As the sun had touched the horizon, the four-man crew of the huge flying-boat started their four J-75 turbojets. Engines whining at idle, they cast off from the buoy and water-taxied out of the lagoon. Turning into the ocean breeze, the throttles were pushed forward into full afterburner and the 80-ton behemoth accelerated with a high, white rooster tail whipped up by the exhausts. The carefully shaped vee-hull rode higher and higher until the P6M was airborne. In its watertight, rotary, bomb bay was a nuclear weapon transferred from a submarine only hours before. Flying just below the speed of sound, the seaplane bomber would reach its target deep in the Soviet Union at midnight, drop its deadly load and return to a different, obscure, ocean island where another U.S. Navy submarine was waiting with fuel and another bomb.
The foregoing was not some farfetched Hollywood plot, but the planned mission of the revolutionary Martin P6M SeaMaster. The aircraft never did fly the deep strikes it was designed for, but the story of its development and testing represents the pinnacle of a long history of military seaplanes.
Seaplane Strike Force
After congressional hearings on unification and strategy for the Army, Navy, and newly founded (1947) Air Force, a decision was made to develop a Seaplane Strike Force (SSF) capable of delivering nuclear weapons, especially mines, to combat the rising threat of Soviet submarines. The operational requirement was issued by the Chief of Naval Operations in July of 1951 and sought proposals for a high-speed mine layer as the centerpiece of the SSF. The two archrivals of seaplane manufacture, Consolidated (Convair) and the Glenn L. Martin Company competed for the lucrative contract. The Navy’s design branch allowed them an unprecedented 180 days due to the exceptional technological challenges. The final choice of the Glenn L. Martin Company was more political than technical. Competition was deemed the key to progress, and Convair already had two advanced seaplane contracts; the R3Y four-engine turboprop Tradewind transport and the F2Y jet fighter Sea Dart. A $30,861,000 contract was signed in October 1952, and the Martin Company in Baltimore, MD, began construction of two XP6M-1 “SeaMaster” prototypes. (Officially written as one word with two capitals.)
Because the SeaMaster program was classified confidential during development, the rollout on the first day of winter 1954 had no audience of employees, invited guests, and, most importantly, no media coverage. After the company photographers were finished, the prototype was towed on contemporary beaching gear (basically clip-on wheels) to the seaplane hangar at Strawberry Point for three months of load tests. The Navy liked what it saw and awarded Martin a $180 million dollar contract for six pre-production YP6M-1 SeaMasters, including production tooling, structural test articles, and a separate structural test facility.
Scorched and slanted
Ground testing revealed the first design flaw. The SeaMaster was designed with afterburners to give the Allison J71 turbojets extra thrust for takeoffs. When lit the “shock diamonds” from the inboard afterburners impacted, scorched, and fatigued the fuselage. After trying many ideas on a specially built test rig, Martin engineers determined the best solution was to cant the engines five degrees outboard. Since it meant a major redesign of intakes and nacelles, the decision was to fly the two prototypes using the outboard afterburners only and wait to put the slanted engines on the pre-production YP6M. The scorching problem put the schedule six weeks behind and increased pressure for the first flight.
On the water, in the air… at last
Unknown to watching residents in the homes on the shores of Frog Mortar Creek, Middle River, and Chesapeake Bay, the first water trial of the huge, sleek, exotic-looking, dark blue aircraft was cut short for a malfunctioning hydroflap. Later, the first attempted takeoff was aborted because of a ruptured bomb bay door seal—the first of many. The next for a J71 engine failure—also the first of many.
Two days before the first flight, aviation pioneer and entrepreneur Glenn L. Martin died. At the time, only the superstitious believed his death an evil omen—later many would.
On July 14, 1955, the first four-engined jet seaplane in the world was finally airborne. The flight was limited to 1+45 because that was all the 16mm film could record of the test panel instruments. While chief test pilot George Rodney described the flight as “uneventful,” an airframe shaking at 300 knots appeared for the first, but not the last, time.
