for Submariners

by Hamilton 1:1 Communications, LLC

Thresher / Permit Class
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SSN-594 Thresher / Permit class


In 1956 Admiral Arleigh Burke, then CNO, requested that the Committee on Undersea Warfare of the National Academy of Sciences to study the effect of advanced technology on submarine warfare. The result of this study, dubbed "Project Nobska" was an increased emphasis on deeper-diving, ultraquiet designs utilizing long-range sonar. The Permit class was based on Project Nobska’s recommendations. Hull streamlining, reduction in sail dimensions by approximately 50%, quieting of the propulsion plant and an increase in test depth all led to a dramatic advance in submarine operational capabilities and stealth.

The SSN-594 Permit class was the world's first modern, quiet, deep-diving fast attack submarines, integrating such advanced features as a hydrodynamically shaped hull, a large bow mounted sonar array, advanced sound-silencing features, and an integrated control/attack center with the proven S5W reactor plant. These submarines were a major advance over previous submarine designs, and established the pattern of all successive American attack submarine classes, in several extremely important respects:

  • They were the first submarines to have hulls constructed of High Yield-80 (HY-80) steel alloy, which allowed operations at substantially greater depths than previous submarines.
  • They were the first submarines to have raft mountings for turbines, motors and other equipment, resulting in substantially quieter operations.
  • They were the first submarines to have a large bow-mounted sonar requiring the installation of torpedo tubes amidships, aft of the forward crew compartment.

Although they were larger than the previous SSN 585 Skipjack class, and used the same nuclear power plant, their hull design did not compromise their underwater speed. Designed for prolonged periods submerged, they were limited only by the amount of food that she could carry, and were capable of sustained operation at high speed.

These submarines were originally designated the THRESHER class, but the USS Thresher (SSN 593) was lost 200 miles off the coast of New England on 10 April 1963. According to investigators, a seawater pipe in the aft engine spaces broke, spraying water into the engine room and shorting one of the main electrical bus boards. The sub lost electrical power and couldn't operate the reactor. Darkness, a sea mist, and sheer terror inhibited the crew from manually actuating the valves. The aft part of the sub filled up with water and tilted down. With no power to get back on line, the sub drifted down to crush depth and imploded. A ghastly death for an entire crew, and one the US Navy vowed never to allow happen again.

The ill-fated USS Thresher (SSN-593) and her crew did not suffer in vain. Out of that terror and the lessons learned grew the SubSafe Program. Through this program, every submarine in the US Fleet, every pressure hull integrity-related system aboard those subs, and every pressure-related part within those systems must be certified as being 100% safe for use on a submarine. The goals are to ensure that in case of a casualty, the ship and its crew can be recovered and to ensure that the integrity of the material used on the ship can operate at design test depth. Directly related to the Thresher tragedy, sea-connected joints can no longer be brazed; they must now be welded. The SubSafe program brought other controls, too. Now when an emergency arises aboard a sub, all vital equipment which sailors would need quick access to in the event of an emergency is clearly marked and easily accessible. At all times an operator is one second away from flipping the emergency main ballast tanks to vent, so the sub can rise to the surface.

The Navy took other steps to ensure such a tragedy never occur again. Following the recommendations of a special Presidential Deep Submergence Review Group, the Deep Submergence Rescue System was developed in the mid-1960s. The deep submergence rescue vehicles Mystic (DSRV 1) and Avalon (DSRV 2) of the Deep Submergence Unit are the genesis of that program.

The last three units of this class [Flasher, Greenling, Gato] were modified during construction to incorporate lessons learned from the loss of the Thresher. Fitted with heavier machinery and a larger sail, they were ten feet longer than the other units of the class to correct stability problems caused by weight growth.

The SSN 605 Jack was fitted with an experimental direct-drive propulsion system coupled with a pair of counter-rotating propellers. Click on link: Detailed on USS Jack (SSN-605) page


General Dynamics' Electric Boat Division
594, 595, Mare Island Naval Shipyard;
596, 607, 621, Ingalls Shipbuilding;
603, 604, 612, New York Shipbuilding;
605, 606, Portsmouth Naval Shipyard; 613-615
4,200 tons submerged
3,540 tons Light Displacement
278 feet
297 feet SSN-605
292 feet SSN-613-615
Beam: 3
2 feet
28 ft Maximum Navigational Draft
Official - 20-plus knots,
Actual - 30 knots [35 mph] submerged,
Actual - 15 knots [17 mph] tactical
Operating Depth:
Official: 400 feet
Actual: 1300 feet [400 meters] test depth
Actual: 1900 feet [600 meters] collapse depth
High Yield-80 (HY-80) steel alloy
Power Plant:
One S5W nuclear reactor
Two steam turbines, 15,000 shp
One shaft,
MK 48 Torpedoes, four torpedo tubes
UGM-84A/C Harpoon
MK 57 deep water mines
MK 60 CAPTOR mines
BQQ-5 bow-mounted sonar
TB-16 Towed Sonar Array
Processing systems:
Mark 113 Fire-control system
Unit Operating Cost Annual Average
$10,000,000 (Source FY 1996 VAMOSC)