Last October, the Puget Sound Naval Shipyard completed the year-long dismantling of the US Navy nuclear-powered attack submarine USS Narwhal (SSN-671). Commissioned in 1969, the submarine was a one-of-a-kind, an experimental design that would become one of the most successful boats of her era. She was the quietest attack submarine in the U.S. fleet in the 1970s and was routinely deployed to watch Soviet submarines in the Atlantic and around Soviet-supported satellite states in the Mediterranean Sea.

Sound is the enemy of a stealthy submarine. The technologies developed for the Narwhal have helped make US submarines among the quietest in the world’s navies. What made the Narwhal so quiet?

The submarine was built with a prototype ultra-quiet natural circulation reactor design. As described by The Drive, the engineers designed Narwhal’s unique S5G power plant with the reactor vessel situated low in the submarine’s hull and the steam generators situated above the reactor to facilitate a natural flow of the reactor’s primary coolant. This Pressurized-Water Reactor (PWR) design heated its primary loop in the reactor and convection moved the water into the steam generator near the top of the loop where energy is exchanged into the secondary system. With less energy, the primary coolant flows back down towards the reactor vessel in a return loop. 

In many conditions, this allowed natural circulation so that the noisy high-pressure pumps could be secured, removing a major sound source from a wide range of operations. 

The shipyard also installed a revolutionary, direct-drive main engine turbine that was mechanically linked to the shaft. This system was larger than the original reduction gear design, 12 feet in diameter and 30 feet in length, resulting in a slightly wider submarine than the Sturgeon class, but also a significantly quieter submarine overall.

Additionally, two seawater scoops were added to the exterior of the hull that forced water into the main seawater system as the sub moved forward, effectively supplying all engine room seawater demands without pumps. 

The Narwhal served from 1969 until 1999 when the Navy decommissioned her from active duty service. The sub deployed 17 times, and according to the Federation of American Scientists, she received numerous medals, including the Navy Unit Commendation, three Meritorious Unit Commendations, and five Battle Efficiency “E” awards.

Thanks to Alaric Bond for contributing to this post.