Three large U-cruisers, designated Type 139, were ordered from Germaniawerft of Kiel, in August 1916. Displacing nearly 2,000 tons, and with a surface speed of, they were armed with 24 torpedoes and two 15 cm deck guns, and had a cruising range of around. They carried a large enoughcomplement to furnish captured vessels with prize crews and their intended purpose was to capture or destroy merchant ships on the surface; their large-calibre deck guns and comparatively high speed allowed them to engage even armed merchant vessels. Unlike the earlier Type U-151 submarines, the Type 139 was designed from the outset for war service. Four bow and two stern torpedo tubes were fitted, but the main armament was the two 15 cm deck guns, which could be laid on target by a rangefinder on the aft section of the bridge. The conning tower's command centre was protected by of armour against the guns typically carried by enemy merchant ships, while the pressure hull was thicker than usual at, so as to increase diving depth. The superstructure was also raised by so that a shell hitting it would not penetrate the pressure hull. A Type 139 U-boat cost 8.7 million Marks at the time, the 15 cm guns accounting for around 7% of the cost. Three submarines of this type,,, and, were ordered. The later "Project 46" specified even more powerful U-cruisers, of a similar displacement to the Type 139 boats, but with an increased surface speed of, and with two 88 mm deck guns in addition to the two 150 mm guns. An even larger U-cruiser was proposed under "Project 47", but never reached construction; it would have displaced 2,500 tons, had a top speed of, and been armed with four 150 mm guns as well as six torpedo tubes, two of which would have fired to the side.
Service
The Type 139 submarines were dispatched on long-range missions, south across the Equator, and to the west across the Atlantic, operating independently. Lothar von Arnauld de la Perière commanded U-139, the first of the class, and named the submarine Kapitänleutnant Schweiger, after Walther Schwieger, who had sunk in 1915. Under von Arnauld, U-139 sank five small ships from May 1918, and sank the last ship to fall to a Type 139 U-boat in World War I on 14 October 1918.
