SM UC-21

	March 18, 1917, Imperial German Navy U-boat SM UC-21 sinking American tanker steamer Illinois
History
German Empire
Name	UC-21
Ordered	29 August 1915
Builder	Blohm & Voss, Hamburg
Yard number	271
Launched	1 April 1916
Commissioned	12 September 1916
Fate	Missing since September 1917
General characteristics
Class & type	Type UC II submarine
Displacement	417 t (410 long tons), surfaced; 493 t (485 long tons), submerged;
Length	49.35 m (161 ft 11 in) o/a; 39.30 m (128 ft 11 in) pressure hull;
Beam	5.22 m (17 ft 2 in) o/a; 3.65 m (12 ft) pressure hull;
Draught	3.68 m (12 ft 1 in)
Propulsion	2 × propeller shafts; 2 × 6-cylinder, 4-stroke diesel engines, 500 bhp (370 kW); 2 × electric motors, 460 shp (340 kW);
Speed	11.6 knots (21.5 km/h; 13.3 mph), surfaced; 7.0 knots (13.0 km/h; 8.1 mph), submerged;
Range	9,430 nmi (17,460 km; 10,850 mi) at 7 knots (13 km/h; 8.1 mph), surfaced; 55 nmi (102 km; 63 mi) at 4 knots (7.4 km/h; 4.6 mph), submerged;
Test depth	50 m (160 ft)
Complement	26
Armament	6 × 100 cm (39.4 in) mine tubes; 18 × UC 200 mines; 3 × 50 cm (19.7 in) torpedo tubes (2 bow/external; one stern); 7 × torpedoes; 1 × 8.8 cm (3.5 in) Uk L/30 deck gun;
Notes	35-second diving time
Service record
Part of:	Flandern Flotilla; 14 November 1916 – 30 September 1917;
Commanders:	Oblt.z.S. Reinhold Saltzwedel; 15 September 1916 – 9 June 1917; Oblt.z.S. Werner von Zerboni di Sposetti; 10 June – 30 September 1917;
Operations:	11 patrols
Victories:	95 merchant ships sunk ; (131,844 GRT); 3 auxiliary warships sunk ; (2,219 GRT); 5 merchant ships damaged ; (11,826 GRT); 1 warship damaged ; (778 tons); 1 merchant ship taken as prize ; (148 GRT);

SM UC-21 was a German Type UC II minelaying submarine or U-boat in the German Imperial Navy (German: 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 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.