In 1915 Austro-Hungarian authorities ordered construction of four submarines based on an outdated, though proven, Dutch design in the U-20-class (U-XX). This group was to succeed the aging U-14-class and serve to strengthen the Navy service in the ongoing World War - which now spanned multiple fronts and was being battled in the air, on the land and at sea. Construction of the class spanned from 1915 to 1917 with two boats completed at Pola and two at Flume. Of the four, two were ultimately lost in action.
U-20, the lead ship of the class, was ordered on March 27th, 1915 and built by the Pola Navy Yard with her keel laid down on September 29th of that year. She was launched to sea on September 18th, 1916 and formally commissioned into service on October 20th, 1917. The vessel displaced 173 tons when surfaced and 210 tons submerged. It featured a running length of 127.1 feet, a beam of 13 feet and a draught down to 9 feet. Her propulsion came from 1 x diesel engine outputting 450 horsepower for surface travel and 1 x electric motor of 160 horsepower for undersea travel - both arrangements driving 1 x shaft. Speeds reached 12 knots when surfaced and 9 knots when submerged while range (surfaced) was out to 1,400 nautical miles. Internally, she was crewed by eighteen personnel made up of officers and submariners. Her armament was 2 x 17.7" (45cm) torpedo tubes fitted to the bow with two torpedo reloads carried. For surface work she was outfitted with 1 x 66mm (2.6") deck gun. An 8mm machine gun was used for local defense.
From the outset the U-20 and its sisters were not wholly valued by Austro-Hungarian naval commanders due to their rather obsolete nature. Nevertheless, the boats were readily available and could be produced in short order - any attack submarine was better than none at this point in the growing war. The design was a compact one as submarines went, relying on a modestly-sized crew with limited inherent offensive capabilities with acceptable performance figures.
Her baptism was swift to get her to the fighting front but her early days were marred by a collision with Admiral Spaun, an Austro-Hungarian light cruiser during a March 1917 trial run. This forced her into repairs which lasted some seven months - and thus delayed her commissioning date until later in the year. Patrol work greeted her early going as the boat saw no notable action during its tours in the Adriatic Sea. The Italian Navy submarine F-12 identified U-20 as a potential target during early July 1918 and sent U-20 underwater. Finally forced to surface, U-20 was torpedoed by aggressor which caused catastrophic damage to the Austro-Hungarian boat - leading to her sinking on July 4th, none aboard were spared. It was not until the 1960s that her remains were located and portions raised while other components were scrapped.
Her recovered conning tower resides at the Museum of Military History in Vienna, Austria.
The failed U-20-class was itself succeeded by the wartime U-27-class based on a German design. This group numbered eight boats and, again, two were lost - though only one during the war itself. Production of these units spanned 196 to 1917 and featured similar crew sizes and armament while managing slightly better war records than the U-20 group - which failed to sink any enemy vessels for their part in the conflict. U-23 was the other U-20-class boat lost during World War 1. The remaining two were given up as war prizes following the November 1918 Armistice.