" In the mid-1950s, the U.S. Navy was still pushing nuclear propulsion out to the submarine fleet. USS Nautilus, the world’s first nuclear submarine, had just been commissioned in 1954, and nine classes of submarines were created, including the Sailfish, Barbel, Skate and Skipjack classes, before the Navy felt it had a design worthy of mass production. Preceding classes of nuclear submarines were built in small batches, but Thresher would be the first class to build more than five. Altogether fourteen Threshers would be built.

The Threshers were designed to be fast, deep-diving nuclear attack submarines. They were the second class, after the pioneering Skipjack class, designed with the new streamlined hull still in use today. They were the first submarines to use high strength HY-80 steel alloy, which was used through the 1980s on the Los Angeles class.
The submarines were just 278 feet long, with a beam of thirty-one feet. They weighed 4,369 tons submerged, making them about 30 percent larger than the Skipjacks. Their S5W pressurized water reactor drove two steam turbines, which turned a single propeller to a combined thirty-thousand-shaft horsepower. This gave them a surface speed of twenty knots, and thirty knots submerged. This was a noticeable improvement over the underwater speed of the older Skate class, which could manage only twenty-two knots underwater.

The ship primary sensor was a BQQ-2 bow-mounted sonar, the first bow-mounted sonar in any American attack submarine. This necessitated moving the four torpedo tubes amidships, an arrangement that is carried on to this day in the Virginia-class subs. The submarines could carry Mark 37 homing torpedoes, Mark 57 deep-water mines, Mark 60 CAPTOR mines and the SUBROC antisubmarine weapon. The Thresher would be a powerful addition to the U.S. Navy’s submarine fleet. "

http://nationalinterest.org/blog/the-buzz/the-tragic-reason-why-americas-nuclear-attack-submarines-are-19566