The third was completely constructed by Whitehead's at Fiume and purchased by Austria-Hungary to bolster their U-boat fleet after the outbreak of World War I.

They instead opted to order two submarines each of designs by Simon Lake, Germaniawerft, and Electric Boat for a competitive evaluation.

[1] The ships were powered by twin 6-cylinder gasoline engines while surfaced, but suffered from inadequate ventilation which resulted in frequent intoxication of the crew.

[1] The first two boats, U-5 and U-6, were ordered by the Austro-Hungarian Navy for evaluation and were partially assembled in the United States, shipped to Fiume, and riveted together by Whitehead & Co., which, author Edwin Sieche reports, "caused a lot of trouble".

When the surface performance of the electric motors proved disappointing in trials, SS-3's power-plant was rebuilt to match the gasoline/electric combination used in U-5 and U-6.

During their early years, each boat was demonstrated to a foreign naval delegation; U-5 to a Peruvian detachment in 1911, U-6 to a Norwegian group in 1910.

[9] At the beginning of World War I in August 1914, U-5 and U-6 comprised half of the operational U-boat fleet of the Austro-Hungarian Navy.

[3] U-12 damaged, but did not sink, the largest ship torpedoed by any of the U-5 class when she hit the French battleship Jean Bart on 21 December 1914.

[1] U-12 was sunk with the loss of all hands when she hit a mine near Venice in August 1915,[15][Note 3] while U-6 was scuttled by her crew in May 1916 after becoming trapped in an anti-submarine net that was a part of the Otranto Barrage.

[15] U-5 herself sank after hitting an Austro-Hungarian mine during a training exercise, but was raised, repaired and recommissioned before the war's end.

She was commissioned into the Austro-Hungarian Navy in April 1910, and served as a training boat—sometimes making as many as ten cruises a month—through the beginning of the First World War in 1914.

[3] The submarine scored most of her wartime successes during the first year of the war while under the command of Georg Ritter von Trapp.

She was laid down in 1909 and launched in March 1911 and featured improvements in the electrical and mechanical systems from the Holland design of her older sister boats, U-5 and U-6.

[16] While searching for targets in the vicinity of Venice in August 1915, U-12 struck a mine that blew her stern off, and sank with all hands,[15] becoming the first Austro-Hungarian submarine sunk in the war.