Flight testing proceeded normally over the next few months: the flight envelope was expanded, the shake investigated with tuft tests, the radical weapons door (similar to the bomb bay of the B-57 Canberra) was rotated open, and takeoffs at full operating weight, 160,000 pounds, were made. In November a press and VIP day was held with the public rollout of SeaMaster number 2 and flybys of number 1. A week later the XP6M reached .95 mach and 46,000 feet; beating the Air Force’s B-47 in payload, ceiling and speed.
The wings touched
Three Navy test pilots from Patuxent River arrived for familiarization flights. Because of low clouds and poor visibility, the first scheduled day was spent with water “flights” only (Martin numbering counted water work as flights. The XP6M’s first time in the air was its sixth “flight”). The next day was also cold and gloomy, but the cloud bases were about 10,000 feet so low-profile flights were possible.
The first of the day went well and the SeaMaster taxied in to a sea buoy for a pilot switch.
The SeaMaster had completed three of the planned seven longitudinal stability runs and was over the Potomac when witnesses saw pieces of airplane tear off in a cloud of mist which exploded into a fireball.
The SeaMaster had suddenly and violently pitched nose down. The negative G-force buckled the wing spars and carry-through attachments. The wings went so far under the fuselage that the outer panels and tip floats smashed into each other. The engines were torn from their mounts. The actuators and hinges failed and the entire horizontal tail came off. The fuselage broke at the bomb bay. A cloud of fuel from the ruptured tanks detonated, separated the forward fuselage and sent a fireball into the crew compartment.
One parachute floated into the cold water. There was another chute—a streamer—which was unnoticed in the mass of flaming debris. Large circles of seething water roiled to the surface where large chunks had hit for an hour after the explosion.
The ensuing investigation was hindered because a circuit breaker had popped, disabling the radio and wire, cockpit voice recorder. There was no chase aircraft because the flight was not considered experimental. As the wreckage was recovered, it was reassembled in a hangar at Patuxent River. A team of 100 experts investigated all major systems. The official accident report was issued in April 1956. Six possible causes were given with a long list of recommendations. The actual cause is unknown to this day.
The flight test engineer’s body with severe injuries to his head and chest was recovered immediately. He had probably been hit by falling wreckage. His automatic connections were undone, which resulted in a streamed chute. The flight deck section was hoisted to the surface 11 days later with both pilots in their seats, face curtains unpulled and the rudder pedals bent by their feet from the estimated 100 G impact. The flight engineer’s remains with the parachute attached were located ten miles up the Potomac River over three months later. Unconscious when he hit the water, he had drowned.
To show support for the program, the Secretary of the Navy made a publicized visit to the Martin plant. Admirals made statements about the importance of the P6M. The pressure was on to qualify the SeaMaster.
The second XP6M, which had been proudly displayed to the public, had obviously been grounded pending the results of the crash investigation. Many changes were made, especially in the tail, and the second aircraft had its early navigation and weapons systems removed and instrumentation installed to complete flying qualities testing. This time, besides the photopanel, telemetry including strain gauges was installed. A SeaMaster was airborne again in May of 1956 and testing resumed. There was still aerodynamic shaking and buffet when an engine was shut down or quit, but in August, Martin received a contract for 24 P6M-2s.
On November 9, the company yacht, Glenmar III, was filled with Navy VIPs to watch the second XP6M’s thirty-third flight. A pair of slats had been fastened on the bottom of the horizontal tail to lock the slaving elevator and stabilizer together in an attempt to eliminate the shake. The SeaMaster used the prototype beaching vehicle to roll into Frog Mortar Creek, took off near the Glenmar and climbed to altitude. The pilots reported there was less of a shake, so ground control asked for a second run to .9 Mach in a five-degree dive at 22,000 feet. As the run ended, the chase pilot in an FJ Fury watched amazed as the SeaMaster pitched violently up into the beginning of a loop. He frantically yelled, “Redbird, Redbird, you’re breaking up. Bail out!” The copilot reached for his face curtain and pulled. The pilot briefly fought for control before he also ejected. The flight engineers in the back both had their heads down when the Gs hit. Moments passed as they tried to make sense of the noise, smoke, and radio calls before they also ejected. Afterwards, the project test pilot said, “I pulled back on the thing and then there was, as far as I can remember, nothing but a sharp pitch-up at that point … which it really pitched hard. I applied full-forward stick with no tendency to stop the thing at all. I was pulling so many Gs that my chin was down on my chest … there was no recovery.”
