[Note 1] UB-5 was initially assigned to the Flanders Flotilla in March 1915 and sank five British ships of 996 gross register tons (GRT) under the command of Wilhelm Smiths.

[5][Note 2] UB-5 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] UB-5 soon joined the other UB I boats then comprising the Flanders Flotilla (German: U-boote des Marinekorps U-Flotille Flandern), 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 (including the English Channel), 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.

On 15 April, 6 nautical miles (11 km; 6.9 mi) from the North Hinder lightship, UB-5 scored her first success when she torpedoed and sank the British steamer Ptarmigan.

[16] Even though none of the boats sank any ships, by successfully completing their voyages they helped further prove the feasibility of defeating the British countermeasures in the Straits of Dover.

Holtzendorff's directive from 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.

[20] Shortly after this cessation, UB-5 was transferred to the Baltic Flotilla (German: U-boote der Ostseetreitträfte V. U-Halbflotille) on 9 October.

[1] According to authors R.H. Gibson and Maurice Prendergast, submarines assigned to training duties were "war-worn craft" unfit for service.