SM U-21 was a notable German Type U 19 U-boat constructed for the Imperial German Navy shortly before World War I. Built at the Kaiserliche Werft in Danzig between 1911 and October 1913, she measured 64.15 meters (210 feet 6 inches) in length overall, with a beam of 6.10 meters (20 feet) and a height of 8.10 meters (26 feet 7 inches). Her displacement was 650 tonnes (640 long tons) when surfaced and 837 tonnes (824 long tons) submerged. U-21 featured a propulsion system comprising two 8-cylinder 2-stroke MAN diesel engines for surface travel and two electric double motors with batteries for submerged operation. This made her the first German submarine equipped with diesel engines, marking a significant development in U-boat technology. She could reach a top speed of 15.4 knots on the surface and 9.5 knots underwater. Armament included four 50-centimeter (19.7-inch) torpedo tubes—two in the bow and two in the stern—carrying a total of six torpedoes. She was initially armed with a machine gun, which was replaced in 1914 by an 8.8 cm (3.5 inch) SK L/30 deck gun, with a second gun added in 1916. Her complement consisted of four officers and twenty-five sailors. U-21’s service history was distinguished by several notable achievements. She became the first submarine to sink a ship with a self-propelled torpedo when she destroyed the cruiser HMS Pathfinder off the Firth of Forth in September 1914, causing the ship to explode and sink with 261 casualties. Throughout 1914 and 1915, she sank numerous transports in the English Channel and Irish Sea, adhering to cruiser rules by stopping ships and ensuring crew safety before sinking them. She transferred to the Mediterranean in 1915, where she supported Ottoman efforts during the Gallipoli Campaign, sinking the British battleships HMS Triumph and HMS Majestic—both significant victories that temporarily disrupted Allied naval operations in the area. In 1916, U-21 served under the Austro-Hungarian Navy as U-36, due to Germany’s restrictions on attacking Italian ships. She continued her patrols, sinking various Allied and neutral ships, and gaining a reputation for effective torpedo tactics. Later, she returned to Germany in 1917 to join unrestricted submarine warfare, sinking numerous merchant vessels across European waters. She spent her final years as a training submarine and survived the war, but she sank accidentally in 1919 while under tow to Britain. Over her career, U-21 sank 40 ships totaling 113,580 gross register tons, marking her as a significant vessel in early submarine warfare history.

This description has been generated using GPT-4.1-NANO based on the Vessel's wikidata information and then modified by ShipIndex.org staff.