World War 1 (1914-1918) would mark the last recorded actions of the Austro-Hungarian Navy, a fighting force in service since 1786. In 1904 a committee was arranged to draw up plans for an indigenous submarine but it and others were passed on during a 1905 review. In the end, the service elected for an improved version of the existing German U-1 which became the U-3 for the Austro-Hungarians. From this meager beginning evolved several other submarine types prior to The Great War and all were based on foreign designs like the American C-class. With the assassination of Archduke Ferdinand in 1914 to officially begin the conflict, the Austrian-Navy was put on alert - though just six boats made up its submarine force and two of these were not even operational.
Pressed to strengthen its undersea fighting force amidst the growing war commitment, the Navy pushed for construction of a design based on the U-20-class which were built from the period spanning 1915 to 1917 and commissioned from 1916 to 1918. Four were completed including "SM U-21". Despite their generally obsolete status by the time the war arrived, any boat was better than none and local industry was charged with delivering the submarines for wartime service (construction having been split between Austrian and Hungarian firms which did not help matters much).
U-21 displaced 170 tons (long) when surfaced and 210 tons (long) when submerged. She held a length of 127.1 feet with a beam of 13 feet and a draught down to 9 feet. Power was from a combination diesel-electric propulsion system with the diesel units used for surface running and battery-driven motors for undersea running. Power from the diesel installation totaled 450 horsepower with 160 available from the electric motors. These drove a single shaft at the stern. Maximum speed was 12 knots when surfaced and 9 knots when submerged with ranges out to 1,400 nautical miles (estimated) and 23 nautical miles respectively. Internally the boat was crewed by eighteen personnel made up of officers and sailors. Armament centered on 2 x 450mm torpedo tubes in bow-facing, bow-mounted positions. Beyond this, the vessel could call upon a 66mm deck gun and an 8mm machine gun for surface attacks.
U-21 was ordered on March 27th, 1915 and saw her keel laid down shortly thereafter - such was the expediency asked of Austro-Hungarian industry to deliver viable submarines. She was launched on August 15th, 1916 and officially commissioned on August 15th, 1917. She marked the first boat of her class to enter service.
As with other Austro-Hungarian submarines of World War 1, the impact of U-21 in the conflict proved quite minimal. From the outset she was beset with self-inflicted damage owing to both her design and generally inexperienced crews. Damage forced her to be laid up early in her career and a leak ended one of her wartime patrols. During a run in July of 1918, the boat encountered an issue with her diesels and had to be towed to Pola for repairs and it was in this state that she remained until the Armistice of November 1918. Following the Austro-Hungarian surrender, U-21 was passed on to Italy as war reparations which led to the submarine being stripped of her value and ultimately scrapped in 1920.