The Stranraer – Mitchell’s last flying-boat.

Supermarine Stranraer (from a painting by the author)

After Supermarine had received orders for the Scapa, the Air Ministry issued another specification, R.24/31, for another general purpose coastal patrol flying-boat but capable of carrying a 1,000 lb. greater load and of maintaining height on one engine with 60% of fuel on board. An enlarged and substantially altered version of the Scapa had to be projected but only the Saunders-Roe A.27 was accepted. Built as the 'London', it replaced the Southamptons and Scapas of Nos. 201 and 202 Squadron respectively. 

   At about this time, the Singapore III was 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.Thus, given an economic situation in which orders for 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 R.24/31 proposal 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 engines chosen initially to pull the new machine’s thinner aerofoil through the air, and to give it the required one-engine performance required by the Air Ministry specification, were 820 hp Bristol Pegasus IIIMs providing a combined 590 hp more than the Scapa's Kestrels. The two engines were to be mounted with the same thrust line  and in streamlined fairings but, being air-cooled radials, did not incur the extra weight and drag penalties of the Scapa's radiators; long-chord Townend drag-reducing rings surrounded the cylinder heads and their oil coolers formed part of the top centre-section leading edge.

   An extra depth of the new hull allowed the top of the enclosed cockpit to form a continuous line with the midship gunner’s cockpit which was now placed in the centre of the hull top – in the Southampton and Scapa there had been two midship gunner’s cockpits, offset from the centre-line. And now, for the first time, Supermarine were able to install the second rear gunner in a more faired-in cockpit in the tail. This had been proposed for the unsuccessful Southampton X prototype, with its wingspan of 79 feet, and so presented little difficulty for the new 85 footer.

The new prototype, K3973, was test flown by Summers on the 27th of July, 1934 and delivered in very short time to MAEE, Felixstowe for service assessment. The performance of the aircraft was such that an order for seventeen aircraft, K7287 to K7303, was placed with Supermarine by the following year. The standard service machine was fitted with the more powerful, 920 hp Pegasus X engines and could outperform all contemporary flying-boats. It had a maximum speed of 165 mph, making it the fastest biplane flying-boat to enter RAF service, yet had a stalling speed of only 51 mph. Its maximum ceiling was 20,000 feet and it could climb to the first 10,000 feet at a thousand feet per minute. As it was necessary to withhold these performance details because of the 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’.

The machine entered service as the 'Stranraer' and it must have been gratifying for Supermarine to see the Saunders-Roe London flying-boat 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 Scapas with No. 228 Squadron.

Whilst it must be acknowledged that the Stranraer, being a fabric covered biplane, was an obsolescent type, as a stop-gap it was actually operated in World War II, 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 Lockheed Hudson. No. 240 Squadron was also equipped with the Stranraer and made the last operational patrol of the type on 17 March, 1941.

   In addition, the Royal Candian Air Force also adopted the Stranraer and 40 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. They were finally replaced on active service by the Canadian Catalina, the Canso, in March 1941. 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. The last one of these Stranraers served in these spartan regions until 1958.

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For reference sources, see my Blogpost: “Source Material and References" – an extended bibliography is included in my R.J.Mitchell at Supermarine; Schneider Trophy to Spitfire which also provides material for wider reading, grouped according to specific areas of interest.