The USS Thresher Lost, April 10, 1963

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Today in 1963, the USS Thresher, an American nuclear-powered attack submarine, sank in the Atlantic Ocean 220 miles east of Cape Cod, Massachusetts. Everyone on board-129 officers, enlisted men and civilian technicians-went down with the ship. The disaster shocked the world and changed the way the US Navy operates and maintains its submarine fleet.

The Thresher was the lead ship in what was planned to be a 14-ship class when she was commissioned in August, 1961. She used a proven nuclear reactor design (the S5W built by Westinghouse) and carried four torpedo tubes amidships to make way for new and powerful bow-mounted sonar equipment. Her normal crew compliment was 16 officers and 96 enlisted men. Thresher could dive to 1,300 feet and run at over 30 knots (or 35 miles per hour) submerged. She was designed to hunt and kill Soviet submarines and surface warships and was the finest war machine her country could produce for that task.

In the spring of 1963, the Thresher was coming out of her first refit period and was made ready for post-overhaul trials. On April 9, the sub and her escort, the submarine rescue ship USS Skylark, headed for open ocean off the coast of Massachusetts. The next morning, she began deep-diving tests, staying in contact with the Skylark via underwater telephone.

Communications between the two ships soon became garbled. From what could be made out, Skylark reported that the Thresher had experienced some sort of difficulty and was still diving. Finally, a short message was understood clearly “...minor difficulties, have positive up-angle, attempting to blow.” It was the last message the submarine would ever send. Two days later, the Navy announced to the world that Thresher and aboard her were lost.

The water in the area of the sinking is 8,400 feet deep, far deeper than any normal submarine could go. The Navy used the deep-diving bathyscaphe Trieste and oceanographic surface ships to find the wreckage. Eventually, the ship was found to be in six major sections; smaller debris was found in an area of about 134,000 square meters. 

A Naval Court of Inquiry determined that the ship probably sank due to a failure in the saltwater intake and piping system on the ship, which was not welded but used silver brazing to hold pipe joints together. Post-overhaul tests using ultrasound equipment found that 14% of the Thresher's brazed joints were problematic, but this was not considered a large enough risk at the time to warrant repair. The failure of one or more brazing joints at test depth could have caused the submarine to take on more water than the ship's ballast tanks, which create buoyancy, could have compensated for.

Later tests also showed that moisture in the sub's high pressure air flasks, which are used to blow seawater from the ballast tanks, could have caused ice to form inside the piping to the ballast tanks, leading to the flasks' inability to clear seawater from the ballast tanks. This would have made it impossible for the Thresher to surface.

Another problem, this one operational, could have contributed to the loss of Thresher. The officers and enlisted men tasked with running the ship's nuclear reactor received two years of intensive training in the Navy's Nuclear Power Program. At that time, the thinking was that in the event the reactor was shut down due to an electrical short or manually, it was imperative to keep the reactor warm so it could be restarted quickly. This means the secondary side of the reactor plant, which produces the steam which ultimately drives the vessel, would need to be cut off, leaving the submarine with no propulsion. Thresher's Reactor Control Officer was not on the boat the day of the loss---he was home with his wife who was recovering from an accident. Thus, the reactor plant was supervised that day by an officer only recently graduated from Nuclear Power School. Although we have no way of knowing if the rules were followed, it would have been drilled into every Power School officer to close the main steam valves leading to the ship's twin turbines if the reactor SCRAMed, meaning shut down. This likely occurred as seawater entering the aft of the boat shorted out electrical panels. Once shut, those valves had to be opened by hand. With Thresher sinking tail-first past her crush depth, it would not have been humanly possible to open those valves before pressure from the ocean outside crushed the sub. Would a more experienced Reactor Control Office have saved the ship by using all the steam at his disposal to drive the ship to the surface? We'll never know.

As a result of the loss of the Thresher, the Navy instituted the SUBSAFE program designed to ensure proper construction and maintenance of any component of a submarine which comes into contact with seawater. No SUBSAFE-certified ship has ever been lost.

The US Navy continues to monitor the area of the Thresher sinking to ensure that harmful levels of radiation are not released into the area. To this day, the nuclear fuel remains intact in the reactor and radiation remains typical of worldwide background levels.

The loss of the USS Thresher resulted in design changes to the other 13 ships of the class. The second ship of the class, the USS Permit, assumed the role of class leader after loss of Thresher. The last Permit-class submarine was retired from the US Navy in 1994.