The GRP domes improved the bow sonar sphere performance; though for intelligence gathering missions, the towed-array sonar was normally used as it was a much more sensitive array. Noise reduction Several Sturgeon boats and related submarines were modifications of the original designs to test ways to reduce noise. was outfitted with Raytheon Harmonic Power Conditioners which eliminated an electrical bus noise problem that was inherent in the class. This was done by harmonic conditioning of the power system. This successful feature was later outfitted on the entire class. and among others were outfitted with SHT (Special Hull Treatment) during a non-refueling overhaul, which reduced noise and the submarine sonar profile. , a one-ship class, was completed using a turbo-electric system for main propulsion rather than a reduction gear drive from the steam turbines. The massive motor and associated generators required her to be lengthened to . The Lipscombs trial of turbo-electric propulsion was not considered successful due to lower speed - top speed was , 5 knots slower than the Thresher/Permits - and a lack of reliability, and she was decommissioned in 1989. , the quietest submarine of her era, was had a similar but distinct design to the Sturgeon-class submarines. The Narwhal (SCB 245) and the Sturgeon class (SCB 188A) were developed simultaneously. Forward the Narwhal resembled the Sturgeon-class submarines, but with a relocated diesel generator and slightly greater beam. Aft she had the natural circulation S5G reactor and a direct-drive turbine, along with several other quieting features. Unlike the Sturgeon class, the Narwhal did not fully comply with SUBSAFE regulations due to her unique main seawater system. Variants Beginning with , units of this class had a longer hull, giving them more living and working space than previous submarines. received an additional hull extension containing cable tapping equipment that brought her total length to . A number of the long hull Sturgeon-class SSNs, including Parche, L. Mendel Rivers, and Richard B. Russell were involved in top-secret reconnaissance missions, including cable tap operations in the Barents and Okhotsk seas. Parche received nine Presidential Unit Citations for successful missions. A total of seven boats were modified to carry the SEAL Dry Deck Shelter (DDS). The DDS is a submersible launch hangar with a lockout chamber attached to the ship's midships weapons shipping hatch, facilitating the use of SEAL Delivery Vehicles. DDS-equipped boats were tasked with the covert insertion of special