The four men landed on the ground near Odessa, Delaware. The wreckage of the XP6M was scattered over 15 square miles of farm fields. (One quick-thinking farmer plowed a strip around a burning hunk of fuselage to save the rest of his barley crop.) There were no injuries.
There was no need to reconstruct the wreck as the cause of this crash was found immediately. The telemetry ran for 55 seconds after the pitch-up; the photopanel film and other recordings were found intact. The modified tail had set up forces that overcame the hydraulic actuator in compression. The load factor had reached 9.5 G before the massive aircraft slowed enough near the top of the loop for the stabilizer to behave normally, but it was too late as the XP6M was in a flat spin and coming apart. The engineers had made an error in converting hinge moment data from wind tunnel models to the actual aircraft. The computations had been double-checked at Martin and by six other agencies. As the chief of experimental test said, “We all just missed it.” A massive understatement considering the impact of a second crash. SeaMaster development continued, but now the YP6M program had to be successful … or else.
The six follow-on YP6M-1 SeaMasters achieved their design goals. One year after the second prototype’s crash, Ship No. 1 was rolled out. The tail actuator was now two cylinders for redundancy with improved hydraulics. The scorching problem was eliminated by canting the J-71 A-6 engines out 5° and the intakes were larger and repositioned. The production beaching vehicle enabled the SeaMasters to power themselves in and out of the water and taxi on land.
However, there were still problems. The airframe shake and engine-out buffet remained. The wet wing had persistent fuel leaks (eventually it was discovered the sulfur in standard JP-4 fuel was destroying the sealant used.) Spray ingestion was flaming out the engines in heavy seas or at heavy weights. (One attempted fix had a large NACA-designed deflector fastened to the side of the nose. It was called “the surf board” and painted day-glo.)
Weapons, both mines and conventional bombs, and navigation tests were successful. An aerial refueling (ARF) unit and camera package were built in response to changing requirements. The SeaMaster got to show off with low, high-speed passes at an Annapolis graduation. Twenty hours of Navy trials went well. SeaMasters were flying almost daily. Then, ominously, the order for the P6M-2 was cut from 18 to eight aircraft. Some 1,500 employees were laid off. Soon after, the Navy conducted a utilization review of the SSF program, which resulted in cancellation of further YP6M tests. One by one, the SeaMasters were grounded. The Navy was heading away from using tactical seaplanes. Only a speedy and decisive introduction of the P6M-2 version could save the program.
To help expedite the development process, Martin rearranged the order of completions and Ship No. 9 was the first P6M-2 rolled out. There was no publicity—probably more from shame than for security. Heavy ice on the waters around the Martin plant delayed the beginning of tests until February 1959.
Every improvement of the Dash-2 had seemed to create a new problem. The airline-style cockpit windows were changed to a full bubble canopy. While pilot visibility was much better, sunlight beating in made the cockpit into an oven. There were times when the pilots had to take turns as the control yokes were too hot to hold for long.
The engines were changed to Pratt & Whitney J-75s with 15,800 pounds of thrust each. (The same engine that powered the F-105 Thunderchief.) Afterburners were no longer needed. The engine cant was reduced and the intake area increased. With more thrust available, the SeaMaster’s maximum weight increased to 195,000 pounds. However, at the higher weight, the hull rode deeper and the floats at the ends of the drooped wings dug into the water badly enough that capsizing was a real possibility. During the takeoff run, spray ingestion was worse. Water-flaps were added to the bottom of the wing tips. Auxiliary air-doors were added to the engine nacelles. Finally, the wing was changed to almost level—a major manufacturing adjustment—and an aerodynamic alteration. Problems at high speed, which the YP6M had eliminated, returned. The change in wing angle that kept the tips out of the water meant that when returning from a flight and not weighing much, the SeaMaster did not stay level enough to fit into the beaching vehicle.
