The "fleet boat" that first entered service in 1938 was a landmark in submarine design. There was no breakthrough technology involved. This model was just the synthesis of a number of good ideas that had been tried individually in earlier models, or "classes", of submarine.

That same year, the DC-3 was introduced into commercial passenger airline service. The fleet boat and the DC-3 were both built in very large numbers in World War II, primarily because they were both the most recent proven design. Both models were superseded in the 1950s, by nuclear propulsion for submarines and by jets for airliners. But both vehicles were still doing yeomen's work in the least glamorous tasks of their respective fields well into the 1960s, and even the 1970s.

Parallel with the development of nuclear reactors in submarines was an extension of the operating envelope, the limits of speed and depth at which it was possible to maneuver. Every step along the way an added performance capability in a submarine required a modest shaving of the safety margins available in the hull. Each and every step along the way was minor, apparently trivial. Eventually, a new, streamlined class of submarine was launched, known for a few brief years as the "Thresher" class.

The first boat of the class spent a few years in operational service, and then went into Portsmouth Naval Shipyard in New Hampshire for overhaul. Upon completion of the repairs in 1963 the Thresher went out to conduct sea trials. Several shipyard workers (yardbirds) went out on trials aboard the submarine. They are all, crew and yardbirds alike, still on that sea trial. They are now at a depth of 13,000 feet.

The investigation found the immediate cause of the sinking. But the investigation also found that the overall concept of a submarine had not been re-evaluated since 1938. The 150% reserve buoyancy at test depth that was available on a fleet boat was now down to 6% on the Thresher class. When a comparatively minor problem occurred on Thresher, the guys never stood a chance.

The same problem of design philosophy did not occur in the development of airplane design. The FAA acted as an independent regulator of aircraft licensing. But the Navy did not subject its own ship designs to independent, outside review.

The SUBSAFE program came out of the Thresher incident. The class of submarines got a new name, so no one would have to write to his mom that he got a great new assignment to a Thresher class boat. New safety features were retrofitted into the existing nuclear powered submarines. Things were better.

One of the items on the retrofit program was to reduce the number of pipes entering the pressure hull, and to put quick-acting, hydraulically operated valves on the few pipes that did penetrate the hull. If an internal sea water line burst, the valve could be shut in time to prevent massive flooding, and loss of the boat and crew.

Of course, there was a standard, a specification, for the system, and the valve was required to shut within a matter of seconds after it was actuated. The valve was tested periodically, and witnesses signed to verify the proper operation of the valve.

One afternoon at the Officers' Club after work, an old classmate and I were talking. He was serving on one of our older nuclear submarines. They had tested the valve on his boat that day. It did not pass the test.

This valve was never used, except to test it, you understand. It stayed open for months at a time, accumulating rust and barnacles. The first time that they tested the valve, it took three times too long to shut. If this had been an emergency at depth, all would have been lost. This needed fixing.

The solution was to cycle the valve open and shut, open and shut, open and shut, until it was free enough of corrosion and biota that it could shut quickly enough. Then the certificate was signed, verifying the safety of the system.

In real life, things were unsafe as soon as the next generation of barnacles and rust began to form on the valve. On paper, all was fine.

In the back of my mind, I filed this little story away under the caption, "Close Enough for Government Work."