Prototype The first S.25, now named the Sunderland Mark I, flew from the
River Medway on 16 October 1937 with Shorts' chief
test pilot,
John Lankester Parker at the controls. The deeper hull and installation of nose and tail turrets gave the Sunderland a considerably different appearance from the Empire flying boats. The prototype was fitted with
Bristol Pegasus X engines, each providing 950 hp (709 kW ), as the planned Pegasus XXII engines of 1,010 hp (753 kW) were not available at the time. The 37 mm gun, originally intended as a primary anti-submarine weapon, was dropped from the plans during the prototype phase and replaced with a
Nash & Thompson FN-11 nose turret mounting a single
.303-inch (7.7 mm) Vickers GO machine gun. The turret could be winched back into the nose, revealing a small "deck" and demountable marine
bollard used during mooring manoeuvres on the water. The change of armament in the nose to the much lighter gun moved the
centre of gravity rearwards. After the first series of flights, the prototype was modified to have a wing that was swept 4.25° to the rear, thereby moving the
centre of pressure into a more reasonable position in relation to the new centre of gravity. This left the engines and wing floats canted out from the aircraft's centreline. Although the wing loading was much higher than that of any previous Royal Air Force flying boat, a new
flap system kept the takeoff run to a reasonable length.
Sunderland Mark I The RAF received its first Sunderland Mark I in June 1938 when the second production aircraft (
L2159) was flown to
230 Squadron at
RAF Seletar,
Singapore. By the outbreak of war in Europe, in September 1939,
RAF Coastal Command was operating 40 Sunderlands. The main offensive load was up to of
bombs (usually ),
mines () or other stores that were hung on traversing racks under the wing centre section (to and from the
bomb room in the fuselage). Later, depth charges (usually 250 lb) were added. By late 1940, two
Vickers K machine guns had been added to new hatches that were inserted into the upper sides of the fuselage just aft of the wing, with appropriate slipstream deflectors. A second gun was added to the nose turret. New constant speed propellers and deicing boots were installed as well during 1940. The Sunderland had difficulty in landing and taking off from rough water, but, other than in the open sea, it could be handled onto and off a short chop, by a skilled pilot. Many rescues were made, early in the war, of crews that were in the Channel having abandoned or ditched their aircraft, or abandoned their ship. During May 1941, during the
Battle of Crete Sunderlands transported as many as 82 armed men from place to place in one load. Steep ocean swells were never attempted, however a calm ocean could be suitable for landing and takeoff. Beginning in October 1941, Sunderlands were fitted with ASV Mark II "Air to Surface Vessel"
radar. which featured a revised hull configuration which had been tested on a Mark I the previous June. This modification improved seaworthiness, which had suffered as the weight of the Sunderland increased with new marks and field changes. In earlier Sunderlands, the hull "step" that allows a flying boat to "unstick" from the surface of the sea was an abrupt one, but in the Mk III it was a curve upwards from the forward hull line. The Mark III turned out to be the definitive Sunderland variant, with 461 built. Most were built by Shorts at Rochester and Belfast, a further 35 at a new (but temporary) Shorts plant at White Cross Bay, Windermere; while 170 were built by Blackburn Aircraft. The Sunderland Mark III proved to be one of the RAF Coastal Command's major weapons against the U-boats, along with the
Consolidated Catalina. As the U-boats began to use
Metox passive receivers the ASV Mk II radar gave away the presence of aircraft and the number of sightings diminished drastically. The RAF response was to upgrade to the ASV Mk III, which operated in the 10 cm band, with antennas that could be faired into fewer more streamlined blisters. During the Mk III's life there were a large number of almost continuous improvements made, including the ASV Mk IIIA and four more machine guns in a fixed position in the wall of the forward fuselage just behind the turret (developed on RAAF aircraft first) with a simple bead and ring sight for the pilot. Sunderlands with upgraded ASV Mk III equipment were designated Mk IIIA. Relative to the Mark III, the Mark IV had a stronger wing, larger
tailplanes and a longer fuselage with some changes in hull form for better performance in the water. The armament was heavier with .50 inch (12.7 mm) machine guns and
20 mm Hispano cannon. The changes were so substantial that the new aircraft was redesignated the
Short Seaford. Thirty production examples were ordered; the first delivered too late to see combat and only eight production Seafords were completed and never got beyond operational trials with the RAF.
Sunderland Mark V The next production version was the Sunderland Mark V, which evolved out of crew concerns over the lack of power of the Pegasus engines. The weight creep (partly due to the addition of radar) that afflicted the Sunderland had resulted in running the Pegasus engines at combat power as a normal procedure and the overburdened engines had to be replaced regularly. Australian Sunderland crews suggested that the Pegasus engines be replaced by
Pratt & Whitney R-1830 Twin Wasp engines. The 14-cylinder engines provided 1,200 hp (895 kW) each and were already in use on RAF Consolidated Catalinas and
Douglas Dakotas, and so logistics and maintenance were straightforward. Two Mark IIIs were taken off the production lines in early 1944 and fitted with the American engines. Trials were conducted in early 1944 and the conversion proved all that was expected. The new engines with new Hamilton Standard's Hydromatic constant-speed
fully feathering propellers provided greater performance with no real penalty in range. In particular, a Twin Wasp Sunderland could stay airborne if two engines were knocked out on the same wing while, in similar circumstances, a standard Mark III would steadily lose altitude. Production was switched to the Twin Wasp version and the first Mark V reached operational units in February 1945. Defensive armament fits were similar to those of the Mark III, but the Mark V was equipped with new centimetric ASV Mark VI C radar that had been used on some of the last production Mark IIIs as well. A total of 155 Sunderland Mark Vs were built with another 33 Mark IIIs converted to Mark V specification. With the end of the war, large contracts for the Sunderland were cancelled and the last of these flying boats was delivered in June 1946, with a total production of 777 aircraft completed.
Transport variants In late 1942, the
British Overseas Airways Corporation (BOAC) obtained six Sunderland Mark IIIs, which had been de-militarised on the production line, for service as mail carriers to
Nigeria and
India, with accommodation for either 22 passengers with 2 tons of freight or 16 passengers with 3 tons of freight. Armament was removed, the gun positions being faired over, and simple seating fitted in place of the bunks. As such they were operated by BOAC and the RAF jointly from
Poole to
Lagos and
Calcutta. Six more Sunderland IIIs were obtained in 1943. Minor modifications to the engine angles and flight angle resulted in a significant increase in the cruise speed, which was a relatively unimportant issue for the combat Sunderlands. In late 1944, the RNZAF acquired four new Sunderland Mk IIIs already configured for transport duties. In the immediate postwar period, these were used by New Zealand's
National Airways Corporation to link South Pacific Islands in the "Coral Route" before
TEAL Short Sandringhams took over after 1947.
Post-war civilian use At the war's end BOAC obtained more Mark IIIs and gradually came up with better accommodation for its passengers, in three configurations. Bench seats were removed, and civil-standard seating installed: The H.1 configuration had 16 seats on one deck, while the H.2 had an additional promenade deck, and the H.3 had 24 seats, or sleeping berths for 16. These conversions were given the name
Hythe and BOAC operated 29 of them by the end of the war. In February 1946, the first of these,
G-AGJM, made a 35,313-mile route survey from Poole to Australia, New Zealand, Hong Kong, Shanghai and Tokyo in 206 flying hours. It was the first British civil flying boat to visit China and Japan. A more refined civilian conversion of the Sunderland was completed by the manufacturer as the postwar
Short Sandringham. The Sandringham Mk. I used Pegasus engines while the Mk. II used Twin Wasp engines. ==Operators==