SM U-2 or U-II was the second ship of the of submarines or U-boats built for and operated by the Austro-Hungarian Navy ( or ). U-2 was designed by American naval architect Simon Lake of the Lake Torpedo Boat Company, and constructed at the navy yard in Pola. She was one of two Lake-designed submarines purchased as part of a competitive evaluation of foreign submarine designs after domestic proposals were rejected by the Navy.

Ordered on 24 November 1906, U-2 was laid down in July 1907 before being launched in April 1909. She was long and displaced between depending on whether she was surfaced or submerged. An experimental design, U-2 included unique features such as a diving chamber and wheels for traveling along the seabed. Originally powered by gasoline engines for surface running, it was discovered during her sea trials throughout 1909 and 1910 that these engines were found to be incapable of reaching the submarine's contracted speed and posed a risk of poisoning the ship's crew. U-2 was commissioned in June 1911 and served as a training boat through 1914, though she was mobilized briefly during the Balkan Wars. U-2s design has been described by naval historians as a failure that was rendered obsolete by the time she was commissioned into the Austro-Hungarian Navy. Despite these criticisms, tests of her design provided information which the Navy used to construct subsequent submarines.

At the beginning of World War I, U-2 was in drydock awaiting new batteries and replacement diesel engines. This refit resulting in a changed bow for the ship as well as changes in her displacement. As part of this overhaul, U-2 was also fitted with a new conning tower. She returned to service as a training boat in June 1915. U-2 conducted reconnaissance cruises out of Trieste until she was declared obsolete in early 1918. The ship continued to serve in a training role at the submarine base on Brioni, but was at Pola at the end of the war. Facing defeat in October 1918, the Austro-Hungarian government transferred its navy to the newly formed State of Slovenes, Croats and Serbs in order to avoid having to hand its ships over to the Allied Powers. Following the Armistice of Villa Giusti in November 1918 however, U-2 was seized by Italian forces and subsequently granted to the Kingdom of Italy under the Treaty of Saint-Germain-en-Laye in 1920. Italy chose to scrap the submarine, and she was broken up at Pola later that same year without ever having sunk or damaged any vessels during her career.

Background Prior to 1904, the Austro-Hungarian Navy had shown little to no interest in submarines, preferring to instead observe other navies experiment with the relatively new type of ship. In early 1904, after allowing the navies of other countries to pioneer submarine developments, Constructor General (German: Generalschiffbauingenieur) of the Austro-Hungarian Navy Siegfried Popper ordered the Naval Technical Committee (German: Marinetechnisches Kommittee, MTK) to produce a submarine design. Technical problems during the initial design phase, however, further delayed any proposals from MTK for nearly a year.

After observing the MTK design submitted in early 1905, the Naval Section of the War Ministry (German: Marinesektion) remained skeptical. Additional proposals submitted by the public as part of a design competition were all rejected as well by the Austro-Hungarian Navy as being impracticable. As a result, the Navy decided to purchase designs from three different foreign firms for a class of submarines. Each design was to be accompanied by two submarines in order to test each ship against one another. This was done in order to properly evaluate the different proposals which would come forward. Simon Lake, owner of the Lake Torpedo Boat Company of Bridgeport, Connecticut, was chosen as one of the participants in this design competition by the Navy. After being recommended by Popper, Lake traveled to Austria-Hungary in 1906 to negotiate the details of his contract with the Navy, which formally ordered plans for the building of two boats—including U-2—on 24 November.

Design

Although intended to serve as an experimental design when initially ordered, U-2 and her sister ship U-1 would be the first submarines of the Austro-Hungarian Navy. However, both ships would prove to be a disappointment. U-2 and her sister ship were described by the naval historians David Dickson, Vincent O'Hara, and Richard Worth as "obsolete and unreliable when completed and suffered from problems even after modifications". René Greger, another naval historian, wrote that U-2 and her sister ship "proved a total failure".

Despite these criticisms and shortcomings, the experimental nature of the submarine provided valuable information for the Austro-Hungarian Navy, and Lake's designs did address what the Navy was asking for when ordering the submarine class. John Poluhowich writes in his book Argonaut: The Submarine Legacy of Simon Lake that U-2 was "completed to the satisfaction of Austrian officials".

U-2 was constructed in line with Austro-Hungarian naval policy at the time, which stressed coastal defense and patrolling of the Adriatic Sea. As a result, she was intended to spend most of her time on the surface. Lake envisioned U-2 and her sister ship only submerging while conducting surprise attacks or in order to escape pursuit from an enemy warship. Following the onset of World War I, however, it became clear that Austro-Hungarian U-boats were best suited for offensive operations, namely raiding Allied shipping in the Adriatic and Mediterranean Seas.

General characteristics

Built with a double hull, U-2 had an overall length of , with a beam of and a draft of . She was designed to displace while surfaced, but while submerged she displaced . U-2 was designed for a crew of 17 officers and men.

