SM UC-31 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 7 August 1916. She was commissioned into the German Imperial Navy on 2 September 1916 as SM UC-31."SM" stands for "Seiner Majestät" () and combined with the U for Unterseeboot would be translated as His Majesty's Submarine. In 13 patrols UC-31 was credited with sinking 38 ships, either by torpedo or by mines laid. UC-31 was surrendered on 26 November 1918 and broken up at Canning Town in 1922.
Design A Type UC II submarine, UC-31 had a displacement of when at the surface and while submerged. She had a length overall 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 48 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-31 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.Fate31 December 1916Protector200Sunk4 January 1917Lonclara1,294Sunk25 January 1917O. B. Suhr1,482Sunk28 January 1917Alexandra179Sunk29 January 1917Shamrock173Sunk29 January 1917Thistle167Sunk31 January 1917Ravensbourne1,226Sunk24 February 1917Beneficent1,963Sunk29 March 1917Kathleen Lily521Sunk30 March 1917Harberton1,443Sunk4 April 1917Helga839Sunk5 April 1917N. J. Fjord1,425Sunk6 April 1917HMT Strathrannoch215Sunk11 April 1917Quaggy993Sunk12 April 1917Dina Hinderika200Sunk12 April 1917Neptunus209Sunk12 April 1917Union152Sunk12 April 1917Voorwaarts147Sunk14 April 1917Spray1,072Sunk15 April 1917Brothertoft155Sunk6 May 1917Poseidon I98Sunk8 May 1917HMY Zarefah279Sunk17 May 1917Aspen3,103Damaged17 May 1917Viken1,825Sunk30 June 1917Lady of the Lake51Sunk1 July 1917Amstelland5,404Sunk2 July 1917Thirlby2,009Sunk3 July 1917Matador3,642Sunk7 August 1917Othalia1,205Damaged11 August 1917Holar548Sunk12 August 1917Bogatyr1,360Sunk8 September 1917Newholm3,399Sunk11 September 1917Cento3,708Damaged16 September 1917Quatre Freres53Sunk13 November 1917Amelie1,135Sunk13 November 1917Australbush4,398Sunk19 November 1917HMT Morococala265Sunk21 January 1918Teelin Head1,718Sunk29 January 1918Ethelinda3,257Sunk1 April 1918Ardglass4,617Sunk5 April 1918Cyrene2,904Sunk10 April 1918HMS Magic1,025Damaged
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