[Note 1] UB-13 spent her entire career in the Flanders Flotilla and sank 11 merchant ships, about half of them British fishing vessels.

[4][Note 2] UB-13 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.

[8][Note 3] On 26 April, UB-13 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.

[9] Submarines of the Flanders Flotilla sank over 14,000 tons of merchant vessels in June 1915,[11] and UB-13's first ship sunk, Dulcie, contributed almost one-seventh of that total.

The British steamer Dulcie, listed at 2,033 gross register tons (GRT), was headed from Dunston for Le Havre with a load of coal when Becker torpedoed her 6 nautical miles (11 km; 6.9 mi) east of Aldeburgh.

[13] On 27 and 28 July, Becker and UB-13 sank three British fishing vessels while patrolling between 15 and 30 nautical miles (28 and 56 km; 17 and 35 mi) off Lowestoft.

[13][14] All three of the sunken ships were smacks—sailing vessels traditionally rigged with red ochre sails[10]—which were stopped, boarded by crewmen from UB-16, and sunk with explosives.

His 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.

[16] On 20 February 1916, under the command of Kapitänleutnant Karl Neumann, who replaced Becker in December 1915,[17] UB-13 captured a Belgian ship named Z10 David Marie and retained her as a prize.

Germany initially tried to implicate British mines or torpedoes, but relented when confronted with evidence that it was one of their own torpedoes—which had been assigned to UB-13[Note 5]—that had sunk Tubantia.

Further, they reported, UB-13 had fired that specific torpedo at a British warship on 6 March—ten days before Tubantia was sunk[27]—which would have been under her previous commander, Kapitänleutnant Neumann.

The U.S. Minister to the Netherlands, Henry van Dyke, writing in Fighting for Peace in 1917, called this explanation "amazing" and derided it:[28]This certain U-boat had fired this particular torpedo at a British war-vessel somewhere in the North Sea ten days before the Tubantia was sunk.

Author Dwight Messimer, in his book Verschollen: World War I U-boat Losses, reports that the British had deployed a new explosive anti-submarine net at position 51°33′N 2°45′E / 51.550°N 2.750°E / 51.550; 2.750 in the early morning hours of 24 April.

[32] However, according to authors R. H. Gibson and Maurice Prendergast, in their book The German Submarine War, 1914–1918, UB-13 fouled the anchor cable of the British naval drifter Gleaner of the Sea on 24 April, and was depth charged by E.E.S.. Then for good measure, the British destroyer Afridi deployed explosive sweeps against the submarine.