The USS S-21 (SS-126) was an S-18-class submarine of the United States Navy, measuring approximately 219 feet 3 inches (66.8 meters) in length with a beam of 20 feet 8 inches (6.3 meters) and a mean draft of 17 feet 3 inches (5.3 meters). It displaced about 930 long tons (940 metric tons) on the surface and 1,094 long tons (1,112 metric tons) when submerged. The vessel's hull was designed for a maximum diving depth of 200 feet (61 meters). Powered by two 600-horsepower NELSECO diesel engines for surface navigation, each driving a propeller shaft, and electric motors delivering 1,175 horsepower for submerged propulsion, S-21 could reach speeds of 14.5 knots on the surface and 11 knots underwater. Its armament comprised four 21-inch (533 mm) torpedo tubes in the bow, with a total of twelve torpedoes, including eight reloads, and a single 4-inch (100 mm)/50 caliber deck gun. Constructed by Bethlehem Shipbuilding Corporation's Fore River Shipyard in Quincy, Massachusetts, her keel was laid on December 19, 1918. She was launched on August 18, 1920, and commissioned on August 24, 1921. After initial operations from New London, Connecticut, she was decommissioned in 1922 but was recommissioned in 1923, resuming patrols along the U.S. Northeast coast, the Caribbean, and the Panama Canal area. Notably, in 1928, S-21 participated in the first gravimetric measurements conducted aboard a U.S. ship at sea, contributing valuable data about Earth's shape and gravity anomalies in the Caribbean Sea. This expedition involved a route from Virginia to Puerto Rico, with measurements that influenced subsequent U.S. geophysical investigations. During the 1930s, S-21 operated extensively from Pearl Harbor, Hawaii, until 1938, after which she returned to the U.S. East Coast. Following the outbreak of World War II, she conducted patrols near Panama and later was transferred to the Royal Navy in 1942, where she served as HMS P.553 until 1944. The vessel was returned to the U.S. Navy and was ultimately used as a target, sinking in 150 feet of water off Cape Elizabeth, Maine, on March 23, 1945.

This description has been generated using GPT-4.1-NANO based on the Vessel's wikidata information and then modified by ShipIndex.org staff.