Quest for Performance: The Evolution of Modern Aircraft
Chapter 8: Boats in the Sky
Twilight of an Era, 1945-
[208] Although the Boeing 314, last of the four-engine, commercial flying boats developed in the United States, was first flown in 1938 and scheduled commercial operations of these aircraft ended in 1946, limited flying-boat development continued for some years following the end of World War II. The Grumman Mallard, for private and short-haul use, and the Albatros, for military missions, have already been mentioned. An ambitious flying-boat project begun during the war, the Hughes H-4 Hercules, reached fruition in 1947. With a wing of 320-foot span and an area of 11 430 square feet (the Boeing 747 has a wing area of 5500 square feet), the H-4 was, and is, the largest (in terms of dimensional size) airplane ever built. Powered by eight Pratt & Whitney 28-cylinder radial air-cooled engines of 3000 horsepower each, the aircraft [209] was unique in being constructed almost entirely of wood. This material was used to conserve strategically important aluminum alloys during World War II. One flight at an altitude of 70 feet and of about 1 mile in length was made by the aircraft at the hands of pilot Howard Hughes in November 1947. It never flew again but was preserved by the eccentric Mr. Hughes in an environmentally controlled hangar at Long Beach, California, for over 30 years. Today, the aircraft, an experiment that somehow failed, can be seen by the public, along with the ex-luxury liner Queen Mary, at Long Beach.
An interesting postwar experiment that received greater success than the Hughes H-4 was the turboprop-powered Convair R3Y Tradewind transport. Originally conceived as a patrol boat, this 123 500-pound aircraft was equipped with four 5100 - shaft- horsepower Allison engines, each driving two three-blade contrarotating propellers. Ultimately used by the U.S. Navy as passenger and cargo transports in the 1956-58 time period, a total of 11 of these aircraft were built; some were used for experimental purposes, others were lost, and the remainder were retired in 1958 because of persistent propeller and gear-box problems.
During the postwar period, two large, new flying boats, both built by Martin, successfully served with the U.S. Navy. These were the JRM Mars cargo transport and the P5M Marlin patrol boat. Based on the earlier XPB2M-1 patrol bomber (later converted to a transport), 20 of the JRM transports were ordered in January 1945. Following the cessation of hostilities, however, the order was reduced to six aircraft. First flight took place in July 1945, and the last of the six flying boats had been delivered by the fall of 1947. The last one completed, designated JRM-2, was heavier and had more powerful engines than the JRM-1, for which data are given in table IV.
With its full cantilever wing mounted at the top of the hull and the four radial engines located in the leading edge of the wing, the JRM had a configuration that, by the 1940's, had become nearly standard for large flying boats. The JRM-1 Hawaiian Mars is shown taking off from the water in figure 8.23. With a wing span of 200 feet and a gross weight of 145 000 pounds (165 000 pounds for the JRM-2), the Mars was the largest operational flying boat ever developed in the United States. Equipped with four Wright R-3350-8 double-row, 18-cylinder engines of 2200 horsepower each at takeoff, the JRM-1 was capable of maximum and cruising speeds of 222 and 153 miles per hour. With split trailing-edge flaps and a wing loading of 39.4 pounds per square foot, the aircraft had an estimated stailing speed at gross weight of 88 miles per hour.

view of a Martin JRM-1 landing on the water
[210] Figure 8.23 - The 145 000-pound Martin JRM-1 Mars cargo transport, 1945. [mfr via Martin Copp]

