U16 or U-16 may refer to:
SM U-16 or U-XVI was a U-10-class submarine or U-boat of the Austro-Hungarian Navy (German: Kaiserliche und Königliche Kriegsmarine or K.u. K. Kriegsmarine) during World War I. U-16 was constructed in Germany and shipped by rail to Pula where she was assembled and completed in September 1915. She was commissioned in October 1915.
For most of her career, U-16 operated in the Adriatic out of Kotor patrolling off the Albanian coast. The U-boat sank one small sailing ship in November and seized another in December. U-16 carried Field Marshal Svetozar Borojević of the Austro-Hungarian Army as an observer while performing diving tests after engine repairs in January 1916. A water leak in March flooded U-16's batteries and put the U-boat out of action for about six weeks of repairs. While operating off Vlorë in mid-October 1916, U-16 sank an Italian destroyer acting as a convoy escort. U-16 was sunk in the ensuing action, but sources are not clear on the exact cause of her sinking. In all U-16 sank three ships with a combined tonnage of 711. She was the only U-10-class submarine sunk during the war.
German submarine U-16 was a Type IIB U-boat of Nazi Germany's Kriegsmarine that served during World War II. It was launched on 28 April 1936, under the command of Heinz Beduhn, with a crew of 23. Its last of four commanders was Horst Wellner.
German Type IIB submarines were enlarged versions of the original Type IIs. U-16 had a displacement of 279 tonnes (275 long tons) when at the surface and 328 tonnes (323 long tons) while submerged. Officially, the standard tonnage was 250 long tons (250 t), however. The U-boat had a total length of 42.70 m (140 ft 1 in), a pressure hull length of 28.20 m (92 ft 6 in), a beam of 4.08 m (13 ft 5 in), a height of 8.60 m (28 ft 3 in), and a draught of 3.90 m (12 ft 10 in). The submarine was powered by two MWM RS 127 S four-stroke, six-cylinder diesel engines of 700 metric horsepower (510 kW; 690 shp) for cruising, two Siemens-Schuckert PG VV 322/36 double-acting electric motors producing a total of 460 metric horsepower (340 kW; 450 shp) for use while submerged. She had two shafts and two 0.85 m (3 ft) propellers. The boat was capable of operating at depths of up to 80–150 metres (260–490 ft).