Martin Models 230 and 237

P5M-1 & 2 Marlin Patrol Planes, P7M SubMaster
Variants/Specifications

Two-engine flying boats powered by huge Wright R-3350 engines were proposed by Consolidated in 1939, Martin in 1940, and Boeing in 1942. Although the Martin PBM-4 was never built, single prototypes of the XP4Y-1 Corregidor and PBB-1 Ranger demonstrated greater speed and range than the underpowered PBM-3. Priority for the R-3350 (and for Boeing), however, went to the B-29. In 1944 some thought was given to building the Boeing-designed Ranger at Middle River as the Martin Model 230, but instead Martin gained Navy support for its own design, Model 237. This incorporated the Mariner's familiar gull wings and nacelle bomb-bays as well as a number of new features. Most noticeable were a high conventional tail in place of the PBM's pinwheel, and a hull with a longer afterbody in place of the traditional "step" beneath the tail. The idea was supposedly based on the Japanese Kawanishi H8K "Emily" of World War II. Besides allowing for more interior space, the longer hull improved the plane's seaworthiness by reducing the amount of "porpoising" between wave crests. This was further improved by reversable-pitch propellers and underwater "hydroflaps" that allowed greater maneuverability on the water. Several of these features were part of a proposed PBM-6 design proposed in 1945, but the Model 237 was really a new plane. It was given the designation P5M in the Navy's revived "Patrol" classification; Martin named it the Marlin.

The last PBM-5 that came off the production line in 1947 was built with a new hull and tail as the Marlin's prototype. Like the P4M-1 Mercator, it was heavily armed with twin 20-millimeter cannons in bow and tail turrets, plus two more .50-caliber machine guns in a dorsal turret. The XP5M-1 first flew in May 1948; orders for 167 production P5M-1's began the following November. This differed from the prototype in several respects: more powerful turbo-compound engines, streamlined pylons for the wingtip floats, a raised cockpit for greater visibility, and a bulbous nose radome in place of the bow turret. The dorsal turret was deleted.

The differences indicated changes in the plane's principal mission of hunting enemy submarines. As technological improvements in subs allowed them to spend more time under water, the chance of catching one on the surface and fighting it out there were reduced. Instead of guns, Marlins carried increasingly sensitive radars and complicated underwater detection equipment. Most noticeable were the Magnetic Anomaly Detection (MAD) gear atop the tails and "doghouse" direction-finder housings behind the cockpit which appeared on 80 modified P5M-1S's later in the 1950's. These were also equipped with the "Julie and Jezebel" system of droppable sonobuoys that broadcast reports back to a computerized Automatic Integrated Display System (AIDS) in the plane, which tracked targets for the crew's Tactical Coordinator. Bomb-bays were altered to accommodate antisubmarine torpedoes and nuclear depth charges.

Most of these changes were standard on the 116 P5M-2's built between 1954 and 1960. The most visible innovation in the new model was a T-tail borrowed from Martin's XB-51 design. Not only was it lighter, but the new tail kept the horizontal stabilizers clear of spray and provided a more streamlined fairing for the MAD boom. Martin hydrodynamic engineers contributed a bluff bow with low chine line that was better able to keep spray down and out of the P5M-2's engines.

Marlins entered service with Navy patrol squadrons in 1952 and 1953, just missing the Korean War. Up to ten "patrons" and one training squadron concentrated on antisubmarine warfare, though Pacific Fleet Marlins patrolled the dangerous Formosa Strait during the middle 1950's. The Coast Guard ordered seven P5M-1G's and four P5M-2G's for air-sea rescue work, but found the big planes difficult to maintain. They handed them back to the Navy for use as trainers. The French Navy received ten P5M-2's in 1959 under a lend-lease program; after five years of patrols from Dakar in West Africa they also returned to the U.S. Navy in 1964.

Actual combat for the Marlin came at the very end of its service life, off Vietnam. Between 1964 and 1967 three squadrons based at Sangley Point Naval Air Station in the Philippines operated from tenders anchored in Cam Ranh Bay near Saigon, patrolling the Southeast Asian coasts as part of Operation Market Time. They occasionally attacked small surface vessels supplying enemy forces, using rockets mounted under their wings or machine guns fired from open doors (tail turrets having been removed earlier in favor of more anti-submarine gear).

In 1955 Martin designed a successor to the Marlin, initially designated P5M-3. 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 competing 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. The last operational Navy flying-boat missions were flown in by Marlins 1967. A single survivor is preserved at the National Museum of Aviation History in Pensacola.