The Type UB I submarine (sometimes known as the UB-1 class) was a class of small coastal submarines (U-boats) built in Germany at the beginning of the First World War. 20 boats were constructed, most of which went into service with the German Imperial Navy. Boats of this design were also operated by the Austro-Hungarian Navy (Kaiserliche und Königliche Kriegsmarine or K.u.K. Kriegsmarine) and the Bulgarian Navy. In the Austro-Hungarian Navy, it was called the . Built to meet the need for small maneuverable submarines able to operate in the narrow, shallow seas off Flanders, the vessels were intended to be quickly constructed, then shipped by rail and assembled at their port of operation. The design effort began in mid-August 1914 and by mid-October the first 15 boats were ordered from two German shipyards. The German Imperial Navy subsequently ordered an additional pair of boats to replace two sold to Austria-Hungary, who ordered a further three boats in April 1915. A total of 20 UB Is were built. Construction of the first boats for Germany began in early November 1914; all 20 were completed by October 1915. Several of the first boats underwent trials in German home waters, but the rest were assembled and tested at either Antwerp or Pola. The German boats operated primarily in the Flanders, Baltic, and Constantinople Flotillas. The boats were about long and displaced when surfaced and while submerged. All had two bow torpedo tubes and two torpedoes, and were equipped with a deck-mounted machine gun. In 1918 four of the surviving German boats were converted into coastal minelayers. Of the seventeen boats in German service, two were sold to Austria-Hungary, one was sold to Bulgaria, and nine were lost during the war. One of the five Austro-Hungarian boats was sunk and another mined and not repaired. The five surviving German boats, the four surviving Austro-Hungarian boats, and the Bulgarian boat were all turned over to the Allies after the end of the war and were broken up. Design In the earliest stages of the First World War the German Army's rapid advance along the North Sea coast found the German Imperial Navy without submarines suitable to operate in the narrow and shallow seas off Flanders. By 18 August 1914, two weeks after the German invasion of Belgium, the planning of a series of small coastal submarines had already begun. The German Imperial Navy stipulated