U-21 formed the third of the four-strong class of U-19 U-boats in service with the Imperial German Navy heading into World War 1 (1914-1918). The class represented a prewar German attack submarine design and the first in German naval service to feature diesel engines as standard propulsion. The class was pushed into action with the outbreak of war in July of 1914 and netted its first kill in September - U-21 becoming the first submarine in naval history to record the destruction of an enemy surface ship by way of a self-propelled torpedo (HMS Pathfinder). U-21 was built by Kaiserliche Werft Danzig and saw her keel laid down in 1910. Her hull was launched on February 8th, 1913 and she was formally commissioned on October 22nd of that same year.
As completed U-21 displaced at 720 tons (short) when surfaced and 923 tons when submerged. She held a length of 210.5 feet with a beam measuring 20 feet and a draught of 11.8 feet. Drive power was through 2 x MAN 8-cylinder diesel engines driving two shafts for surface travel and 2 x AEG Motordynamos for use submerged travel. Maximum speed (surfaced) was 15.5 knots (9.5 knots submerged). U-21's crew complement totaled 29 and included four officer-level personnel. Armament was 4 x 500mm torpedo tubes and 1 x 88mm SK L/30 deck gun.
As World War 1 began to evolve among its many players, U-21 found herself stationed near the Heligoland archipelago off the coast of northwest Germany. War patrols soon became a way of life for U-boat crews as the British Royal Navy fleet remained a constant threat to German operations in the North Sea and the Atlantic. Her first patrol commenced in August though no action was reported duirng that service and through her second war patrol. It was not until September 5th that U-21 met HMS Pathfinder near the Isle of May when the German submarine first spotted its prey. After a period of maneuvering, U-21 managed a successful torpedo hit on Pathfinder which, in turn, ignited one of Pathfinder's magazines and caused her to explode. The attack killed 261 British sailors. During 1916, U-21 was given a second 88mm deck gun for improved surface attack capabilities.
From September 1914 to June 1917, U-21 managed a lengthy kill record that began with Pathfinder and ended with the Swedish ship "Baltic". Over her wartime career, U-21 completed eleven total patrols and netted 38 merchant vessels along with four enemy warships. Total merchant tonnage equaled nearly 88,000 tons along with 34,575 tons from the warships sunk. Her career took her from the North Sea in 1914 to the Mediterranean during 1915-1917 where she and her crew were called to support a new German ally in Turkey. During the Gallipoli Campaign, U-21 proved instrumental in threatening Allied warships from concentrating useful close support fire for landing ground forces -sinking two (of eighteen present) pre-Dreadnought battleships in the action. She formed a portion of the Black Sea Flotilla and then saw herself commissioned as part of the Austro-Hungarian Navy to allow her free reign in attacking Italian ships - Germany was not yet formally at war with Italy at this point but the Italians were at war with German ally Austria-Hungary. She served the Austro-Hungarian flag as "U-36" until the Italian declaration of war on Germany on August 27th, 1916. For 1917, she was recalled to the North Sea to once-again target British warships. In 1918, she was made part of III U-boat Flotilla and later served in the training role for the rest of the war - which ended with the Armistice of November 11th, 1918.
With the German surrender, U-21 was collected by the British and it was during the operation to bring her to British waters under tow that U-21 took on water and sunk - ending her ocean-going career in full.