Shortly after World War I, Supermarine received two Air Ministry contracts to Specification 21/22, one in 1921 for a five-seat military seaplane and the other in 1922 for a commercial seaplane, the former named Scylla and the latter Swan.
The aircraft were equal-span, two-bay biplanes with monoplane tail units and triple fins and rudders. The raised cockpit seated the two-crew side-by-side.
The Scylla N174 emerged as a monoplane, but was apparently converted to triplane configuration in 1923, when it carried out taxi trials at Felixstowe. There is no evidence that it ever flew.
The Swan N175 was completed as planned in 1924, but as a maritime reconnaissance flying-boat biplane. After completing its flight trials, it was converted to civilian use. An Air Ministry Specification R18/24 was issued for a development, later built by Supermarine as the first Southampton.
Engine: 2 x 2 x 450hp Napier Lion IIB
Max take-off weight: 5820 kg / 12831 lb
Wingspan: 20.90 m / 69 ft 7 in
Length: 17.06 m / 56 ft 12 in
Height: 7.95 m / 26 ft 1 in
Max. speed: 175 km/h / 109 mph
Ceiling: 3089 m / 10150 ft
Range: 1400 km / 870 miles
Crew: 2
Passengers: 10
All Aero 