The Sturgeon-class (colloquially in Navy circles, the 637 class) attack submarine (SSN) were the "work horses" of the submarine attack fleet throughout much of the Cold War. They were phased out in the 1990s and early 21st century in favor of the Los Angeles and Virginia classes.
All Sturgeons were designed to surface through ice, with a reinforced sail and diving planes capable of rotating all the way to vertical. Those in Squadron 4 all had modified designs. Sunfish had a polymer bow, which allowed better sonar reception. Batfish had rubber insulation mounted on its hull for quieter operation. Narwhal, which was nearly a sub-class of its own, was completed with an S5G reactor which was cooled using natural convection rather than pumps. The Glenard P. Lipscomb was a trials submarine which was completed using a large electric motor for main propulsion rather than direct drive from the steam turbines. The Lipscomb’s trial of electric propulsion was not considered successful due to lack of reliability and she was decommissioned in 1989.
Beginning with Archerfish (SSN-678), units of this class had a 10-foot (3.0m) longer hull, giving them more living and working space than previous submarines. Parche (SSN-683) received an additional 100-foot (30m) hull extension containing cable tapping equipment that brought her total length to 401 feet (122m). A number of the long hull Sturgeon-class SSNs, including Parche, Rivers, and Russell were involved in top-secret reconnaissance missions, including cable tap operations in the Barents and Okhotsk seas.
A total of six boats were modified to carry the SEAL Dry Deck Shelter, indicated below by "(DDS)". The DDS is a submersible launch hangar with a hyperbaric chamber attached to the ship's weapon shipping hatch. DDS-equipped boats were tasked with the covert insertion of special forces troops.