The Type U 66 was a class of five submarines or U-boats operated by the German Imperial Navy () during World War I. The class is alternately referred to as the U-66-class or the Type UD. The class was built by Germaniawerft of Kiel to their 506d design as the U-7-class for the Austro-Hungarian Navy. The five boats were sold to the Imperial Germany Navy at the beginning of World War I when it was thought impossible for the submarines to reach the Mediterranean for delivery to Austria-Hungary. The Austro-Hungarian Navy, after competitively evaluating six submarines of three foreign designs, selected the Germaniwerft 506d or Type UD design over a design from Whitehead & Co. for the U-7 class. The boats, numbered U-7 to U-11, were designed to be long and displace between when surfaced and submerged. They were to be armed with five torpedo tubes and a deck gun. For propulsion the design called for twin diesel engines for surface running and twin electric motors for subsurface movement. The Austro-Hungarian Navy ordered the boats in February 1913 and construction began on the first boats in November. After the outbreak of World War I in August 1914, the Austro-Hungarian Navy became convinced that delivery of the still-unfinished submarines to the Mediterranean via Gibraltar would be impossible. As a result, they sold the five boats to the Imperial German Navy in November 1914. The German Navy assigned the numbers U-66 to U-70 to the five submarines and had them redesigned and reconstructed to their specifications. These changes, which included a larger deck gun, increased the displacement of the U-boats by almost surfaced and nearly submerged. All five boats saw active service, and four sank 15 or more ships. Only , sunk six days into her first war patrol in March 1916, had no successes. Two other boats, and disappeared in 1917. The remaining two U-boats, and , were surrendered to the United Kingdom and were broken up by 1921. Background In 1904, the Austro-Hungarian Navy, after allowing the navies of other countries to pioneer submarine developments, ordered the Austrian Naval Technical Committee ( or ) to produce a submarine design. When the Navy rejected the January 1905 MTK design and other designs submitted as part of a public competition as impracticable, they instead opted to order two submarines each of designs by Simon Lake, Germaniawerft, and John Philip Holland for a