SM UC-21 was a German Type UC II minelaying submarine or U-boat in the German Imperial Navy () 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."SM" stands for "Seiner Majestät" () and combined with the U for Unterseeboot translates as His Majesty's Submarine.
In eleven patrols UC-21 was credited with sinking 98 ships, either by torpedo or by mines laid. They included the British hospital ship , 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 when at the surface and while submerged. She had a total length of , a beam of , and a draught of . The submarine was powered by two six-cylinder four-stroke diesel engines each producing (a total of ), two electric motors producing , and two propeller shafts. She had a dive time of 35 seconds and was capable of operating at a depth of .
The submarine had a maximum surface speed of and a submerged speed of . When submerged, she could operate for at ; when surfaced, she could travel at . UC-21 was fitted with six mine tubes, eighteen UC 200 mines, three torpedo tubes (one on the stern and two on the bow), seven torpedoes, and one Uk L/30 deck gun. Her complement was twenty-six crew members.
Summary of raiding history DateNameNationalityTonnageMerchant ship tonnages are in gross register tons. Military vessels are listed by tons displacement.Fate28 November 1916Clematis22Sunk28 November 1916Lady of the Lake91Sunk28 November 1916Vulcan27Sunk28 November 1916HMD Pelagia84Sunk30 November 1916Draupner1,126Sunk30 November 1916Eggesford4,414Damaged30 November 1916Therese165Sunk1 December 1916King Bleddyn4,387Sunk2 December 1916Demetrios Inglesis2,088Sunk2 December 1916Robinson186Sunk2 December 1916Uribitarte1,756Sunk3 December 1916Aiglon280Sunk3 December 1916Louise155Sunk3 December 1916Verdun184Sunk4 December 1916Pallas1,202Sunk5 December 1916Nexos1,013Sunk6 December 1916Gerona1,328Sunk7 December 1916Avristan3,818Sunk8 December 1916HMT Dagon250Sunk8 December 1916Falk1,379Sunk8 December 1916Marjolaine163Sunk8 December 1916Modum2,937Sunk17 December 1916Margaret54Sunk18 January 1917778Damaged19 January 1917Joseph Rosalie138Sunk19 January 1917Marietta Di Giorgio988Sunk19 January 1917Tremeadow3,653Sunk20 January 1917Kisagata Maru No. 32,588Sunk20 January 1917Jotunfjell2,492Damaged21 January 1917Victoire290Damaged21 January 1917Leontine124Sunk21 January 1917Saint Pierre127Sunk22 January 1917Bearnais301Sunk22 January 1917Precurseur364Sunk22 January 1917Steinmachos1,175Sunk24 January 1917Dan1,869Sunk24 January 1917Gladiateur23Sunk24 January 1917Loire III27Sunk24 January 1917Marie 325Sunk24 January 1917Quebec3,346Sunk24 January 1917Vega195Sunk25 January 1917Myrdal2,631Sunk1 February 1917Sainte Helene2,128Sunk10 February 1917Beechtree1,277Sunk11 February 1917Dernes738Sunk12 February 1917Nordcap332Sunk13 February 1917Progreso1,620Sunk14 February 1917Longscar2,777Sunk14 February 1917Mar Adriatico2,410Sunk15 February 1917Aline30Sunk15 February 1917Marion Dawson2,300Sunk16 February 1917Niobe1,319Sunk16 February 1917Pollcrea1,209Damaged17 February 1917Cabo1,254Sunk17 February 1917Silene171Sunk18 February 1917Triumph52Sunk19 February 1917Rutenfjell1,844Sunk10 March 1917Asbjørn3,459Sunk12 March 1917Alice Charles41Sunk12 March 1917Arethuse40Sunk13 March 1917Girda1,824Sunk13 March 1917Vivina3,034Sunk14 March 1917Blaamanden954Sunk14 March 1917La Marne133Sunk15 March 1917Eugene Robert98Sunk15 March 1917Fleur D’Esperance24Sunk15 March 1917Frimaire1,778Sunk15 March 1917Petit Jean21Sunk16 March 1917Anais130Sunk16 March 1917Madeleine Davoust148Sunk16 March 1917Ronald3,021Sunk18 March 1917Illinois5,225Sunk17 April 1917HMHS Donegal1,885Sunk19 April 1917Cilurnum3,126Sunk20 April 1917Georgios3,124Sunk21 April 1917Emile Et Charlotte41Sunk21 April 1917Ville De Dieppe1,254Sunk22 April 1917Capenor2,536Sunk22 April 1917Percy Birdsall1,127Sunk22 April 1917Valerie2,140Sunk24 April 1917Barnton1,858Sunk25 April 1917Baigorry2,161Sunk26 April 1917Boy Denis41Sunk22 May 1917Jeune Albert25Sunk23 May 1917Harwood Palmer2,885Sunk23 May 1917Lesto1,940Sunk26 May 1917Aristides2,179Sunk26 May 1917Norhaug1,245Sunk27 May 1917Efstathios3,847Sunk28 May 1917Hiram598Sunk28 May 1917Urna2,686Sunk28 May 1917Waldemar1,267Sunk30 May 1917Sørland2,472Sunk2 June 1917Tonawanda3,421Damaged7 June 1917Hafnia1,619Sunk29 June 1917Lauwerzee47Sunk4 July 1917Bestevaer1,044Sunk4 July 1917Roelfina148Captured as prize11 July 1917Coquimbo1,759Sunk15 August 1917Phoebe3,956Sunk17 August 1917Pontoporos4,049Sunk19 August 1917Therese & Marie1,615Sunk31 August 1917Marques De Mudela1,930Sunk16 September 1917Ann J. Trainer426Sunk23 September 1917St. Dunstan730Sunk
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