U-2 was derived from an earlier concept for a submarine intended for peaceful sea exploration. As a result, she had several features typical of Lake's earlier designs. These including a diving chamber under the bow and two variable pitch propellers. The diving chamber was intended for manned underwater missions such as destroying ships with explosives and severing off-shore telegraph cables, as well as for exiting or entering the submarine during an emergency. This diving chamber ultimately proved its usefulness during the sea trials of U-2 and her sister ship when the crew of one submarine forgot to bring their lunches on-board before conducting an underwater endurance test. A diver from shore was able to transport lunch for the crew without the submarine having to resurface.

Lake's design also called for two retractable wheels that, in theory, could allow travel over the seabed. The design also placed the diving tanks above the waterline of the cylindrical hull, which necessitated a heavy ballast keel for vertical stability. The location of the diving tanks also necessitated the use of pumps to flood the vessel. The propulsion system for U-2 consisted of two shafts which were powered by two gasoline engines for surface running and two electric motors for running while submerged. The gasoline engines could produce , while the electric motors had an output of . This gave U-2 a range of while traveling at when surfaced, and while traveling at when submerged. For underwater steering and maneuverability, the design featured four pairs of diving planes. These planes provided the submarines with a considerable amount of maneuverability.

The submarine had three torpedo tubes—two in the bow, one in the stern—and could carry up to five torpedoes, but typically carried three. In 1917, U-2 was fitted with a deck gun, which was ultimately removed in January 1918 when the submarine resumed training duties.

Construction and commissioning

U-2 was laid down on 18 July 1907 at the navy yard (German: Seearsenal) at Pola. Construction on the submarine was delayed by the need to import the American-made engines. As the second ship of her class, U-2 was launched on 3 April 1909.

Upon completion of the submarine, the Austro-Hungarian Navy evaluated U-2 in sea trials throughout 1909 and 1910. These trials were considerably longer than other sea trials due to the experimental nature of the submarines and the desire of Austro-Hungarian naval officials to test every possible aspect of the ship. During these trials, extensive technical problems with the gasoline engines of both submarines were revealed. Exhaust fumes and gasoline vapors frequently poisoned the air inside the boat and increased the risk of internal explosions, while the engines themselves were not able reach the contracted speed, which was supposed to be while surfaced and while submerged. Indeed, the engine problems for U-2 were so significant that on multiple occasions her crew had to conduct emergency resurfacing in order to bring fresh air into the ship. Because of the problems, the Austro-Hungarian Navy considered the engines to be unsuitable for wartime use and paid only for the hulls and armament of U-2. While replacement diesel engines were ordered from the Austrian firm Maschinenfabrik Leobersdorf, they agreed to a lease of the gasoline engines at a fee of $4,544 USD annually for both U-2 and her sister ship U-1.

Flooding the diving tanks, which was necessary to dive, took over 14 minutes and 37 seconds in early tests, but was later reduced to 8 minutes. Despite the engine problems, U-2 and her sister ship had the best performance in diving and steering among the U-boats under evaluation by the Navy. At a depth of her hull began to show signs of stress and was in danger of being crushed. As a result, the commission overseeing U-2s sea trials set her maximum dive depth at this level. While surfaced, the shape of U-2s hull resulted in a significant bow-wave, which led to the bow of the ship dipping under the water while surfaced. In order to correct this problem, the deck and bow casing of the submarine was reconstructed in January 1915. Other tests proved the use of U-2s underwater wheels on the seabed to be almost impossible.

Service history

Pre-war U-2 was commissioned into the Austro-Hungarian Navy on 22 June 1911 and served as a training boat—conducting up to ten training cruises per month—through 1914. Despite being used primarily for training duties, U-2 was mobilized in late 1912 during the Balkan Wars. By the end of November 1912, the threat of conflict between Austria-Hungary, Italy, Serbia, and Russia, coupled with allegations of Serbian mistreatment of the Austro-Hungarian consul in Prisrena led to a war scare in the Balkans. Both Russia and Austria-Hungary began mobilizing troops along their border, while Austria-Hungary began to mobilize against Serbia. During the crisis, the entire Austro-Hungarian Navy was also fully mobilized. U-2 and her sister ship U-1 were both ordered to join the rest of the Austro-Hungarian fleet assembling in the Aegean Sea in the event of a war with Serbia and Russia.

By December 1912, the Austro-Hungarian Navy had, in addition to U-2 and U-1, a total of seven battleships, six cruisers, eight destroyers, 28 torpedo boats, and four submarines ready for combat. The crisis eventually subsided after the signing of the Treaty of London, and the Austro-Hungarian Army and Navy were subsequently demobilized on 28 May 1913. After demobilization, U-2 resumed her duties as a training vessel.

World War I