USS S-21 (SS-126)

USS S-21 (SS-126) was a first-group (S-1 or "Holland") S-class submarine of the United States Navy. Her keel was laid down on 19 December 1918 by the Bethlehem Shipbuilding Corporation in Quincy, Massachusetts. She was launched on 18 August 1920 sponsored by Mrs. Thomas Baxter, and commissioned on 24 August 1921 with Lieutenant R. P. Luker in command.

Service history
Following operations from New London, Connecticut, S-21 was decommissioned and returned to her builder on 31 March 1922. After she was reacquired by the Navy, S-21 recommissioned at Groton, Connecticut, on 14 September 1923 with Lieutenant Commander Palmer H. Dunbar, Jr., in command. From 1923 into 1930, S-21 operated off the northeastern coast of the United States. From January into April 1924, she visited the Panama Canal, Saint Thomas, United States Virgin Islands, and Trinidad. Departing from New London on 25 November, she visited Hawaii, from 27 April to 25 May 1925, before returning to New London in July. Following duty in the Panama Canal area, from February through April 1926, she visited Kingston, Jamaica, from 20 to 28 March 1927. The submarine operated in the Panama Canal area, from February into April 1928. From 4 October 1928 until 30 November 1928 S-21 was the vessel used in the first gravimetric observations at sea made by the United States.

S-21 served again in the Panama Canal area from March into April 1929, and from January through February 1930. Departing New London on 22 October that year, S-21 sailed via the Panama Canal and California to Pearl Harbor, arriving on 7 December. From 1931 into 1938, S-21 operated from Pearl Harbor, with the period 18 November 1932 to 24 January 1934 spent in reserve.

Departing Pearl Harbor on 15 October 1938, she sailed via California and the Panama Canal to Philadelphia, Pennsylvania, arriving on 11 December. Following overhaul she arrived at New London on 25 March 1939. She remained at New London with a partial crew from 1 June of that year until 1 September 1940, when she was returned to full duty.

On 9 December, two days after the Pearl Harbor attack, S-21 got underway for the Panama Canal Zone. Arriving ten days later, she conducted defensive patrols in the Pacific Ocean approaches to the canal through May 1942. Her second such patrol, 24 January to 7 February, was cancelled to allow her to participate in search and rescue operations for submarine USS S-26 (SS-131), which had been rammed by submarine chaser PC-460 on the first day of the patrol.

In June 1942, S-21 returned to New London. On 14 September, she was decommissioned and was transferred to the United Kingdom. As HMS P.553, she served in the Royal Navy until returned to the United States Navy, at Philadelphia, on 11 July 1944. Subsequently used as a target, she was sunk off northern New England on 23 March 1945.

USS S-21: First U.S. Gravity Measurements at Sea
The first U.S. gravity measurements at sea were made from the USS S-21 assisted by Eagle Boat 35 and Eagle Boat 58. After the first gravity measurements at sea had been made in 1926 from a submarine of the Royal Navy the Carnegie Institution proposed the U.S. take Caribbean Sea measurements using a gravimeter, or gravity meter, designed by Dr. Felix Vening Meinesz. Such measurements required the stability and lack of motion only attainable at sea on a submerged submarine. With assistance from the National Academy of Sciences, the U. S. Coast and Geodetic Survey, the U.S. Naval Observatory and U.S. Navy Hydrographic Office the expedition was arranged. Special chronometers necessary for the observations were furnished by the Naval Observatory. Dr. Meinesz accompanied the expedition with Elmer B. Collins representing the Hydrographic Office aboard S-21 with the Eagle Boats serving as tenders.

The route was from Hampton Roads starting 4 October 1928 and return by way of Key West, Florida, Galveston, Texas, Guantanamo, Cuba, the Virgin Islands and Puerto Rico with the return to Hampton Roads on 30 November 1928. The first of forty-nine gravity measurement stations was located at 36°56.7′N 76°19.19′W. The gravity measurements were accompanied by a sonic depth sounding with results published on a chart produced by the U.S. Navy Hydrographic Office. The gravity measurements were utilized in determining the shape of the Earth and of particular interest were the negative gravity anomalies. The results of this expedition, particularly the significant negative anomalies, created interest in a second expedition resulting in the Navy-Princeton gravity expedition to the West Indies in 1932 using the submarine USS S-48. That was in turn followed by a third measurement effort using USS Barracuda (SS-163) on the Gravimetric Survey Expedition.