SM U-73 was one of 329 submarines serving in the Imperial German Navy in World War I. She engaged in the commerce war as part of the First Battle of the Atlantic. U-73 has the distinction of being responsible for laying the underwater mine that later led to the sinking of the largest ship sunk during World War I, the 48,158 ton hospital ship Britannic.
Operations After completion at Danzig in November 1915, U-73 was commissioned by Kapitänleutnant Gustav Sieß. She joined the Kiel School, where she remained until February 1916, conducting trials and crew training. She then left for the North Sea and was attached to the 1st Half Flotilla. Her activities were monitored throughout the war by Room 40, and most of her recorded movements are based on that information.National Archives, Kew: HW 7/3 Her first operational cruise began 1 April 1916, when she left Heligoland Bight, bound for the Mediterranean by way of the North Sea. En route, she attacked one steamer in the Atlantic and laid mines off Lisbon and Malta. On 27 April 1916 she laid a minefield of 22 mines outside the Grand Harbour of Valletta in which four ships were sunk: the battleship ; the sloop ; HMT Crownsin, sunk 4 May 1916 with the loss of 11 men; and the yacht HMY Aegusa. On arriving in Cattaro on about 1 May (the date is uncertain), U-73 joined the Pola-Cattaro Flotilla.
The minelaying cruises of U-73 in the Mediterranean cannot be reconstructed. The battleship HMS Russell hit two of the mines and sank. On 7 October 1916 she is reported to have left Pola in Croatia, and the French put down to her the mine sunk off Cape Male on 12 October, as well as a minefield in the Gulf of Salonika, and mines in the Gulf of Athens on which two Greek ships were blown up. It seems certain U-73, still commanded by Sieß, laid the mine by which the hospital ship HMHS Britannic (currently the largest passenger ship resting on the seafloor and the largest ship sunk during World War I) was lost, only one hour after U-73 laid the mine. It is possible the hospital ship HMHS Braemar Castle was also damaged by one of her mines. U-73 suffered from constant machinery trouble that was common with her class. At the end of October 1918, now in the hands of Kptlt. Fritz Saupe, she was scuttled at Pola in Croatia.
Summary of raiding history DateNameNationalityTonnageMerchant ship tonnages are in gross register tons. Military vessels are listed by tons displacement.Fate11 April 1916Inverlyon1,827Sunk17 April 1916Terje Viken3,579Sunk27 April 19161,250Sunk27 April 191614,000Sunk28 April 1916HMY Aegusa1,242Sunk4 May 1916HMT Crownsin137Sunk3 August 1916HMS Clacton820Sunk9 August 1916Lorenzo Donato140Sunk24 October 1916Propontis700Sunk31 October 1916Kiki Issaias2,993Sunk14 November 1916Burdigala12,009Sunk20 November 1916Spetzai788Damaged20 November 1916Sparti961Damaged21 November 1916 48,158Sunk23 November 1916HMHS Braemar Castle6,318Damaged21 December 1916Murex3,564Sunk23 December 1916Thistleban4,117Sunk4 January 1917Peresvyet13,500Sunk12 March 1917Bilswood3,097Sunk29 September 1917R 23515Sunk30 September 1917Midlothian1,321Sunk30 September 1917Nicolosa50Sunk1 October 1917Ludovicos50Sunk19 October 1918Almerian3,030Sunk
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