The submarine had been laid down in Kiel in November 1913 as U-7, the lead ship of the U-7 class for the Austro-Hungarian Navy (German: Kaiserliche und Königliche Kriegsmarine or K.u.K.

They became convinced after the outbreak of war in August 1914 that none of these submarines could be delivered to the Adriatic via Gibraltar, and sold the entire class, including U-7, to the German Imperial Navy in November 1914.

As a part of the Baltic and IV Flotillas, U-66 sank 25 ships with a combined gross register tonnage of 73,847 in six war patrols.

A postwar German study offered no explanation for U-66's loss, although British records suggest that she may have struck a mine in the Dogger Bank area.

[10] In A Naval History of World War I, author Paul G. Halpern reports on part of the German response, which was an experiment involving U-66.

As in the first submarine offensive, U-boats were sent independently around Scotland to patrol the Irish Sea and the western entrance to the English Channel.

On that date she was in the vicinity of Fastnet Rock and came upon the 3,890-ton British refrigerated cargo ship Zent headed from Garston to Santa Marta in ballast.

U-66 torpedoed Zent 28 nautical miles (52 km; 32 mi) from Fastnet and sank the ship with the loss of 49 crewmen;[14] the master and nine sailors were rescued and landed at Queenstown.

[15] Over the next two days, U-66 dispatched two French sailing vessels, the 151-ton Binicaise,[16] and the 397-ton fishing smack Sainte Marie west of the Isles of Scilly.

[12] The 4,341-ton Eastern City was sailing from Saint-Nazaire to Barry Roads in ballast when she was shelled by U-66 and sent to the bottom 18 nautical miles (33 km; 21 mi) from Ushant;[21] all of her crew survived and were landed by 11 April.

[19] U-66's next victim was the 2,888-ton Glenalmond sailing from Bilbao to Clyde laden with iron ore. Torpedoes from U-66 sank the ship 27 nautical miles (50 km; 31 mi) north of Ushant,[22] but all her crew were saved.

U-66 sank the British steamer Margam Abbey 55 nautical miles (102 km; 63 mi) southwest of the Lizard while the ship was en route from Bordeaux to Barry Roads in ballast.

[29] At 09:00 on 31 May, U-66 sent out a wireless report of eight battleships, light cruisers, and destroyers on a northerly course 60 nautical miles (110 km; 69 mi) east of Kinnaird Head.

Inverdruie was carrying a load of pit props from Sandefjord to Hartlepool when she was sunk some 160 nautical miles (300 km; 180 mi) east of Aberdeen.

U-66 tried repeatedly to deal the stricken cruiser a coup de grâce, but narrowly missed with torpedoes on several further attacks.

One depth charge attack blew out all the lights on U-66 and knocked clips off two hatches that caused the boat to flood with a considerable quantity of water before the leaks could be sealed.

[34] Records on U-66 next appear in late 1916, when she is reported as one of the U-boat escorts assisting the German merchant raider Wolf into the North Atlantic.

[35] Wolf, under the command of Karl August Nerger, began a 15-month raiding voyage on 30 November that took the ship into the Indian and Pacific Oceans before a safe return to Germany.

The ship and her general cargo, headed from Göteborg to Hull, were sent to the bottom without loss of life,[37][38] and her crew was safely landed by 14 December.

[40] From the early stages of the war the Royal Navy had blockaded Germany, preventing neutral shipping from reaching German ports.

[42] With the blockade having such dire consequences, Kaiser Wilhelm II personally approved a resumption of unrestricted submarine warfare to begin on 1 February 1917 to help force the British to make peace.

[49] Five days later, U-66 encountered the five-masted bark Neath 28 nautical miles (52 km; 32 mi) south by east of Fastnet Rock.

[50] Equipped with an auxiliary triple-expansion steam engine,[51] Neath was the former German bark R. C. Rickmers which had been seized by the Admiralty at Cardiff in August 1914.

[52] During April 1917, German U-boats sank 860,334 tons of Allied and neutral shipping, a monthly total unsurpassed in either of the two world wars.

[53] U-66's sole contribution to this figure came when she torpedoed the tanker Powhatan 25 nautical miles (46 km; 29 mi) from North Rona in the Outer Hebrides.

The 4,329-ton Ikalis, carrying wheat from New York to Manchester, was torpedoed and sunk 170 nautical miles (310 km; 200 mi) from Fastnet Rock.

[63] U-66 scored another success when she torpedoed and sank the outbound British steamer African Prince on 21 July 60 nautical miles (110 km; 69 mi) north-northwest of Tory Island.

[12] U-66 began her seventh and what was to be her final patrol on the morning of 2 September when she departed from Emden destined for operations in the North Channel.

British records suggest that U-66 may have either struck a mine in an older minefield in the Dogger Bank area, or that a combination of destroyers, submarines, and anti-submarine net tenders sank U-66 sometime between 1 and 11 October.