On 12 March 1917, UB-6 ran aground near the Meuse (Maas) in the Netherlands due to a navigational error by her commander; the submarine and crew were interned by the neutral country and taken to Hellevoetsluis.

[5][Note 2] UB-6 was part of the initial allotment of eight submarines—numbered UB-1 to UB-8—ordered on 15 October from Germaniawerft of Kiel, just shy of two months after planning for the class began.

[9][Note 3] On 19 April, UB-6 joined the other UB I boats then comprising 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.

[10] The UB I boats of the Flanders Flotilla were initially limited to patrols in the Hoofden, the southern portion of the North Sea between the United Kingdom and the Netherlands.

[11] On 1 May, Haecker spotted two old Royal Navy destroyers, Brazen and Recruit, about 30 nautical miles (56 km; 35 mi) southwest of the Galloper light vessel.

Saidieh was en route to Hull from Alexandria with a load of onions and cottonseed when UB-6 sank her at the mouth of the Thames; eight crewmen lost their lives in the attack.

[15] In late June, Korvettenkapitän Karl Bartenbach, head of the Flanders Flotilla, used UB-6 to test a theory that British defenses in the Straits of Dover—anti-submarine nets and mines—were not insurmountable.

UB-6 sailed past Dunkirk on the surface and made Boulogne in the early morning of the 22nd, having to crash dive once during the voyage when discovered by a British destroyer.

[16][17] Even though no ships were sunk during these forays into the English Channel, by successfully completing their voyages, the submarines helped further prove the feasibility of defeating the British countermeasures in the Straits of Dover.

[16] On 12 July, while patrolling between 18 and 23 nautical miles (33 and 43 km; 21 and 26 mi) off Lowestoft, UB-6 attacked five British fishing vessels, sinking four of them.

[14][18] All four of the sunken ships were smacks—sailing vessels traditionally rigged with red ochre sails[19]—which were stopped, boarded by crewmen from UB-6, and sunk with explosives.

[20] Two weeks later, UB-6 torpedoed and sank the 406 GRT Firth 4 nautical miles (7.4 km; 4.6 mi) from the Aldborough Napes Buoy.

Holtzendorff's directive ordered all U-boats out of the English Channel and the South-Western Approaches and required that all submarine activity in the North Sea be conducted strictly along prize regulations.

The 57 GRT smack Crystal was boarded and sunk by explosives 25 nautical miles (46 km; 29 mi) southeast of Southwold on the 27th.

[14] In July, Neumann was succeeded by Oberleutnant zur See Karsten von Heydebreck, a 26-year-old, first-time U-boat captain,[31] who was Voigt's classmate in April 1908 cadet class.

[34] In support of the operation, UB-6 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.

[35][Note 6] This group was to intercept and attack the British light forces from Harwich, should they sortie north to join the battle.

[37] On 10 September, UB-6 was patrolling off the Meuse (Maas) lightship and torpedoed the 400 GRT Norwegian steamer Lindborg, with a general cargo for London; there were no casualties.

[43] 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.

Two days later, UB-6 entered Dutch territorial waters after Steckelberg made a navigational error, and ran aground at the mouth of the Meuse (Maas).