RogerBW's Blog

Martin P6M SeaMaster 21 April 2014

The SeaMaster was to be a flying-boat strategic bomber for the U.S. Navy.

The SeaMaster was the Navy's second attempt to remain relevant in the post-war era where the next war was expected to be nuclear. The first, the carrier United States, had fallen victim to vicious politics and budget cuts (which among other things lead to the B-36 Peacemaker being funded). The third would be the Polaris submarine-launched missile, of which more later.

Clearly the major thing the Navy had to offer over the USAF was survivability. Bomber bases could be destroyed (especially the huge runways needed by the B-36). So why not build a bomber that didn't need a base? A "seaplane striking force" could be used not only for nuclear bombardment but for conventional bombing, mine-laying, and reconnaissance. A group of them could be refuelled and rearmed at sea from a tanker submarine, completely removing the need for bases.

Convair and Martin both submitted proposals, and the Martin one was chosen for further development. The aircraft was a high-wing plane, with four turbojets (initially Allison J71s) mounted close in to the fuselage over the wing (to try to keep them out of sea-spray). The wing had a notable anhedral (down-tilt), and tip tanks doubled as stabilising floats. Many features of the rejected XB-51 prototype were reused in this plane, including the all-moving T tailplane and the traditional Martin rotating bomb bay (which for the SeaMaster had to be sealed against seawater). Defence was a pair of 20mm cannon in a tail turret.

Because the plane had to be able to make a high-speed dash over the target, the engines were afterburning, and early tests showed that they were mounted too close to the fuselage and tended to scorch it. The first prototype was lost when a control system fault set the tailplane to pitch hard down; the aircraft exceeded 9g in a loop and crashed into the Potomac River, killing all four crew. Eleven months later the second prototype was lost: a faulty elevator jack forced the aircraft into a slow climb, though this time the test crew were able to eject safely. However, further pre-production aircraft (with the engines canted outwards to prevent scorching) were ordered and tested.

The next problem was with the engines again; takeoff produced more spray than expected, especially at heavy loads, and the J71s proved unreliable when exposed to it. Continuing control system problems led to porpoising (oscillations due to feedback with lag) at some control settings, and this version of the project was cancelled.

The P6M-2 was a modified version: non-afterburning P&W J75 engines, no anhedral on the wing as increased weight made the plane sit lower in the water, a redesigned canopy for better visibility, an aerial refuelling probe, better control and avionic systems, and a buddy refuelling drogue kit for the bomb bay. However, compressibility effects meant that handling had got significantly worse, especially above Mach 0.8; this was mostly caused by the larger nacelles needed for the new engines. Random small direction and bank angle changes made the aircraft effectively unusable at first. But it could still make nearly Mach 0.9, 5,000 feet off the deck.

However, that third attempt at relevance was coming in, and the SeaMaster was competing for funds against the Polaris missile and submarine. While its problems were slowly being solved, it was well over budget and behind schedule, the community in the Navy that would fight for seaplanes was much smaller than the community that would fight for submarines or aircraft carriers, and manned bombers looked increasingly non-survivable in the "age of the missile".

Martin attempted to sell the basic approach on the civilian market and as a military transport, in an eight-engine airliner/cargo version called the SeaMistress, but nobody bit; the company switched purely to electronics and missiles, never building another aircraft. Two seaplane tenders and one submarine underwent conversion to support P6M squadrons, but testing never got that far.


  1. Posted by John Dallman at 05:20pm on 23 April 2014

    The problem with all seaplanes applied: they need reasonably calm water for takeoffs and landings, and that makes them much more dependent on weather than runway-based aircraft. You can't sell weapons that can't be used for nice, organised training in peacetime.

  2. Posted by RogerBW at 08:13am on 24 April 2014

    That would also have made at-sea replenishment excessively exciting. Initial training could presumably be done at a sheltered harbour or lake or something, but a deployed squadron might well not have had that luxury while trying to maintain readiness.

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