The Douglas Dolphin is an amphibious flying boat of American origin that has served a diverse range of purposes, including private air yacht, airliner, military transport, and search and rescue.
Despite only 58 units being constructed, the Dolphin has proven to be a versatile and reliable aircraft.
The Dolphin was first introduced in 1930 as the “Sinbad,” a flying boat without wheels designed to serve as a luxurious flying yacht.
Despite a lack of demand, Douglas continued to refine the aircraft, and in 1931, it was transformed into an amphibious aircraft capable of landing on both water and land.
The improved version was named the “Dolphin,” and underwent further development, including an increase in length by over a foot, as well as modifications to the empennage, engine nacelles, and wings.
The onset of the Great Depression led to a decline in demand for extravagant aircraft such as the “flying yacht.”
However, Douglas managed to secure the interest of the United States Coast Guard, who purchased not only the Sinbad but also 12 Dolphins. Variants Y1C-21 Douglas Sinbad The original prototype built as a flying boat, intended to be a luxurious flying yacht, first flown in July 1930. No orders were received for the Sinbad which was eventually bought by the U.S. Coast Guard. Dolphin Model 1 The initial two Dolphins built for the Wilmington-Catalina Airline Ltd. as six-seat airliners. Dolphin Model 1 Special The Model 1s redesignated after modification to seat eight passengers. Dolphin Model 3 The third commercial Dolphin built as a luxury transport named Lesgo with seats for two crew and four passengers for Powel Crosley Jr., powered by 2x 300 hp (224 kW) Pratt & Whitney Wasp Junior A engines. Later impressed into the RAAF as A35-3. Dolphin 113 One aircraft named Jade Blanc V for French clothing manufacturer Armand Esders similar to the RD-4, powered by 2x 550 hp (410 kW) Pratt & Whitney R-1340-S1H1 Wasp engines. Dolphin 114 A single Dolphin built to order for Philip K. Wrigley, powered by 2x 450 hp (336 kW) Pratt & Whitney Wasp SC1 engines. Dolphin 116 One aircraft for the Armada Argentina (Argentine Navy), powered by 2x 450 hp (336 kW) P&W R-1340-96. Dolphin 117 One aircraft initially named Rover, bought by William E. Boeing, which ended up in CAA (forerunner of the current Federal Aviation Administration) service. Dolphin 119 Two aircraft built for Alfred Gwynne Vanderbilt Jr. and William Kissam Vanderbilt II and operated from the yacht Alva. One of the two (which is unknown) joined the RAAF as A35-2. Dolphin 129 Two aircraft ordered by Pan American Airways for its subsidiary (at that time) China National Aviation Corporation, powered by 2x 450 hp (336 kW) Pratt & Whitney Wasp S3D1 engines. Dolphin 136 A single Dolphin, powered by 2x 450 hp (336 kW) Pratt & Whitney Wasp Junior SB engines, ordered by Standard Oil and later impressed by the RAAF as A35-1. FP-1 Several C-21 aircraft loaned to the U.S. Treasury Department for border patrols during Prohibition. FP-2 The two Y1C-26 Dolphins during a brief attachment to the U.S. Treasury Department. FP-2A The designation used by those Y1C-26A aircraft that were attached to the U.S. Treasury Department. FP-2B The two C-29s when in use by the U.S. Treasury Department. RD-1 One aircraft, powered by 2x 435 hp (324 kW) Wright R-975E radial engines, operated by the U.S. Navy. RD-2 Four Dolphin aircraft of two distinct types. One Aircraft similar to the Y1C-21 and powered by 2x 500 hp (373 kW) Pratt & Whitney R-1340-10 engines, for the U.S. Coast Guard. Two were U.S. Navy VIP staff transports similar to the Y1C-26 powered by 2x 450 hp (336 kW) Pratt & Whitney R-1340-96 engines. The final aircraft was completed for the U.S. Navy as the first presidential aircraft, for Franklin D. Roosevelt, powered initially by 2x 410 hp (306 kW) Pratt & Whitney R-1340-1 engines and later by 2x 500 hp (373 kW) Pratt & Whitney R-1340-10 engines, seating five though it was reportedly never used by the President. RD-3 A utility transport version of the RD-2, six of which were built for the U.S. Navy, powered by 2x 500 hp (373 kW) Pratt & Whitney R-1340-4 or by 2x 500 hp (373 kW) Pratt & Whitney R-1340-96 engines. RD-4 Ten aircraft for the U.S. Coast Guard, powered by 2x 420 hp (313 kW) Pratt & Whitney Wasp C1 engines. OA-3 C-21 aircraft redesignated. OA-4 C-26 aircraft redesignated. OA-4A Y1C-26A aircraft redesignated. OA-4B C-26B aircraft redesignated, one of which was fitted with an experimental fixed tricycle undercarriage. OA-4C Four OA-4A and one OA-4B aircraft modernized in 1936. Y1C-21 Eight aircraft for the USAAS, similar to the Navy’s RD-1, powered by 2x 350 hp (261 kW) Wright R-975-3 engines. Y1C-26 Two aircraft for the USAAS with increased dimensions, fin area and fuel capacity (from180 US gal (681 L) to 240 US gal (908 L)). Powered by 2x 300 hp (224 kW) Pratt & Whitney R-985-1 engines. Y1C-26A Eight aircraft for the USAAS differ from the Y1C-26 only in minor details. C-21 Y1C-21 aircraft redesignated. C-26 Y1C-26 aircraft redesignated. C-26A Y1C-26A aircraft redesignated. C-26B Four aircraft powered by 2x 400 hp (298 kW) Pratt & Whitney R-985-9 engines. C-29 Two Dolphins, powered by 2x 550 hp (410 kW) Pratt & Whitney R-1340-29 engines, were essentially similar to the Y1C-26As. Specifications RD-3 Dolphin Crew Two Capacity Six passengers Length 45 ft 3 in (13.79 m) Wingspan 60 ft (18 m) Height 15 ft 2 in (4.62 m) Wing area 637 sq ft (59.2 m2) Empty weight 6,764 lb (3,068 kg) Gross weight 9,734 lb (4,415 kg) Fuel capacity 240 US gal (908 L) Powerplant 2 × Pratt & Whitney R-1340-4 Wasp 9-cyl, Air-cooled radial piston engines, 450 hp (340 kW) each or 2x 450 hp (336 kW) Pratt & Whitney R-1340-96 Wasp engines Performance Maximum speed 149 mph (240 km/h, 129 kn) at sea level Cruise speed 105 mph (169 km/h, 91 kn) Range 692 mi (1,114 km, 601 nmi) Service ceiling 15,100 ft (4,600 m) Rate of climb 806.5 ft/min (4.097 m/s) Time to altitude 5,000 ft (1,524 m) in 6 minutes, 12 seconds Wing loading 16.4 lb/sq ft (80 kg/m2) Power/mass 0.093 hp/lb (0.204 kW/kg).
Sources McDonnell Douglas aircraft since 1920, Volume 1-René J Francillon. San Diego Air and Space Museum Archive. The National Museum of the U.S. Air Force.
