Most of R.J. Mitchell’s seaplanes were designed for military purposes, although naval reconnaissance rather than bombing had been the main requirements from the Air Ministry. Foreign orders were more specifically aggressive: the Scarab amphibian bomber for Spain and the Nanok torpedo carrier for Denmark. All the aircraft were, basically, traditional biplanes and Mitchell was also to design two other military biplane flying boats, closer to the outbreak of the Second World War, despite the obsolescence of their configuration.
The first of the new designs, which was to become known as the Scapa, was ordered as a successor to the long-serving Southampton, to be supplied with a metal hull and superstructure and with the new Rolls-Royce Kestrel engines. In the relatively depressed economic situation in Britain and with other companies offering multi-engined flying boats, a more aerodynamically efficient version of the Southampton with the cheapness of using only two of the new, efficient engines seemed a good proposition, and so Supermarine offered the last Southampton, S1648, as the proposed prototype, without extra cost to the Air Ministry.
The new aircraft was designated the Southampton IV and so it might have been regarded as, essentially, an ‘improved Southampton’, particularly as its water-planing lines resembled that of the previous aircraft. Indeed, tank testing now available via the new parent firm, Vickers, did not suggest any real need to depart from the earlier basic shape – a compliment to the intuitive design of the hull of 1925. In fact, a widening behind the step, to counteract spray hitting the tail surfaces, was abandoned when it was found to produce an unpleasant pitching on take-off.
In other respects, the new machine was, effectively, a new design. ‘Stretching’ the Southampton with a lengthened bow and a deeper forefoot, allied to a flatter top-decking, allowed for more useable space within, as well as significantly altering the appearance of the new machine. Also, the slab-sided approach of the Air Yacht and of the slightly later Walrus (another reflection of the economic situation?) was evident, although the upward sweep of the tail and other curvatures restored something of the elegance of the Southampton hull.
The superstructure was even more clearly a departure. The redesign meant that the sweep back of the mainplane outer sections, in response to the changing service loads of later Southamptons, was no longer necessary and the earlier two-bay structure and warren girders were now reduced to a much simpler arrangement, whereby two of the four struts also supported the engines. These engines were now positioned directly under the top wing and the previous triple fins were now succeeded by two units with extended rudders.
Beverley Shenstone, Mitchell’s aerodynamicist, considered the resulting aircraft ‘perhaps the cleanest biplane flying boat ever built, with minimum struttage and clean nacelles faired into the wing’. He did not mention the very ‘boxy’ radiators which projected on either side to the rear of these nacelles, although this positioning least compromised the overall lines of the new design.
The new prototype made its first flight on 8 July 1932, and it was delivered at the end of October to the Marine Aircraft Experimental Establishment, Felixstowe, for service testing. In the following May, the prototype was flown to Malta, for overseas acceptance trials with No. 202 Squadron and these included a long-distance flight to Gibraltar and a cruise to Port Sudan via Sollum, Aboukir, and Lake Timsah. On its return, the Scapa took part in the 1934 fly-past of ‘the competition’ at the Hendon RAF display with, as Penrose reported, ‘the clean Supermarine twin-engined Scapa leading, followed by the four-engined Short Singapore, triple engined Blackburn Perth, the distinctive gull-winged Short Knuckleduster, Saro R24/31 London and the three Saro Cloud trainers’.
The Hendon event was clearly designed to impress foreign governments with Britain’s military capabilities and, during the 1936–1939 period, the Scapa fulfilled its required purpose with anti-submarine patrols to protect neutral shipping during the Spanish Civil War. Some of the aircraft of No. 202 squadron were later transferred to No. 204 Squadron and were sent to Egypt during the Italian-Abyssinian confrontation. There was also a single Scapa, attached to No. 228 Squadron, whose contribution to developing war preparations was its involvement in early radar trials.
Soon after Supermarine had received orders for the Scapa, the Air Ministry issued another specification which was unlikely to be matched by a simple development of this last type. This latest specification was for another general-purpose coastal patrol flying boat, of robust and simple construction with low maintenance costs, but capable of carrying a 1,000lb greater load and of maintaining height on one engine with 60 per cent of fuel on board.
An enlarged and substantially altered version of the Scapa had to be projected and a specification was submitted, alongside one from Shorts and one from Saunders-Roe. Only the last, the A27 London, was accepted and it was later ordered to replace the Southamptons and Scapas of Nos 201 and 202 Squadron respectively. The Short Singapore III had also been ordered as a replacement for other Scapas with Nos 204 and 240 Squadrons.
This Short machine had about the same speed as the Scapa and was powered by twice as many engines, and another Short machine, the Saraband, was only a few mph faster with six engines. Thus, given an economic situation in which orders for these larger flying boats were likely to be kept to a minimum, it seemed a distinct possibility that a performance from Mitchell’s smaller, twin-engined proposal, if significantly better than that of the Saunders-Roe London, might still stand a chance of winning some contracts, given the growing calls for British rearmament.
Another reason for anticipating orders for the proposed new design was not simply based on the good performance figures that the Scapa had returned but on Mitchell’s having come to believe in the virtue of employing a thin wing – for other than Schneider Trophy racers – contrary to the generally perceived wisdom of the day. The eventual outcome was an Air Ministry contract for a flying boat that outclassed all of its contemporaries of similar configuration.
