The Supermarine Seagull was a flying boat produced by the British aircraft manufacturer Supermarine. It was developed by Supermarine’s chief designer R.J. Mitchell from the experimental Supermarine Seal II.

Development of the Seagull started during 1920. In June 1921 it was evaluated for military applications but was rejected, and so Supermarine developed the aircraft as a private venture. During February 1922, an initial order for two aircraft was placed by the Air Ministry; subsequent production of the Seagull is believed to have assisted Supermarine to survive during a period when the aircraft industry struggled to attract customers.

The Seagull was used by the British Fleet Air Arm for gunnery spotting and reconnaissance duties. It was operated by the Royal Australian Air Force for similar purposes. During the early 1930s, when the type was being replaced by the more successful Supermarine Walrus, a number of Seagulls were re-used for civilian purposes.

The origins of the Seagull are heavily interconnected with the Supermarine Seal and the Supermarine Commercial Amphibian. Work on the Seal, which started in 1920, sought to build upon the Commercial Amphibian. A range of alterations and improvements were incorporated, many of which were drawn from the Commercial’s official review. The Commercial Amphibian was later designated as the Seal Mk I—and the new design, initially known as the Seal Mk II, later became the Seagull.

N158, the prototype of the Seagull, flew for the first time in May 1921. On 2 June 1921, having completed manufacturer’s trials, it was handed over for service trials with the Royal Air Force. Attention was paid to the aircraft’s seaworthiness and handling characteristics at low speeds, including its relatively low landing speed. Trials revealed that the rudder gave poor yaw characteristics. New designs were tested until a fin extension was found to solve the issues, which was incorporated into the prototype. Having been sufficiently impressed by the aircraft’s performance, two aircraft were ordered by the Air Ministry in February 1922. The prototype was shown at the 1922 RAF Display at Hendon Aerodrome.

The Supermarine Seagull was an amphibian flying boat, powered by a single Napier Lion engine. This engine was mounted in a nacelle slung from the aircraft’s upper wing and powered a four-blade propeller in a tractor configuration. The Seagull employed gravity feed to supply fuel for the engine, and was the first single-engined flying boat to use this method.

The lower wing was set in the shoulder position and had two bays. The floats were attached to the lower wing near the wingtips via struts, their positioning maximised buoyancy. For land operations, the Seagull was equipped with a retractable undercarriage; pilots lacked aids such as indicators or alarms, thus were reliant on training and memory to deploy the undercarriage when applicable. For easier stowage on board ships, the wings were designed to be folded, which necessitated mounting the wings in a relatively forward position on the fuselage.

The fuselage had an oval cross-section and had a planing bottom with two steps. The interior of the fuselage was divided into several watertight compartments. The three-man crew—pilot, observer, and radio-operator—each had an open cockpit. The pilot was seated in a relatively forward position, at a distance from the other crew members, being directly ahead of the fuel tanks; the cockpit was provisioned with a single retractable machine gun. The radio operator was located just behind the wing, while the rear gunner position was further back still. The majority of the aircraft was constructed from wood.[9] Previous Supermarine aircraft had incorporated the fuel tanks into the hull; with their removal to the upper wing, the Seagull’s crew gained full access within the interior of the aircraft.

On 4 July 1922, the name Seagull was adopted for the type. The prototype was designated as the Mk I; the subsequent production aircraft were given the designation Seagull Mk II. The two versions were similar, although an alternative powerplant in the form of a single more powerful Napier Lion III engine, a reduced wingspan, and a larger fin were used in the Mk II. Production of the Seagull (with serial numbers in and around N9642–N9647) began in 1922. In total, 25 aircraft were built for the Air Ministry and the Royal Navy; their production has been seen as critical to the survival of Supermarine at this time, and the first orders were intentionally built in small batches to ensure that the firm received enough business to stay operational.

In 1925, construction of the improved Mk III began for the Royal Australian Air Force (RAAF), following an order being placed in January of that year. This type featured a more powerful Napier Lion V engine, and the radiators were redesigned so as to be able to operate in tropical regions. The RAAF received six Mk IIIs were between 1926 and 1927. In 1928, one of the Seagull Mk IIs (the so-called Mk IV) was rebuilt with Handley-Page leading edge slots and twin fins and rudders.

Seagull III were delivered to the RAAF in 1926-27 to replace Fairey IIID. They were flown by No 101 Fleet Co-operation Flight, an RAAF unit under the operational control of the RAN. They flew briefly from the RAN seaplane carrier HMAS Albatross and subsequently from cruisers. Under-powered, incapable of catapult launch the Seagulls were withdrawn from service in 1934.

Powerplant: 1 × Napier Lion IIB W-12, 492 hp (367 kW)
Wingspan: 46 ft 0 in (14.02 m)
Length: 37 ft 9 in (11.51 m)
Height: 14 ft 0 in (4.27 m)
Wing area: 593 sq ft (55.1 sq.m)
Empty weight: 3,820 lb (1,733 kg)
Gross weight: 5,691 lb (2,581 kg)
Maximum speed: 98 mph (158 km/h, 85 kn) at 3,000 ft (914 m)
92 mph (80 kn; 148 km/h) at 6,500 ft (1,981 m)
Endurance: four hours, 30 minutes
Service ceiling: 9,150 ft (2,790 m)
Time to altitude: 3,000 ft (914 m) in7 minutes 43 seconds
Armament: Guns: One × .303 in (7.7 mm) Lewis gun
Crew: 3 (pilot, wireless operator & observer)