Before the Thresher - the most advanced submarine of its era - sank 50 years ago, 16 submarines were lost in accidents and collisions.

Mechanical systems failed, torpedoes malfunctioned and naval ships crashed, claiming an average of one sub every three years from 1915 to 1963.

But it was the April 10, 1963, sinking of the Thresher, the first in a new class of nuclear-powered submarines, and the loss of all 129 men on board that so rocked the Navy that its leaders resolved never to let it happen again.

"It served as an awakening to the Navy that we had a problem," Rear Adm. David M. Duryea, the deputy commander for undersea warfare, said in a recent interview. "If you look at the reaction to the sinking, within two months we realized we needed to do something different. Within two months we developed a program that became known as SUBSAFE, a program that instituted a fundamental change in how we did business."

The Navy has never lost a SUBSAFE-certified submarine.

Quality assurance procedures were "inadequate" before the submarine safety standards were implemented, according to a statement from the Navy.

When the recently commissioned Thresher returned to the Portsmouth Naval Shipyard, where it was built in 1962, for an overhaul, there was a new ultrasonic method for inspecting pipe welds.

But this method was used only when there was enough time, according to the Navy, and mechanics conducted most of the inspections. Their supervisors checked their work. Unlike today, third-party inspectors were not used extensively.

The mission of SUBSAFE is to provide the "maximum reasonable assurance" that submarines will remain watertight and, in the event of flooding, will be able to surface.

The Navy has said a silver-brazed joint in a seawater pipe in the engine room on the Thresher (SSN 593) failed during deep-diving tests east of Cape Cod, and the seawater shorted out an electric panel that triggered the reactor to shut down. The submarine couldn't get to the surface because of a design flaw in the system that blows water out of the main ballast tanks to lighten the ship. With no propulsion, and with the added weight of the water, the ship sank below crush depth and imploded.

Sixteen officers, 96 enlisted men and 17 civilian technicians were on board.

It was the first time a nuclear submarine had been lost, and it remains the world's worst submarine disaster, in terms of lives lost.

In the aftermath, the Navy created the safety program and changed the design of its submarines. The flaw in the emergency main ballast tank blow system was fixed by removing strainers that had been placed on pipes to keep debris out of the tanks. Ice had formed on the strainers on the Thresher and had blocked high pressure air from getting into the tanks.

Vital electrical equipment was better protected from water, and flexible pipe connections were removed.

Today, all submarines are certified through SUBSAFE or they don't go to sea, Duryea said.

James M. Noonan, director of SUBSAFE and quality assurance at Electric Boat, said he is focused on the hull, the support structure and all systems that are integral to watertight integrity and recoverability. In shipyard vernacular, Noonan said, his job is to "keep water out of the people tank."

Each hull joint is welded by a trained, qualified welder, who has many years of experience and regular eye exams, he said. The joints are then examined using radiography. If the EB team concludes they are solid and if the Navy agrees, records - a lot of records - are maintained so they can be checked by outside auditors, he added.

The only way to be 100 percent certain the weld is good would be to cut the weld apart and look, Noonan said, but then the submarine would be destroyed.

Noonan talks to EB employees, particularly new hires, about the Thresher and tells them lives could be at stake if they don't do their work perfectly.

Noonan was in third grade in North Adams, Mass., when the Thresher sank. He remembers his teacher telling the class a submarine had been lost off the coast of Massachusetts. It was a watershed event, he said, and "as a people, we've learned our lesson."

"A U.S. nuclear submarine is the most complex machine built by humankind, and it's still one of the safest," he said. "And quite frankly, that's our challenge every day, too, to keep the record intact since the Thresher."

The USS Scorpion sank in 1968, but it was not certified through SUBSAFE. The Scorpion was built before the Thresher sank, and the design changes had not been incorporated.

The USS San Francisco was severely damaged in 2005 when it struck an undersea mountain. Duryea and Noonan both said SUBSAFE was critical to that submarine's safe return.

After so long without another loss, Duryea said, he worries people will become complacent, ignorant or arrogant. There constantly are new suppliers of submarine parts and new employees who have to learn the standards and the safety culture, he added.

Duryea said he doesn't predict any major changes to the program and, despite cuts to the defense budget, funding for it is not at risk. He said he couldn't say how much it costs to certify each submarine because the standards are maintained throughout a submarine's life.

"The Navy SUBSAFE community remembers the crew members and the shipyard workers who were on the Thresher," he said. "And every day, we work to make sure that it does not happen again. Those folks are in our memories every day."

j.mcdermott@theday.com