The engines chosen initially to pull the new machine’s thinner aerofoil through the air, and to give it the one-engine performance required by the Air Ministry specification, were 820hp Bristol Pegasus IIIMs, providing an additional 590hp more than the Scapa’s Kestrels. These two engines were to be mounted with the same thrust line and in streamlined fairings but, being air-cooled radials, they did not incur the weight and drag penalties of the Scapa’s radiators. Long-chord Townend drag-reducing rings now surrounded the cylinder heads and their oil coolers formed part of the top centre-section leading edge. Against these improvements, there was the additional 12 per cent increase in wing area of the new machine. This extra drag and weight was added to by the two-bay strut arrangement required to support the extra 10ft of wingspan that was needed to meet the new specification’s load carrying requirements.
The larger size, nevertheless, did have some aerodynamic advantages. The extra depth of the hull allowed the top of the enclosed cockpit to form a continuous line with the midships gunner’s cockpit, which was now placed in the centre of the hull top (in the Southampton and Scapa there had been two mid-ship gunner’s cockpits, offset from the centre line). Now, for the first time, Supermarine had built a larger service aircraft which made it possible to install the second rear gunner more sensibly, in a faired-in cockpit in the tail. This had been proposed for the unsuccessful Vickers/Southampton X prototype, with its wingspan of 79ft, and so presented little difficulty for the new 85ft craft. Its ‘general purpose’ character was evidenced by the fitting of carriers below the inner sections of the lower wings for up to four 250lb bombs or extra fuel tanks. The flatter fuselage section between the lower wings was even more convenient than that of the Scapa for transporting supplies, such as a spare engine.
By the time that the designing and the constructing of the new prototype was well under way, the Australian Seagull V order had not yet been completed and the Scapa flying boat contract was also being fulfilled. Nevertheless, the new prototype, K3973, was test flown by Summers on 27 July 1934 and delivered in very short time to the Marine Aircraft Experimental Establishment at Felixstowe, for service assessment.
The performance of the aircraft was such that an order for seventeen aircraft, K7287–K7303, was placed with Supermarine by the following year. The standard service machine was fitted with the more powerful 920hp Pegasus X engines, giving it a maximum speed of 165mph and making it the fastest biplane flying boat to enter RAF service – yet with a stalling speed of only 51mph.
Its maximum ceiling was 20,000ft and it could climb to the first 10,000ft in ten minutes – the Scapa had taken twice as long, two years earlier. As it had been necessary to withhold these performance details because of the developing international situation, company publicity had to be content with the by-no-means despairing comment that the aircraft ‘passed all its tests brilliantly’ and went on to claim that:
The outstanding feature of this flying boat is that the performance obtained during a series of extended service trials, whether in respect of speed, climb, ceiling or take-off, is unequalled by any other British flying boat. All the specification requirements were exceeded by large margins.
As the new aircraft had become quite distinct from the earlier Scapa from which it was developed, a new name was chosen and the machine entered service as the ‘Stranraer’. It must have been gratifying for Supermarine to see the Saunders-Roe London flying boat then replaced by their new aircraft with Nos 201 and 240 Squadrons, and to see another rival company’s aircraft, the Singapore III, superseded by the Stranraer with No. 209 Squadron. Other machines replaced the company’s own Scapas with No. 228 Squadron and so the total of Stranraers ordered from Supermarine, including the prototype, came to eighteen.
It thus came about that the Stranraer was actually operated in the Second World War, serving with No. 228 Squadron, when it was needed to patrol the North Sea. Additionally, some of the Stranraers of this unit were transferred to No. 209 Squadron and, fitted with extra fuel tankage under one wing and bombs under the other, they conducted patrols against enemy shipping between Scotland and Norway, until replaced in April 1941 by the ubiquitous Short Sunderland. No. 240 Squadron was also equipped with the Stranraer and made the last operational patrol of the type on 17 March 1941, after which it was replaced by the American-built Catalina.
In addition to the British Stranraers, the Royal Canadian Air Force also adopted them; forty were built by Canadian Vickers and these saw a great deal more service than their British counterparts – in the battle against the U-boats in the Atlantic. The last RCAF Stranraer was retired as late as 20 January 1946, and fourteen of the aircraft were sold to the civil sector, particularly to private airline companies in Canada where the lakes of the Northern Territory provided ready-made runways. The last of the Stranraers served in these regions until 1958.
However, the longevity and performance of the Stranraer does not conceal the fact that, in this larger flying boat category, the influence of Mitchell was relatively short-lived – in many ways, a result of the lack of official encouragement for his Air Yacht (monoplane) approach to the reconnaissance flying boat type and of the cancellation of the Type 179 Giant cantilever monoplane.
Meanwhile, the early and single-minded approach of Shorts to all-metal aircraft had paid dividends and finally led to the military development of another cantilever monoplane type, the Sunderland, which dominated the wartime long-range sea patrol provision, with a total of 741 being built. In addition, Short’s clean, streamlined civilian version had monopolised the flying boat provision on the Imperial Airways routes just prior to the outbreak of the war and represented a major step forward in flying boat design, without a rival aircraft from Supermarine.