She was launched and commissioned as SM UB-8 in the German Imperial Navy in April when the Austrians opted out of the agreement.

The U-boat sank only one ship, SS Merion, was disguised by the British Admiralty as a Royal Navy battlecruiser as part of a decoy operation.

However, in July 2011 Viceadmiral Manushev, Commander of the Bulgarian Navy, announced that the submarine, discovered in 2010 at the sea bottom near the town of Varna, is UB-8.

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

The process of shipping a UB I boat involved breaking the submarine down into what was essentially a knock down kit.

[12] As time dragged on, the Austrian U-3 and U-4 were still not ready,[Note 3] and eventually Admiral Anton Haus, the head of the Austro-Hungarian Navy, reneged on his commitment because of the overt hostility from neighbor and former ally Italy.

[10][Note 4] With the change of heart from the Austrians, Germany resolved to retain UB-8 and send her to the aid of the Turks.

[4] Because of her limited range, UB-8 would not have been able to make the entire journey to Turkey, so on 2 May, she was towed by the Austrian cruiser SMS Novara from Pola down the Adriatic and through the Straits of Otranto.

The watch officer, on the conning tower with the helmsman and a lookout, was able to partially close the hatch before the entire submarine slipped below the waves, depositing the three men in the water.

Voigt ordered the interior hatch to the control room sealed and all the ballast tanks filled with compressed air to increase buoyancy.

Voigt circled back for the missing crewmen but only the watch officer and helmsman were recovered; the lookout had drowned.

[16] On 29 May 1915, UB-8 came upon an Allied convoy near Lemnos, and, enticed by the prospect of hitting what he identified as the Royal Navy battlecruiser HMS Tiger, Voigt allowed five fully laden transport ships to pass unmolested.

[17] When he had a clear shot, Voigt launched one of his torpedoes at the stationary ship and hit it, sending debris into the air.

[18][Note 6] Merion, which eventually sank on 31 May, had been outfitted with wood and canvas "guns" and overloaded with cement and stones to approximate the profile of Tiger.

[19] Despite German intentions to use her in the Dardanelles, UB-8 was ineffective because she was hampered by her limited torpedo supply and her weak engines, which made negotiating the strong currents there nearly impossible.

Because Bulgaria had joined the Central Powers, battleships of the Russian Black Sea Fleet, and aircraft from the seaplane carriers Almaz and Imperator Nikolai I began attacks on Varna and the Bulgarian coast on 25 October.

[1] Bulgarian sailors practiced in the pair of boats and technicians were sent to Kiel for training at the German submarine school there.

[6][25] The transfer of UB-8 to the Bulgarian Navy took place on 25 May 1916,[6] but for reasons unreported in sources, UB-7 remained under the German flag.