The keel of USS S-21 (SS-126) was laid down on 19 December 1918 by the Bethlehem Shipbuilding Corporation…a subcontractor of the Electric Boat Company of New York City, New York…at Quincy, Massachusetts. The submarine was christened by Mrs. Thomas Baxter and launched on 18 August 1920. The S-boat was commissioned on 24 August 1921 with Lieutenant R. P. Luker in command.

When commissioned, the S-1 Class coastal and harbor defense submarine was 219’3″ in length overall; had an extreme beam of 20’8″; had a normal surface displacement of 854 tons, and, when in that condition, had a mean draft of 15’11”. Submerged displacement was 1,062 tons. The submarine was of riveted construction. The designed compliment was four officers and thirty-four enlisted men. The boat could operate safely to depths of 200 feet. The submarine was armed with four 21-inch torpedo tubes…installed in the bow. Twelve torpedoes were carried. One 4-inch/50 caliber deck gun was installed. The full load of diesel oil carried was 41,921 gallons, which fueled two 600 designed brake horsepower Model 8-EB-15NR diesel engines manufactured by the New London Ship and Engine Company at Groton, Connecticut…which could drive the boat…via a diesel direct drive propulsion system…at 14.5 knots on the surface. Power for submerged propulsion was provided by a main storage battery, divided into two sixty-cell batteries, manufactured by the Electric Storage Battery Company (EXIDE) at Philadelphia, Pennsylvania…which powered two 750 designed brake horsepower main propulsion motors manufactured by the by the Ridgway Dynamo and Electric Company at Ridgway, Pennsylvania…which turned propeller shafts…which turned propellers…which could drive the submarine at 11 knots for a short period of time when operating beneath the surface of the sea. Slower submerged speeds resulted in greater endurances before the batteries needed to be recharged by the engines and generators.

Following operations from the United States Naval Submarine Base at New London/Groton, Connecticut, USS S-21 (SS-126) was decommissioned and returned to her builder on 31 March 1922…to correct problems discovered during her initial operating period. After the S-boat was reacquired by the United States Navy, the submarine was recommissioned at the Connecticut submarine base on 14 September 1923 with Lieutenant Commander Palmer H. Dunbar, Junior, in command.

From 1923 into 1930, USS S-21 operated in New England coastal waters. From January into April of 1924, the submarine visited the Panama Canal Zone; Saint Thomas in the United States Virgin Islands…and Trinidad in the southern portion of the Caribbean Sea.

Departing from New London/Groton on 25 November 1924, USS S-21 transited to the Territory of Hawaii…and visited, there, from 27 April to 25 May of 1925; then voyaged back to the submarine base at New London/Groton, Connecticut, during July of 1925.

Following duty in the Panama Canal Zone areas, from February through April of 1926, USS S-21 visited Kingston, Jamaica…from 20 to 28 March of 1927.

After operating in the water approaches to the Panama Canal, from February into April of 1928, USS S-21 later visited Saint Thomas in the United States Virgin Islands from 10 to 15 November of 1928.

USS S-21 served, again, in the Panama Canal Zone areas from March into April of 1929; and, from January through February of 1930.

Departing her Connecticut submarine base on 22 October of 1930, USS S-21 transited to the Territory of Hawaii…via the Panama Canal and California…and arrived at Pearl Harbor on 7 December 1930.

From 1931 into 1938, USS 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, USS S-21 transited…via California and the Panama Canal…to the Philadelphia Navy Yard at Philadelphia, Pennsylvania, where she arrived on 11 December 1938. Following overhaul at the Philadelphia Navy Yard, the S-boat transited to the United States Naval Submarine Base at New London/Groton, Connecticut, arriving, there, on 25 March 1939. The submarine remained at New London/Groton, with a partial crew, from 1 June 1939 until 1 September 1940, when she was returned to full duty.

On 7 December 1941, the Japanese struck targets in the Territory of Hawaii…thus plunging the United States into the Second World War as an active participant.

Two days after the Japanese attack on Pearl Harbor, USS S-21 got underway for the Panama Canal Zone. Arriving after a ten-day transit, she commenced conducting defensive patrols in the Pacific approaches to the Panama Canal…and continued to do so through May of 1942.

Her second such patrol, from 24 January to 7 February 1942, was cancelled to allow her to participate in search and rescue operations for USS S-26 (SS-131), which had been rammed by USS PC-460 at night in the Gulf of Panama. USS S-26 sank on 24 January 1942 with the loss of 46 men. Three of the crew survived. Her hull was not salvaged.

During June of 1942, USS S-21 returned to New London/Groton.

On 14 September 1942, USS S-21 was decommissioned and was transferred to the United Kingdom. In the Royal Navy, as His Majesty’s Submarine (HMS) P. 553, the S-boat served in the King’s Navy until returned to the United States Navy at the Philadelphia Navy Yard at Philadelphia, Pennsylvania, on 11 July 1944. Subsequently used as a target, the submarine was sunk off northern New England on 23 March 1945.