- The USS Thresher was supposed to be the first of a new generation of nuclear-powered attack submarines.
- In April 1963, during hull testing, ships on the surface lost contact with Thresher, and the U.S. Navy lost the boat and all hands onboard.
- Naval experts say that no single flaw sank the ship, but rather many smaller flaws came together to seal the submarine's disastrous fate.
Nearly six decades ago, the U.S. Navy experienced one of its worst accidents of the postwar era. The nuclear-powered fast-attack submarine USS Thresher sank to the bottom of the Atlantic in the spring of 1963, along with the 129 souls onboard. Although charges of a coverup were leveled at Navy officials, a release of previously secret documents in 2020 and 2021 dispelled those claims and reinforced a popular belief among naval experts: that a number of factors came together to sink the ship, ranging from inadequate training to poorly designed equipment.
The two decades after World War II were a time of major upheaval for the Navy's submarine fleet. Desperate to get submarines into the water to challenge the Soviet Union, the service also began incorporating new technologies at a breakneck speed.
Nuclear power—which allowed submarines to spend far more time underwater and operate at previously unheard-of speeds while fully submerged—was the primary technology. This prompted a shift away from the traditional submarine hull shape, which was optimized for sailing on the surface, toward a shape better-suited for underwater efficiency. This shape, known as the "teardrop" hull, featured a bulb-shaped bow; a long, featureless hull; and a tapered stern that ended in a propulsor (at the time, a propeller screw).
As a result, when the Navy commissioned USS Thresher in 1961, it looked rather different from the submarines that had won World War II just 16 years earlier. For instance, Thresher and her sister ships used a new high-strength steel called HY-80, bow-mounted sonars, and Westinghouse S5W nuclear reactors that could propel them at speeds of up to 30 knots underwater—a nearly 50 percent improvement over previous subs. Thresher came equipped with four torpedo tubes, two to port and two to starboard, and was equipped with the world's first fire-control computer.
That meant Thresher and her sister ships were faster and more heavily armed than ever. Still, Navy safety standards had not kept pace as the service's underwater submarines dove deeper and sailed faster than ever, setting the stage for the tragedy to come.
On April 9th, 1963 Thresher was conducting dive trials 220 miles east of Cape Cod, Massachusetts; they were a series of tests designed to determine her maximum safe depth. Meanwhile, the submarine rescue ship USS Skylark waited above, monitoring the test, ready to intervene in case of an emergency. At the time of the incident, Thresher was at a depth of 1,300 feet—very deep for an attack submarine at the time. Although Thresher likely would not operate at such depths under combat conditions, it was useful to know how much pressure her design and the new HY-80 steel could hold up to in an emergency scenario.
At 9:13 a.m., about 15 minutes after Thresher reached a depth of 1,300 feet, the sub radioed to Skylark: "Experiencing minor difficulties. Have positive up angle. Am attempting to blow (ballast tanks). Will keep you informed."
"Positive up angle" meant the bow was above level and the sub was in a good position to begin its ascent. "Blowing" the ballast tanks meant ejecting seawater from them, which would make the boat more buoyant and force it to rise. There was nothing in the message to explain what the "minor difficulties" were or to hint at what would come next.
Shortly afterward, Skylark received two more garbled messages, followed by a sound "like air rushing into an air tank." No further communication came from the sub and a search-and-rescue operation commenced. On September 6, the Navy bathyscaphe Trieste found Thresher's hull at a depth of 8,400 feet, the extreme pressure splitting her hull into six pieces. Among the lost souls onboard were 16 officers and 96 enlisted sailors, plus an additional 17 civilian contractors who had been observing the diving tests.
So, why did Thresher sink? It was likely due to several compounding factors, experts say. There were about 3,000 joints brazed with silver on the ship that carried water, including seawater. In 1960, according to the New York Times, silver brazing on the sub USS Barbel failed, almost sinking the ship. Tests indicated that up to 14 percent of the brazing on Thresher was likely inadequate and could fail. Investigators also believed a water leak caused by a brazing failure could have shorted out one of the main electrical bus boards—an electrical design flaw that would have caused a loss of power and sabotaged the crew's efforts to save the boat.
Another likely flaw that sank the boat: excessive moisture in the air system. In a 2003 testimony to Congress, the Navy stated it believed that the moisture led to a buildup of ice in the ballast valves, preventing them from being blown fast enough to counteract flooding. As a result, more water would have flowed into the submarine than flowed out, pushing it farther and farther down toward pressures that would destroy the hull.
The loss of Thresher in 1963, followed by USS Scorpion in 1968, led to the adoption of greater sub safety measures Navy-wide. Since then, two submarines—USS San Francisco and more recently USS Connecticut—have collided with underwater seamounts. Both submarines suffered extensive damage, but both were able to not only surface, but return to port under their own power. While the loss of nearly 250 sailors and civilians is a bitter memory, their legacy was a fleet of stronger, tougher submarines that could get their crews home, avoiding future tragedies.