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. S-21 then operated off the northeastern coast of the United States. She also visited the Panama Canal, Saint Thomas in the United States Virgin Islands, and Trinidad between January and April 1924; departed New London on 25 November 1924 and visited Hawaii from 27 April to 25 May 1925 before returning to New London in July 1925; performed duty in the Panama Canal area from February through April 1926; visited Kingston, Jamaica, from 20 to 28 March 1927; and operated in the Panama Canal area again from February to April 1928.
The results of the expedition, particularly the negative anomalies, created interest in a second U.S. gravimetric expedition at sea, resulting in the Navy-Princeton gravity expedition to the West Indies in 1932 using the submarine USS S-48 (SS-159) and a third measurement effort in 1936–1937 — the Gravimetric Survey Expedition — using the submarine USS Barracuda (SS-163).[4]
1928–1941
After returning from her gravimetric expedition, S-21 resumed operations from New London along the northeast coast of the United States. She also served in the Panama Canal area from March to April 1929 and again from January through February 1930.
Departing New London on 22 October 1930, S-21 transited via the Panama Canal and California to Pearl Harbor, Hawaii, arriving there on 7 December 1930. 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, S-21 transited via California and the Panama Canal to Philadelphia, Pennsylvania, arriving on 11 December 1938. Following overhaul she arrived at New London on 25 March 1939. She remained at New London with a partial crew from 1 June 1939 until 1 September 1940, when she returned to full commission with a full crew.
World War II
The United States entered World War II with the Japanese attack on Pearl Harbor on 7 December 1941. On 9 December 1941, S-21 got underway from New London bound for the Panama Canal Zone. Arriving on 19 December 1941, she conducted defensive war patrols in the Pacific Ocean approaches to the Panama Canal through May 1942, although her second such patrol, scheduled for 24 January to 7 February 1942, was cancelled to allow her to participate in search and rescue operations for the submarine USS S-26, which had sunk on the night of 24 January after the submarine chaserPC-460 accidentally rammed her when the vessels were operating without navigation lights to avoid detection by the enemy.
In June 1942, S-21 returned to New London. On 14 September 1942, she was decommissioned.
Royal Navy
On the day of her decommissioning, S-21 was transferred to the United Kingdom. As HMS P.553, she served in the Royal Navy until returned to the U.S. Navy at Philadelphia on 11 July 1944.
^ abCarter, M. S. (2002). "The 1928 Cruise of the U. S. Submarine S-21 and its Contributions to the Measurement of Gravity". AGU Spring Meeting Abstracts. 2002: U42B–06. Bibcode:2002AGUSM.U42B..06C.
^ abPinsel, Mark I. (1982). 150 Years Of Service On The Seas - A Pictorial History of the U.S. Naval Oceanographic Office from 1830 to 1980. Washington, D.C.: U. S. Government Printing Office. p. 233. Chapter 4 "Gravity at Sea"