In 1955 Martin designed a successor to the Marlin, designated the "P5M-3 / Model 313" and then, with changes including fit of turbojet engine to provide a "boundary layer control" system that would reduce takeoff run, the "P7M-1 / Model 313".
Using advanced hydrodynamics developed for the P6M SeaMaster, the new model was to be capable of operations on or near the sea surface with dipping sonars like those used on ships. Besides a long hull, the new plane was to have a single jet engine mounted atop the fuselage with exhaust nozzles ducted across the wing surface. Described as a "Boundary Layer Control" mechanism or "blown flaps," the system would cut landing/stall speeds to only 43-48 mph, greatly reducing the danger of impact damage in open-sea landings. Ordinary power was to be supplied by four old reliable Wright R-1820 piston engines. A mockup model was built in 1956, designated the P7M SubMaster.
A mockup of the P7M-1was built in 1956, but that was as far as it got. The competing P6Y design from Convair was the Navy's choice, but neither plane was built. Instead, antisubmarine patrol was given over to faster turboprop Lockheed P3V (P-3) Orion landplanes.
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