An acoustics expert and a naval analyst say they've solved the mystery of why the USS Thresher sank 50 years ago.

They say it was not a broken pipe that triggered the chain of events ultimately leading to the sub's destruction, as the Navy contends. Rather, they believe, it was an electrical failure.

Bruce Rule and Norman Polmar recently studied data collected from the Navy's sea floor Sound Surveillance System on the Thresher's final moments.

In 1963, that system was highly classified. It was only briefly discussed at the court of inquiry the Navy convened to look into the circumstances surrounding the loss, and was not mentioned at all during the congressional hearings that followed, said Polmar, an analyst and author of several books on submarines.

That data, as well as underwater radio messages transmitted by the Thresher - declassified over the past two decades - and other evidence, prove that an electrical "bus," similar to a junction box, malfunctioned, Polmar said. It was connected to the reactor's main coolant pumps, and when the pumps stopped, the reactor shut down, he said.

The Navy has said a silver-brazed joint in a seawater pipe in the engine room on the Thresher (SSN 593) failed on April 10, 1963, during deep-diving tests east of Cape Cod, and seawater shorted out an electric panel, triggering the reactor shutdown.

Both sides agree the submarine couldn't get to the surface because of a design flaw in the system that blows water out of the ballast tanks to lighten the ship. The reactor couldn't be restarted quickly enough. With no propulsion, and with the added weight of the water, the ship sank below crush depth and imploded, killing all 129 men on board instantly.

The Navy said in a statement that while the exact reason, or reasons, for the Thresher's loss might never be known, the Navy still believes the joint failed.

Rule analyzed acoustic events related to the loss of the Thresher in 1963 and testified at the court of inquiry. He served as the lead acoustic analyst in the Office of Naval Intelligence for 42 years. He and Polmar describe their research into the Thresher's sinking in a Navy Times article to be published Monday.

"Most people believed the Navy's story of a piping failure, which reflected poorly on the Portsmouth Naval Shipyard, where the Thresher was built," Polmar said.

Polmar said it's important to correct the historical record, but their account probably will not result in any changes to the Navy's approach to keeping submarines safe, given the amount of time that has passed and the fact that submarine designs have changed greatly.

j.mcdermott@theday.com