Supermarine Seal II

Supermarine

Industry	Aerospace
Founder	Hubert Scott-Paine
Headquarters	Woolston, Southampton , United Kingdom
Products	Flying boats, amphibious aircraft

The Supermarine Seal II was a British flying boat crafted by Supermarine following a contract from the British Air Ministry for a prototype three-seat fleet spotting amphibian. Designed by Supermarine’s R.J. Mitchell, the aircraft was intended to operate from Royal Air Force (RAF) aircraft carrier decks. Mitchell integrated feedback gathered after the Supermarine Commercial Amphibian secured second place in the Air Ministry’s 1920 competition.

This model was a biplane featuring its lower wing mounted atop the fuselage. It was equipped with a 450 horsepower (340 kW) Napier Lion water-cooled piston engine, marking it as the first British flying boat to employ a tractor configuration with the propeller positioned ahead of the engine. The pilot occupied a forward cockpit at the nose, with fuel tanks situated directly behind, while a second cockpit was positioned aft of the wing.

The Seal II conducted its maiden flight on 21 May 1921, after which it underwent RAF evaluation. Subsequent modifications included an engine replacement, and the aircraft was redesignated as the Seagull before delivery to the RAF two months later. In November 1921, the Seal II was among several aircraft sold to the Imperial Japanese Navy Air Service and transported alongside the British-led Sempill Mission to Kasuimiguara.

Background

The flying boats constructed by Supermarine at their Woolston, Southampton facility earned a reputation for durability and excellent seaworthiness. Flight magazine praised them as "consistently robust in construction, genuinely boat-built craft—an actual boat that can fly rather than an airplane that merely floats."

The designs of flying boats developed by Supermarine’s R.J. Mitchell, who would later become the company’s chief designer, were presented to the British Air Ministry. Coupled with the strong performance demonstrated by the Supermarine Commercial Amphibian, these factors led to Supermarine securing a contract for a prototype three-seat fleet spotter amphibian. The Air Ministry issued specification 7/20 based on this prototype, calling for a biplane capable of slow-speed landings on Royal Air Force aircraft carriers while maintaining maximum control.

Design

File:Supermarine Seal general arrangement drawings.jpg

Supermarine Seal II general arrangement drawings

File:Supermarine Seal (front view).jpg

Supermarine Seal II (front view)

The aircraft was named Seal II by Hubert Scott-Paine, the owner of Supermarine. Historians generally believe this designation followed Scott-Paine’s temporary naming of the Commercial Amphibian as Seal I.

After the Commercial Amphibian competed at seaplane trials held at Martlesham Heath and Felixstowe in September 1920, Supermarine incorporated feedback from the Air Ministry into the new design. Mitchell completed the initial plans for the Seal II on 25 November 1920.

The Seal II was configured as a two-bay biplane featuring a lower wing mounted atop the fuselage and powered by a 450 hp Napier Lion water-cooled piston engine positioned beneath the upper wing’s center section. It was notable as the first British flying boat to utilize a tractor configuration, with its propeller mounted at the front to pull the aircraft through the air. The wings were designed to fold rearwards. Its fuselage, based on a design by Linton Hope from World War I, had an approximately circular cross-section and was constructed from planked frames over former and longeron structures covered with fabric. The planing surfaces were built separately and divided into watertight compartments.

The Seal’s second cockpit was situated aft of the wing, and it featured an innovative undercarriage retraction system that eliminated reliance on cables. The pilot occupied a covered cockpit located at the nose, with fuel tanks positioned behind him and ahead of the other crew members.

Operational history

The Seal II prototype, bearing the serial number N146, took to the skies for the first time on 21 May 1921. It was formally delivered to the Royal Air Force on 2 June 1921, after which the RAF transported the aircraft to the Isle of Grain for evaluation. The testing phase yielded generally positive results, prompting the RAF to suggest several alterations. These included relocating the fuel tanks from within the hull to the upper wing and enlarging both the rudder and fin. Subsequently, the aircraft received a new powerplant: a 480 horsepower (360 kW) Napier Lion II engine.

Following orders from the Air Ministry early in 1922 for two prototypes built to specification 21/21, Mitchell implemented additional changes to the Seal II. These modifications involved extending the hull by 2 feet (0.61 m), refining the tail surfaces, and reshaping the wings to improve balance. The resulting prototypes were assigned serials N185 and N159 and were renamed Seagulls on 4 July 1921 before their delivery to the RAF. One Seagull was showcased at Hendon during the RAF’s annual Aerial Pageant in June 1922. Additionally, Mitchell designed a variant called the Fleet Spotting Gunnery Amphibian; however, this model never advanced beyond the design stage.

Both a Seal II and a Seagull were transferred to the Imperial Japanese Navy Air Service following Japan’s procurement of aircraft during a fact-finding visit to Britain. These planes were part of the British-led Sempill Mission and arrived at Kasumigaura Air Field, located approximately 40 miles (64 km) northeast of Tokyo, in November 1921. The acquisition was intended to facilitate experimental trials under operational conditions.