SM U-31 was one of the 329 submarines serving in the Imperial German Navy in World War I. U-31 was engaged in the naval warfare and took part in the First Battle of the Atlantic.

U-31 sailed from Wilhelmshaven on 13 January 1915 but disappeared shortly thereafter. It was assumed, correctly, she had struck a mine, and sunk with all hands somewhere in the North Sea.

Design Type U 31 submarines were double-hulled ocean-going submarines similar to Type 23 and Type 27 subs in dimensions and differed only slightly in propulsion and speed. They were considered very good high seas boats with average manoeuvrability and good surface steering.

U-31 had an overall length of , her pressure hull was long. The boat's beam was (o/a), while the pressure hull measured . Type 31s had a draught of with a total height of . The boats displaced a total of ; when surfaced and when submerged.

U-31 was fitted with two Germania 6-cylinder two-stroke diesel engines with a total of for use on the surface and two Siemens-Schuckert double-acting electric motors with a total of for underwater use. These engines powered two shafts each with a propeller, which gave the boat a top surface speed of , and when submerged. Cruising range was at on the surface, and at under water. Diving depth was .

The U-boat was armed with four torpedo tubes, two fitted in the bow and two in the stern, and carried 6 torpedoes. The boat's complement was 4 officers and 31 enlisted.

Wreck discovery The wreck of U-31 had been discovered in 2012 about off the coast of East Anglia during surveys made in preparation for the construction of an offshore wind farm. However, the wreck was not formally identified until 9 September 2015 when the Dutch Lamlash wreck-diving team discovered the hull number engraved on a salvaged item of navigation equipment.

Summary 2012: Wreck found during sonar survey by Fugro for Offshore Windfarm Project by Scottish Power Renewables

2013 and 2014: Wreck surveyed by RNLNavy with divers and sonar in the course of the search for the wreck of HNLMS O13, lost on patrol in June 1940 in the North Sea. This wreck could be classified as a World War I SM U-31 series U-boat

2015: Wreck positively identified by Dutch divers from mv Lamlash - Haarlem as the SM U-31 (Hull number found on a “Fahrt Tabelle” (Manoeuvring Settings Table)).

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