Summary and Key Points: Caleb Larson traces the April 10, 1963, loss of USS Thresher (SSN-593), lead boat of the Permit class born of 1956’s Project Nobska, with all 129 aboard during deep-dive tests off Cape Cod. After reporting “minor difficulties” to rescue ship USS Skylark (ASR-20), a brazed pipe joint likely flooded the engine room, scrammed the reactor and, with iced high-pressure air lines blocking the ballast blow, sent the hull past crush depth. The SUBSAFE program followed.
The USS Thresher SSN-593 Disaster Is a Dark Page in Military History for the US Navy
The USS Thresher was the most sophisticated submarine in the world at the time. It sank because one pipe joint was brazed instead of welded, and 129 men died in a chain of failures that took less than a few minutes.
The USS Thresher (SSN-593) was one of the world’s most advanced submarines in the 1960s.
Its loss marked a consequential chapter in U.S. naval history. After the USS Thresher disaster cost all hands, the incident forced the Navy to take a clear-eyed look at submarine design, construction, and safety.
Project Nobska
The Thresher story began with Project Nobska. The U.S. Navy established the project in 1956, bringing together the nation’s top physicists and naval authorities to reassess undersea warfare in the nuclear age.
How would nuclear propulsion change anti-submarine warfare? How would sailors find some of the most destructive weapons ever built? Project Nobska sought answers to these questions.
USS Thresher. Image Credit: Creative Commons.
USS Thresher Artist Rendering. Image Credit: Creative Commons.
Image: Creative Commons.
Project Nobska first concluded that submarines should place an absolute premium on reducing their acoustic signatures and boosting their sonar performance, rather than purely improving speed.
Second, they should orient their mission toward hunting down Soviet submarines, rather than serving primarily as surface attack platforms. This was a significant shift from previous submarine designs, which emphasized maneuverability and speed. The result was the Permit-class.
Permit-class
The Permit-class featured a teardrop-shaped hull and a large bow-mounted sonar sphere that was, for its time, very powerful. The class also incorporated the acoustic mitigation measures suggested by Project Nobska.
It marked the Navy’s first concerted effort to build a modern class of anti-submarine warfare submarines. On paper, the class was far more sophisticated than its predecessors or contemporaries, but disaster lurked on the horizon.
The Disaster
On April 10, 1963, the USS Thresher was conducting deep-dive tests off the coast of Cape Cod. During one dive, the submarine reported a problem to the USS Skylark (ASR-20), a submarine rescue ship.
Via an underwater telephone connection, the Thresher reported experiencing “minor difficulties.” It attempted to blow its ballast and force a surface.
But just moments later, a garbled, hard-to-follow transmission reached the Skylark suggesting the submarine had lost control. Contact was severed shortly thereafter. The submarine sank, and all hands—129 sailors and civilian personnel—were lost with the sub.
The Failure
It cannot be known with absolute certainty what exactly went wrong. But the Navy’s investigation, later supported by recorded acoustic data, indicated a chain of failures.
Seawater is believed to have leaked into the engine room through a brazed, not welded, pipe joint. Water spraying on electrical equipment may have caused a scram, forcing the reactor to shut down as an automatic precaution.
With propulsion lost, the crew tried to blow ballast tanks to the surface—but failed.
Moisture within the submarine’s high-pressure air system may have caused ice to form within the piping, blocking airflow.
Without power or buoyancy, the submarine sank past crush depth. At extreme depth, the submarine’s hull collapsed.
All on board were most likely killed instantly. The incident was a cascade of failures exacerbated by relatively minor engineering issues. But when combined, they were catastrophic.
The Impact on the US Navy
The incident was among the worst submarine disasters in maritime history.
An exhaustive post-incident investigation found significant gaps in submarine manufacturing.
Quality control was inconsistent—inexcusable in any context, but especially for deep-ocean operations that depend on nuclear reactors for power.
Testing and submarine certification were also lacking, and emergency recovery was inadequate for deep-dive rescues. The investigation was damning, but it kick-started new safety standards that ultimately contributed to the Navy’s shining safety record thanks to the creation of the SUBSAFE program.
SUBSAFE
SUBSAFE changed submarine construction and maintenance. All components face must meet the rigors of sea pressure and are subject to strict certification standards.
The materials used to build the submarines must be traceable to ensure accountability for workmanship. Later submarine designs incorporated redundant, fully tested emergency ballast blow systems.
But the culture changed, too. Concern over speed and cost gave way to engineering discipline and quality control.
The changes paid huge dividends.
Following the implementation of the SUBSAFE program, no U.S. Navy submarines have been lost due to flooding or structural failure—this is a remarkable record and a testament to SUBSAFE’s extreme rigor.
Upward and Onward
Project Nobska pushed the Navy toward a quieter, more sophisticated submarine fleet, but one that relied heavily on complex systems to achieve underwater dominance—without a corresponding increase in reliability and build quality.
Virginia-Class Submarine.
Consequently, the USS Thresher was the world’s most sophisticated submarine, but it was also vulnerable to mishap. Ultimately, the Thresher forced the Navy to seriously reevaluate the safety of its submarine designs and, consequently, the safety of its sailors.
And given the Navy’s post-Thresher record, SUBSAFE was a success—albeit a costly one.
About the Author: Caleb Larson
Caleb Larson is an American multiformat journalist based in Berlin, Germany. His work covers the intersection of conflict and society, focusing on American foreign policy and European security. He has reported from Germany, Russia, and the United States. Most recently, he covered the war in Ukraine, reporting extensively on the shifting battle lines in Donbas and the war’s civilian and humanitarian toll. Previously, he worked as a Defense Reporter for POLITICO Europe. You can follow his latest work on X.