Dog days of August
All eight P6M-2s—some still on the assembly line—had the fixes complete or imminent. While an all-Navy crew was flying a P6M-2 on August 20, 1959, a telegram was delivered to Martin company headquarters. The aviators reported that on landing they were greeted with long faces and words of doom. The cancellation of the entire SSF/SeaMaster program was effective as of the next day. Forty-eight years of seaplane development had resulted in a technologically advanced jet flying boat that never touched salt water. None remain.
Other Jet Flying Boats
Convair’s Sea Dart was a contemporary of the P6M and had some things in common: the Navy ordered the Sea Dart into production before the prototype flew, a YF2Y broke apart on a test flight, water handling was a problem and … the concept did not work. Only two of the five built got into the air before the program was cancelled. Once, in a shallow dive, a YF2Y managed to go supersonic—the only seaplane ever to do so. (There is a claim that a P6M-2 was pushed past Mach 1 “unofficially.”)
A higher speed boat than Martin had in its Marine Division was needed to chase the P6M. Airport manufacturing manager Francis “Fuzz” Furman designed a fast, robust 28-footer and had a local boatyard build two. Powered by dual Chrysler 354 cubic-inch “Hemis” with four-barrel carburetors, “Fuzzy I” could make 50mph. “Fuzzy II” was built of lighter cedar and could reach 60. They worked long hours in all sorts of weather, but the crews did have free time while waiting for the P6Ms to land.
One it-seemed-like-a-good-idea was to capture ducks by creeping up on a flock and when it took off, accelerate and pull alongside. One eager “hunter” grabbed a panicked duck by the tail feathers only to have the bird let its bowels loose in his face.
Not a Normal Beach Buggy
To get the SeaMaster in and out of the water, a contraption called the automated beaching vehicle (BV) was invented. Martin built a prototype but then contracted with Aeronca, the light airplane company, for production. The BV was 43 feet long, 30 feet wide, 13 feet tall and weighed a hefty 25,000 pounds. The four dual-wheel struts went up and down for ground access. Pneumatic outboard buoyancy tanks raised or lowered the cradle for entry or float-off. Air-filled bumpers acted as cushions and floats. The pilots could steer on land via three high-pressure air lines connected inside the P6M for rear wheel differential braking and castering front wheels. On the water, hydroflaps and asymmetrical power allowed maneuvering up to six knots. The float-on and hook-up took less than 10 minutes. During a demonstration, seaplane-experienced, senior navy officers, used to cumbersome hook-on wheels or inefficient launch boats, whooped and hollered like midshipmen at graduation.
The scrapping—which was given many euphemistic names—took two years and 668 employees. What could be used, was. The airframes were stripped of usable equipment. The 46 J71 engines went to U.S. Navy F3H Demons. The J-75s to USAF F-105 Thunderchiefs. Sections of SeaMaster airframes were used for materials testing. However, millions of dollars worth of specialized machine tools were sold as scrap for pennies a pound.
When the last P5M Marlin came off the assembly line in December 1960, it marked the end of 30 years of aircraft production at the Middle River plant. Fortunately, as an aerospace company, Martin had diversified into electronics and missiles. The rockets that launched the early satellites and astronauts into space were Martin products. The company, which is celebrating its 100th anniversary, is now part of the huge Lockheed-Martin conglomerate. Glenn L. Martin would be happy his name survived all the corporate reorganizations.
The only remaining parts of a SeaMaster are two tail sections that survived the Dahlgren Weapons Testing Facility and a pair of tip floats intended for Furman’s Advanced Fuzzy Pontoon Boat; both are now at the G.L. Martin Museum. Otherwise, the P6M survives in the memory of the men and women who built it and the kids who put together the Revell kit of the newest and most exciting jet bomber in the world.
By R.R. “Boom” Powell photos courtesy of stan piet