SM UC-21

SM UC-21 was a German Type UC II minelaying submarine or U-boat in the German Imperial Navy (Kaiserliche Marine) during World War I. The U-boat was ordered on 29 August 1915 and was launched on 1 April 1916. She was commissioned into the German Imperial Navy on 12 September 1916 as SM UC-21.

In eleven patrols UC-21 was credited with sinking 98 ships, either by torpedo or by mines laid. They included the British hospital ship SS Donegal, which UC-21 torpedoed in the English Channel on 17 April 1917, killing 29 already wounded soldiers and 12 crew.

UC-21 disappeared after departing Zeebrugge for the Bay of Biscay on 13 September 1917.

Design
Like all pre-UC-25 Type UC II submarines, UC-21 had a displacement of 417 t when at the surface and 493 t while submerged. She had a total length of 49.35 m, a beam of 5.22 m, and a draught of 3.65 m. The submarine was powered by two six-cylinder four-stroke diesel engines each producing 250 PS (a total of 500 PS), two electric motors producing 460 PS, and two propeller shafts. She had a dive time of 35 seconds and was capable of operating at a depth of 50 m.

The submarine had a maximum surface speed of 11.6 kn and a submerged speed of 7 kn. When submerged, she could operate for 55 nmi at 4 kn; when surfaced, she could travel 9430 nmi at 7 kn. UC-21 was fitted with six 100 cm mine tubes, eighteen UC 200 mines, three 50 cm torpedo tubes (one on the stern and two on the bow), seven torpedoes, and one 8.8 cm Uk L/30 deck gun. Her complement was twenty-six crew members.