SM U-21 was a U-boat built for the Imperial German Navy shortly before World War I. The third of four Type U-19-class submarines, these were the first U-boats in German service to be equipped with diesel engines. U-21 was built between 1911 and October 1913 at the Kaiserliche Werft (Imperial Shipyard) in Danzig. She was armed with four torpedo tubes and a single deck gun; a second gun was added during her career.
In September 1914, U-21 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. She also sank several transports in the English Channel and the Irish Sea later in the year, all in accordance with the cruiser rules then in effect. In early 1915, U-21 was transferred to the Mediterranean Sea to support the Ottoman Empire against the Anglo-French attacks during the Gallipoli Campaign. Shortly after her arrival, she sank the British battleships HMS Triumph and HMS Majestic while they were bombarding Ottoman positions at Gallipoli. Further successes followed in the Mediterranean in 1916, including the sinking of the French armoured cruiser Amiral Charner in February.
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