SM U-5 or U-V was the lead boat of the U-5 class of submarines or U-boats built for and operated by the Austro-Hungarian Navy (German: Kaiserliche und Königliche Kriegsmarine, K.u.K.

The boat 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.

The sinking of Italian troop transport ship SS Principe Umberto in June 1916 with the loss of 1,926 men, was the worst naval disaster of World War I in terms of human lives lost.

U-5 was built as part of a plan by the Austro-Hungarian Navy to competitively evaluate foreign submarine designs from Simon Lake, Germaniawerft, and John Philip Holland.

[2] U-5's design featured a single-hull with a teardrop-shaped body that bore a strong resemblance to modern nuclear submarines.

[7] For surface running, U-5 was outfitted with 2 gasoline engines, but suffered from inadequate ventilation, which resulted in frequent intoxication of the crew;[8] her underwater propulsion was by two electric motors.

[5] In April 1915, Georg Ritter von Trapp assumed command of U-5,[6] and the following month, led the boat in sinking the French armored cruiser Léon Gambetta off Santa Maria di Leuca.

[5] On the night of 26 April, Léon Gambetta was patrolling the Straits of Otranto at a leisurely 6.5 knots (12.0 km/h) without the benefit of a destroyer screen.

The ship was rocked by the explosions of the two torpedoes and went down in ten minutes, taking down with her the entire complement of officers, including Rear Admiral Victor Baptistin Sénès.

[5] In early August, U-5 was sent out from Lissa when the Austro-Hungarian Navy received word from a reconnaissance aircraft that an Italian submarine had been sighted at Pelagosa.

[18] When U-5 surfaced just offshore, Nereide's commanding officer, Capitano di Corvetta Carlo del Greco, cast off the lines and maneuvered to get a shot at von Trapp's boat.

U-5 lined up a shot and launched a single torpedo at the slowly submerging target, striking her, and sending her to the bottom with all hands.