for the 1923 Schneider Trophy contest at Cowes. Supermarine's test pilot Henry Biard flew the aircraft and managed third place, reaching a speed of 151.16 mph (243.27 km/h). The performance of the Sea Lion III led to Supermarine designing seaplanes instead of flying boats as racers.
The Sea Lion II was flown by Henry Biard, who won the race at an average speed of 145.7 mph (234.5 km/h),[2] The victory was the first post-World War I success by a British aircraft in an international competition[5]
For the 1923 Schneider Race, which was held at Cowes on the Isle of Wight, the Sea Lion II was re-engined with a 550 hp (410 kW) Napier Lion and renamed as the Sea Lion III.[2] Mitchell modified the hull so as to reduce drag forces,[6] and he gave the aircraft two bay wings and a larger rudder area.[2] He expected the Sea Lion III to attain speeds in excess of 160 miles per hour (260 km/h);[6] the aircraft managed third place behind the AmericanCurtiss CR-3 seaplanes, reaching a speed of 151.16 mph (243.27 km/h).[2] Supermarine's managing director Hubert Scott-Paine said after the trophy was won by the Americans:[7]
Our drawing office people got all the speed they possibly could out of the machine. We did the best we could and have no regrets. Sea Lion III was 11 mph faster than Sea Lion II, and the credit for this fine performance was due to several people, one of whom is Mr. R.J. Mitchell, who designed both machines.
The British defeat caused Supermarine to abandon using outclassed flying boats as racers, in favour of seaplanes.[7] The Sea Lion III was transferred to the Royal Air Force in 1923.[2]