UB-12 was a little under 28 metres (92 ft) in length and displaced between 127 and 141 tonnes (125 and 139 long tons), depending on whether surfaced or submerged.

[Note 1] UB-12 spent her entire career in the Flanders Flotilla and sank 22 ships, about half of them British fishing vessels.

[13][Note 2] UB-12 was part of the initial allotment of seven submarines—numbered UB-9 to UB-15—ordered on 15 October from AG Weser of Bremen, just shy of two months after planning for the class began.

Like all boats of the class, UB-12 was rated to a diving depth of 50 metres (160 ft), and could completely submerge in 33 seconds.

[16] The submarine was commissioned into the German Imperial Navy as SM UB-12 on 29 March 1915 under the command of Kapitänleutnant (Kapt.)

[17][Note 3] On 18 April, UB-12 joined the Flanders Flotilla (German: U-boote des Marinekorps U-Flotille Flandern),[1] which had been organized on 29 March.

During this campaign, enemy vessels in the German-defined war zone (German: Kriegsgebiet), which encompassed all waters around the United Kingdom, were to be sunk.

[18] On 24 July, Nieland and UB-12 sank four British fishing vessels while patrolling between 30 nautical miles (56 km; 35 mi) east-northeast of Lowestoft.

[19][20] All four of the sunken ships were smacks—sailing vessels traditionally rigged with red ochre sails[21]—which were stopped, boarded by crewmen from UB-12, and sunk with explosives.

The British-registered ship was en route from Hull to Philadelphia in ballast when Kiel torpedoed her without warning a little more than one nautical mile (two kilometers) from the Corton Lightvessel.

[31] In support of the operation, UB-12 and five other Flanders boats set out at midnight 30/31 May to form a line 18 nautical miles (33 km; 21 mi) east of Lowestoft.

[32][Note 4] This group was to intercept and attack the British light forces from Harwich, should they sortie north to join the battle.

[31] A delayed departure of the German High Seas Fleet for its sortie (which had been redirected to the Skagerrak) and the failure of several of the U-boats stationed to the north to receive the coded message warning of the British advance caused Scheer's anticipated ambush to be a "complete and disappointing failure".

The German fleet planned to depart late in the day on 18 August and shell military targets the next morning.

Although U-boats to the north sank two British light cruisers,[Note 6] UB-12 and her group played no part in the action.

[42] Five days later, the British auxiliary minesweeper HMS Duchess of Montrose sank with a loss of 12 men after detonating a mine laid by UB-12 off Gravelines.

[43] On 23 March, HMS Laforey, a destroyer with the Dover Patrol, struck one of UB-12's mines off Cape Gris-Nez and went down with the loss of 59 men.

[52] Under Schöller's command, UB-12 departed Zeebrugge on 19 August to lay mines in the Downs off the Kentish coast, but never returned.

[19] On 27 October, two weeks before the end of the war, the 92 GRT British ship Calceolaria struck one of UB-12's mines near the Elbow Lightvessel and sank.