The second USS Thresher (SSN-593) was the lead boat of her class of nuclear-powered attack submarines in the United States Navy. She was the U.S. Navy's second submarine to be named after the thresher shark.
On 10 April 1963, Thresher sank during deep-diving tests about 220 miles (350 km) east of Boston, Massachusetts, killing all 129 crew and shipyard personnel aboard. Her loss was a watershed for the U.S. Navy, leading to the implementation of a rigorous submarine safety program known as SUBSAFE. The first nuclear submarine lost at sea, Thresher was also the first of only two submarines that killed more than 100 people aboard; the other was the Russian Kursk, which sank with 118 aboard in 2000.
07:47: Thresher begins its descent to the test depth of 1,300 feet (400 m). 07:52: Thresher levels off at 400 feet (120 m), contacts the surface, and the crew inspects the ship for leaks. None are found. 08:09: Commander Harvey reports reaching half the test depth. 08:25: Thresher reaches 1,000 feet (300 m). 09:02: Thresher is cruising at just a few knots (subs normally moved slowly and cautiously at great depths, lest a sudden jam of the diving planes send the ship below test depth in a matter of seconds.) The boat is descending in slow circles, and announces to Skylark she is turning to "Corpen [course] 090." At this point, transmission quality from the Thresher begins to noticeably degrade, possibly as a result of thermoclines. 09:09: It is believed a brazed pipe-joint ruptures in the engine room. The crew would have attempted to stop the leak; at the same time, the engine room would be filling with a cloud of mist. Under the circumstances, Commander Harvey's likely decision would have been to order full speed, full rise on the sail planes, and blowing main ballast in order to surface. Due to Joule-Thomson effect, the pressurized air rapidly expanding in the pipes cools down, condensing moisture and depositing it on strainers installed in the system to protect the moving parts of the valves; in only a few seconds the moisture freezes, clogging the strainers and blocking the air flow, halting the effort to blow ballast. Water leaking from the broken pipe most likely causes short circuits leading to an automatic shutdown of the ship's reactor, causing a loss of propulsion. The logical action at this point would have been for Harvey to order propulsion shifted to a battery-powered backup system. As soon as the flooding was contained, the engine room crew would have begun to restart the reactor, an operation that would be expected to take at least 7 minutes. 09:12: Skylark pages Thresher on the underwater telephone: "Gertrude check, K [over]." With no immediate response (although Skylark is still unaware of the conditions aboard Thresher), the signal "K" is repeated twice. 09:13: Harvey reports status via underwater telephone. The transmission is garbled, though some words are recognizable: "[We are] experiencing minor difficulty, have positive up-angle, attempting to blow." The submarine, growing heavier from water flooding the engine room, continues its descent, probably tail-first. Another attempt to empty the ballast tanks is performed, again failing due to the formation of ice. Officers on the Skylark could hear the hiss of compressed air over the loudspeaker at this point. 09:14: Skylark acknowledges with a brisk, "Roger, out," awaiting further updates from the SSN. A follow-up message, "No contacts in area," is sent to reassure Thresher she can surface quickly, without fear of collision, if required. 09:15: Skylark queries Thresher about her intentions: "My course 270 degrees. Interrogative range and bearing from you." There is no response, and Skylark's captain, Lieutenant Commander Hecker, sends his own gertrude message to the submarine, "Are you in control?" 09:16: Skylark picks up a garbled transmission from Thresher, transcribed in the ship's log as "900 N." [The meaning of this message is unclear, and was not discussed at the enquiry; it may have indicated the submarine's depth and course, or it may have referred to a Navy "event number" (1000 indicating loss of submarine), with the "N" signifying a negative response to the query from Skylark, "Are you in control?"] 09:17: A second transmission is received, with the partially recognizable phrase "exceeding test depth...." The leak from the broken pipe grows with increased pressure. 09:18: Skylark detects a high-energy low-frequency noise with characteristics of an implosion. 09:20: Skylark continues to page Thresher, repeatedly calling for a radio check, a smoke bomb, or some other indication of the boat's condition. 11:04: Skylark attempts to transmit a message to COMSUBLANT (Commander, Submarines, Atlantic Fleet): "Unable to communicate with Thresher since 0917R. Have been calling by UQC voice and CW, QHB, CW every minute. Explosive signals every 10 minutes with no success. Last transmission received was garbled. Indicated Thresher was approaching test depth.... Conducting expanding search." Radio problems meant that COMSUBLANT did not receive and respond to this message until 12:45. Hecker initiated "Event SUBMISS [loss of a submarine]" procedures at 11:21, and continued to repeatedly hail the Thresher until after 17:00. On April 11, at a news conference at 10:30, the Navy officially declared the ship as lost.
