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The Douglas C-21/ OA-3 Dolphin was an amphibious aircraft originally ordered as a bomber leader, but normally used as transport and air-sea rescue aircraft.
The Dolphin was based on the single Douglas Sinbad, a twin engined flying boat intended for sale as an 'air yacht'. The Sinbad was completed in 1930, and was a high-winged monoplane, powered by two engines carried on struts above the wings. The Great Depression meant that it was a commercial failure, but the sole aircraft was bought by the Coast Guard and used from 1931 to 1939.
The Sinbad was followed by the Douglas Dolphin, a very similar aircraft but equipped as an amphibian. It was given main wheels that were attached to the hull sides on hinged struts and to the wings by oleo struts, and that could be retracted above water level. The two engines were carried above the wings, and an auxiliary aerofoil was attached to the top of the engine nacelles to overcome a problem with turbulence. The original design also included auxiliary fins on the tail to help improve directional stability. The Dolphin had an all-metal hull and a plywood covered wooden wing.
The pilot and co-pilot sat side by side in a fully enclosed position level with the wing leading edge, and with a passenger cabin just behind. The Model 1 could carry six and the improved Model 1 Special could carry eight passengers. Fifty eight Dolphins were built in a surprisingly wide range of versions. The US Army purchased some of the original type as the C-21/ OA-3, and a larger number with a modified tail as the C-26/ OA-4. The US Navy also purchased a number as the Douglas RD (R for Transport, Multi-engined, D for Douglas).
Y1C-21
In 1931 the US Army ordered eight Dolphins for use as escort aircraft to accompany bombers on missions over water. The idea was that the amphibians would act as navigational leaders and air-sea rescue aircraft. The order was placed on 10 August 1931. The aircraft were powered by two 350hp Wright R-975-3 engines. Soon after the Y1C-21 was delivered it was followed into service by the Martin B-10 and Martin B-12, both of which were too fast to be led by the Y1C-21. The original plan was abandoned, and the aircraft were used as staff transports, with the designation C-21.
FP-1
Some of the C-21s were loaned to the Treasury Department to take part in border patrols during Prohibition. During this period they were designated as the Douglas FP-1.
OA-3
Later in their careers the C-21s were redesignated as the OA-3 Observation Amphibian. In this role they were used as transport and air-sea rescue aircraft. One aircraft, attached to the 7th Bombardment Group at Hamilton Field, was used to work out air-sea rescue techniques.
One aircraft was destroyed in an accident on 23 November 1938, but the other seven were assigned to the Panama Canal Zone, Hawaii and the Philippine Islands. The last one was withdrawn from service in March 1940.
C-21
Engine: Two Wright R-975-3 radial engines
Power: 350hp each
Crew: 2
Span: 60ft 0in
Length: 43ft 10in
Height: 14ft 1in
Empty weight: 5,861lb
Loaded weight: 8,583lb
Max speed: 140mph at sea level
Climb Rate: 18.5min to 10,000ft
Service ceiling: 14,200ft
Range: 550 miles