A zero-lift drag coefficient of 0.0233 and a maximum lift-drag ratio of 16.4 made the JRM the most aerodynamically efficient of any of the flying boats for which data are given in table IV. An indication of the range potential of the aircraft is given by its record 4375-mile flight from Patuxent River, Maryland, to Natal, Brazil, while carrying a payload of13 000 pounds. (The flight was made by the early patrol-bomber version of the aircraft.)
Accommodations aboard the two-deck aircraft provided for duty and reserve crews of four men each. Included were two shower baths, one for officers and one for enlisted men. Special loading hatches, tie-down rings, and a hoist with a 5000-pound capability were part of the equipment included for cargo handling. As an assault transport, the JRM-1 could carry 132 fully equipped troops and 7 jeeps; or as an ambulance aircraft, 84 stretcher cases and 25 medical attendants could be accommodated.
Early in their operational life, two of the JRM-1 aircraft, the Mars and the Marshall Mars, were destroyed; the remaining four served the Navy until they were retired in 1956. In 1959, these aircraft were purchased by Canadian interests to be converted to water bombers for use in controlling forest fires. In this configuration, the aircraft could carry 6000 gallons of water. The tanks could be replenished in flight by extending scoops and skimming along the surface of a lake or other body of water. The Marianas Mars was lost in a flying accident, and the Caroline Mars (JRM-2) was destroyed by a hurricane. The other two aircraft, the Philippine Mars and the Hawaiian Mars, continue in use [211] today as water bombers - certainly an application never foreseen by its designers but nevertheless a useful occupation in retirement for a good aircraft whose design goes back more than 35 years.
No large, multiengine propeller-driven flying boat has been developed in the United States since the Martin P5M Martin first flew in 1948. (The jet-powered Martin P6M Seamaster flying boat is described in part II.) With a gull wing of the same size as that used on the earlier PBM Mariner, the P5M was, however, a much heavier aircraft equipped with more powerful engines. Although bearing many configuration similarities to the PBM, the P5M had an entirely new, high length-beam ratio hull with a planing-tail afterbody. This new and greatly improved hull form had been extensively studied in both the towing tank and wind tunnels at the NACA Langley laboratory (refs. 36, 37, and 124, for example) and offered the possibility of reducing the unfavorable drag differences between flying boats and landplanes. It was found that by maintaining the product bl2 constant and increasing the value of the length-beam ratio l/b, the water drag and spray characteristics of the hull were little altered and the aerodynamic drag was significantly reduced (l and b are the length and beam of the hull, respectively). The planing-tail afterbody ameliorated the stability problems of porpoising and skipping. As compared with more usual values of 5 to 6, the hull length-beam ratio of the P5M was 8.5, while some of the experimental data in reference 124 are for hulls of length-beam ratio as high as 15.
The P5M-2 version of the Marlin is depicted in figure 8.24 and clearly shows the new hull form. Although the P5M-2 differed from the P5M-1 in a number of respects, the high T-tail of the P5M-2, as compared with a low tail on the P5M-1, immediately identifies the later aircraft. The 76 595-pound gross weight Marlin was powered by two Wright R-3350-18 turbocompound, 18-cylinder, radial air-cooled engines that drove controllable-pitch, fully reversible propellers. (Further details of this engine are given in chapter 6 describing the Lockheed 1049G Super Constellation.) These propellers, together with individually extendible flaplike surfaces below the waterline at the end of the hull, greatly enhanced the maneuverability of the Marlin on the water. Power-boosted controls and spoiler ailerons were other modern features of the aircraft. As compared with its look-alike wartime ancestor, the PBM, the Marlin had a 19-percent lower zero-lift drag coefficient and a 9-percent higher maximum lift-drag ratio. With the low specific fuel consumption of the Wright R-3350 engines, the hypothetical range of the Marlin was over 4800 miles as compared with 3500 miles for the Mariner. The maximum and cruising speeds of the Marlin were....

aerial view of a Martin P5M-2
[212] Figure 8.24 - The Martin P5M-2 Marlin was the last Navy patrol boat; 1953. [Robert L. Lawson via AAHS]


....251 and 159 miles per hour as compared with 202 and 135 miles per hour for the Mariner. Clearly, the Marlin was a more capable aircraft than its well-known predecessor.

The Marlin was primarily an antisubmarine aircraft and, as such, was equipped with a variety of electronic detection equipment. Offensive armament consisted of various combinations of torpedoes, bombs, depth charges, and rockets. A number of these stores could be carried in the elongated engine nacelles. Several power-operated turrets were provided for defense. Like most large, long-range patrol aircraft, the P5M had a galley and provisions for crew rest on long flights.
Of a total of 259 P5M boats built, 145 were the P5M-2 version. The last new one was accepted by the Navy in 1960. After a long and useful career, the P5M was finally retired from Navy service in 1967.
Today, the four-place Lake amphibian flying boat for the private owner, equipped with a 200-horsepower engine, is the only new flying boat offered for sale in the United States. Is the proud era of the flying boat ended, or will new applications of this versatile type of aircraft be found? Perhaps the next few years will provide the answer.