They reluctantly ordered four U-20 boats in 1915 because construction could start immediately, even though the Havmanden-class design was largely obsolete by the beginning of the war.

Political considerations caused the order to be split between Austrian and Hungarian firms, which contributed to construction problems and delays, keeping any of the boats from being operational until the middle of 1917.

The Austro-Hungarian Navy's U-boat fleet at the beginning of World War I consisted of six largely experimental submarines, two of which were not operational.

[3][Note 1] When it became apparent that the war would not be a short one,[4] Austria-Hungary moved to bolster their U-boat fleet by seizing the plans for the Danish Havmanden-class submarines,[5] three of which had been built at Whitehead's in Fiume.

[6] This allocation of contracts, while politically expedient, exacerbated technical problems that resulted in numerous modifications and delays.

U-20, the lead boat of the class, sank in July 1918 when she was torpedoed by the Italian submarine F12 in the Tagliamento estuary.

[5] SM U-20, the lead boat of the class, was laid down on 29 September 1915 at the Pola Navy Yard.

[1] During diving trials in March 1917, U-20 was accidentally rammed by the Austro-Hungarian Navy light cruiser Admiral Spaun, which required seven months of repairs for the U-boat.

U-20 was hit by one torpedo at a range of 650 yards (590 m) and sank with all hands west of Trieste near position 45°29′N 13°02′E / 45.483°N 13.033°E / 45.483; 13.033.

[9] Remains of her crewmen were interred on the grounds of the Theresian Military Academy at Wiener Neustadt.

[1] The still-incomplete boat sank in the harbor at Fiume in June 1917, but was raised, repaired, and relaunched in October.

After three months of repairs for her failed electric motor in mid 1918, U-22 resumed service, patrolling the Montenegrin coast out of Cattaro.

[7] The following day,[9] Linienschiffsleutnant Klemens Ritter von Bézard, U-23's only commanding officer,[12] guided the boat in an attack on the Italian transport Memfi in the Straits of Otranto.