The Supermarine Southampton was the first flying boat designed after the First World War to enter RAF service, and was the first of a series of successful military flying boats designed by Reginald Mitchell. It was based on the single Supermarine Swan flying boat, and was ordered off the drawing board in August 1924 after the Swan impressed in tests. The Southampton would become the second longest serving RAF flying boat (behind the Short Sunderland), entering service in 1925 and remaining in use for over ten years, while the related Stranraer was still in use at the start of the Second World War.
The Southampton was a two-bay biplane. The lower wing was mounted just above the fuselage, and was supported by spar bracing tubes (standard practise was to build the lower wing roots into the hull). The wooden hull of the Mk I was built with an inner fuselage section with the planning bottom and two steps attached to the base. The gap between the two was divided into watertight compartments. On the metal-hulled Mk II this system was replaced by a simple single skin, which helped reduce weight and increase storage space. The two engines were mounted on pylons carried between the wings. The Southampton uses a triple fin and rudder, similar to the one used on the Swan.
Three crew positions were placed ahead of the wings – the bow mooring position with a single Lewis gun was in the nose, followed by twin open tandem cockpits. The engineering and navigation stations were placed below the wing centre-section. Behind the wing were two offset Lewis gun positions.
The Southampton entered service in the summer of 1925 with No.480 (Coastal Reconnaissance Flight). It was best known for a series of long distance flights, carried out partly as flag waving exercises and partly to gain experience in operating flying boats in remote waters. The most famous of these tours lasted for over a year, and saw four aircraft from the Far East Flight travel 27,000 miles between October 1927 and 11 December 1928. During this journey the Southamptons circumnavigated Australia, and visited Hong Kong, Indo-China and Burma, before ending the journey at Singapore, where the flight was reformed as No.205 Squadron.
The first six Southampton Is were ordered off the drawing board in August 1924. They featured a wooden hull with a double bottom, and were powered by two Napier Lion V engines. The first of these six aircraft made its maiden flight on 10 March 1925. A second order for twelve Southampton Is was placed in July 1925, and all 18 aircraft had been delivered by the end of 1926. Most of the Southampton Is were later returned to Supermarine and given metal hulls and new engines to bring them up to the Mk II standard. The Mk I entered service with No.480 (Coastal Reconnaissance Flight), which in January 1929 became No.201 Squadron.
A single metal-hulled Southampton had been ordered alongside the first Mk Is in August 1924. The new hull was 500lb lighter than the wooden equivalent, and also saved another 400lb in water soakage (always a problem with wooden hulls, which could become rather waterlogged after long periods of operation). The Mk II was powered by 500hp Napier VA engines. A combination of the lighter design and new engines increased the range of the aircraft by over 500 miles. The majority of the Mk Is were converted to Mk II standard, and the Mk II accounted for the majority of the 78 Southamptons.
The Mk II was followed by a small number of Mk IIIs and then by a series of prototypes that either failed to enter service, or became known by different names. The Mk IV became the Scapa, the Mk V the Stranraer.
The Mk X was a three-engined version, built in response to an Air Ministry specification of 1927. A single prototype of the Mk X was ordered in June 1928 and began tests in March 1930. The Mk X used a corrugated stainless steel hull, and had wings of unequal size. Tests revealed that the aircraft was a third-heavier than predicted, and performance was disappointed. Even an increase in engine power from three 430hp Armstrong-Siddeley Jaguar IVs to 570hp Bristol Jupiter XFBMs failed to solve the problems, and no further aircraft were ordered.
Stats (Mk II)
Engine: Two Napier Lion VA W-12 inline engines
Power: 500hp each
Wing span: 75ft
Length: 51ft 1in or 49ft 8.5in
Height: 22ft 4in or 20ft 5in
Empty weight: 9,696lb
Loaded weight: 15,200lb
Top speed: 108mph, 95mph at sea level
Range: 770 miles or 540 miles
Service ceiling: 14,000ft or 8,100ft
Armament: Three 0.303in Lewis guns, one in the nose and two amidships
Bomb load: 1,100lb