Supermarine Sea Lion III

Supermarine Aviation Works

Industry	Aviation
Founded	1913 (1913)
Founder	Reginald Mitchell
Headquarters	Southampton , United Kingdom
Key people	Reginald Mitchell (chief designer), Hubert Scott-Paine (managing director)
Products	Aircraft, including Supermarine Sea Lion III flying boat

For the 1919 Schneider Trophy entrant, see Supermarine Sea Lion I. For the 1922 Schneider Trophy entrant, see Supermarine Sea Lion II.

The Supermarine Sea Lion III was a British racing flying boat developed by Supermarine Aviation Works. Conceived by Reginald Mitchell, it represented an evolution of the earlier Supermarine Sea Lion II model. Propulsion was provided by a 550 hp (410 kW) Napier Lion powerplant.

Entered in the 1923 Schneider Trophy competition held at Cowes, the Sea Lion III was piloted by Supermarine’s test aviator, Henry Biard, who secured third place with a top speed recorded at 151.16 mph (243.27 km/h). This outcome influenced Supermarine’s strategic shift from flying boats to seaplanes for future racing designs.

Development

In preparation for the 1922 Schneider Trophy contest, the British manufacturer Supermarine Aviation Works fielded an entry without government backing, unlike their Italian and French counterparts who received state support. Their contender was a modified version of the Supermarine Sea King II fighter, a single-seat biplane amphibian powered by a 300 hp (220 kW) Hispano Suiza engine arranged in pusher configuration, which had its maiden flight in 1921. The design originated at Supermarine’s Woolston facility in Southampton under the leadership of chief designer and engineer Reginald Mitchell. For this model, a 450 hp (340 kW) Napier Lion engine was provided on loan from D. Napier & Son.

1923 Schneider Trophy race

File:1923 Schneider Trophy course.svg

The course of the 1923 Schneider Trophy race

In the 1922 race, pilot Henry Biard flew the Sea Lion II to victory, averaging speeds of 145.7 mph (234.5 km/h), marking Britain’s first international aviation competition win following World War I.

For the subsequent 1923 event at Cowes, held on the Isle of Wight, the aircraft underwent significant enhancements: it was re-engined with a more powerful 550 hp (410 kW) Napier Lion and rebranded as the Sea Lion III. Designer Mitchell refined the hull to minimize aerodynamic drag and introduced two-bay wings along with an enlarged rudder surface. Expectations were high for speeds surpassing 160 miles per hour (260 km/h), though ultimately the aircraft finished third behind American Curtiss CR-3 seaplanes, achieving a peak speed of 151.16 mph (243.27 km/h). Following the race, Supermarine’s managing director Hubert Scott-Paine remarked that their design team had maximized the aircraft’s speed capabilities and expressed pride in their efforts, particularly acknowledging R.J. Mitchell’s contributions to both models.

The disappointing result prompted Supermarine to discontinue pursuing flying boats for racing purposes, opting instead to concentrate on seaplane development. In 1923, the Sea Lion III was subsequently handed over to the Royal Air Force.

Operators

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• Royal Air Force
• Supermarine Aviation Works