SM U-66 was the lead ship of the Type U-66 submarines or U-boats for the Imperial German Navy () during World War I. The submarine had been laid down in Kiel in November 1913 as U-7, the lead ship of the U-7 class for the Austro-Hungarian Navy ( or ). They became convinced after the outbreak of war in August 1914 that none of these submarines could be delivered to the Adriatic via Gibraltar, and sold the entire class, including U-7, to the German Imperial Navy in November 1914.
Under German control, the class became known as the U-66 type and the boats were renumbered; U-7 became U-66, and all were redesigned and reconstructed to German specifications. U-66 was launched in April 1915 and commissioned in July. As completed, she displaced when surfaced and submerged. The boat was long and was armed with five torpedo tubes and a deck gun.
As a part of the Baltic and IV Flotillas, U-66 sank 25 ships with a combined gross register tonnage of 73,847 in six war patrols. The U-boat also torpedoed and damaged the British cruiser in August 1916. U-66 left Emden on her seventh patrol on 2 September 1917 for operations in the North Channel. The following day the U-boat reported her position in the North Sea but neither she nor any of her 40-man crew were ever heard from again. A postwar German study offered no explanation for U-66s loss, although British records suggest that she may have struck a mine in the Dogger Bank area.
Design and construction
After the Austro-Hungarian Navy had competitively evaluated three foreign submarine designs, it selected the Germaniawerft 506d design, also known as the Type UD, for its new U-7 class of five submarines. The Navy ordered five boats on 1 February 1913.
The U-7 class was seen by the Austro-Hungarian Navy as an improved version of its U-3 class, which was also a Germaniawerft design. As designed for the Austro-Hungarian Navy, the boats were to displace on the surface and while submerged. The double-hulled boats were to be long overall with a beam of and a draft of . The Austrian specifications called for two shafts with twin diesel engines ( total) for surface running at up to , and twin electric motors ( total) for a maximum of when submerged. The boats were designed with five torpedo tubes; four located in the bow, one in the stern. The boats' armament was to also include a single /26 deck gun.
U-7 and sister boat were both laid down on 1 November 1913, the first two boats of the class begun. Their construction was scheduled for completion within 29 to 33 months, but neither U-7 nor any of her sister boats were complete when World War I began in August 1914. Because the boats were under construction at Kiel on the Baltic Sea, the Austrians became convinced that it would be impossible to take delivery: the boats would need to be transferred into the Mediterranean past Gibraltar, a British territory. As a result, U-7 and her four sisters were sold to the Imperial German Navy on 28 November 1914.
U-7 was renumbered by the Germans as U-66 when her class was redesignated as the Type U-66. The Imperial German Navy had the submarines redesigned and reconstructed to German standards, increasing the surface and submerged displacements by , respectively. The torpedo load was increased by a third, from 9 to 12, and the deck gun size was upgraded from the size originally specified to SK L/30.
Early career
U-66 was launched on 22 April 1915. On 23 July, SM U-66 was commissioned into the Imperial German Navy under the command of Kapitänleutnant (Kptlt.) Thorwald von Bothmer, a 31-year-old, thirteen-year veteran of the Imperial German Navy. U-66 was assigned to the Baltic Flotilla () on 17 October.
In late September, the British submarine flotilla in the Baltic began a submarine offensive against German ships, intending to deny free passage of cargo, especially iron ore, from neutral Sweden to Germany. In A Naval History of World War I, author Paul G. Halpern reports on part of the German response, which was an experiment involving U-66. The U-boat was towed behind an "innocent-looking vessel" and connected to the host ship by a telephone line in addition to the towline. U-66 was able to cast off at a moment's notice to attack an enemy submarine. Halpern does not report on any encounters by U-66, nor does he provide any insight into the overall effectiveness of the plan. U-66 was not credited with the sinking of any vessels of any kind during this time. On 15 January 1916, she was transferred from the Baltic Flotilla into the IV Flotilla (), where she joined her sister boats and .