Navy, the Austro-Hungarian Navy, and the Bulgarian Navy. In German service, they served primarily in the Flanders Flotilla, the Baltic Flotilla, and the Constantinople Flotilla. German Imperial Navy Flanders Flotilla The first Type UB I to enter service was UB-10, which formed the nucleus of the Flanders Flotilla, on 27 March 1915. By the end of April five more Type UB I boats had become operational. UB-10 was eventually joined in the Flanders Flotilla by UB-2, UB-4, UB-5, UB-6, UB-12, UB-13, UB-16, and UB-17; of these, only UB-2 made the journey to Flanders by sea rather than rail. UB-4 departed on the first patrol from Flanders on 9 April, and was responsible for sinking the first ship sent down by the flotilla. The Type UB I boats of the Flanders Flotilla originally patrolled the area between the United Kingdom and the Netherlands, but began patrolling the English Channel after UB-6 pioneered a route past British antisubmarine nets and mines in the Straits of Dover in late June. Over the Type UB Is' first year of service, UB-4 and UB-13 were both lost, and UB-2 and UB-5 were transferred to the Baltic Flotilla. In March 1917, UB-6 ran aground in Dutch waters and was interned for the rest of the war, along with her crew. The four remaining Type UB Is in Flanders—UB-10, UB-12, UB-16, UB-17—were all converted to minelayers by 1918, having their torpedo tubes removed and replaced with chutes to carry up to eight mines. All but UB-10 were lost in 1918; UB-10, in poor repair and out of service, was scuttled in October 1918 when the Germans evacuated from Flanders. Baltic Flotilla UB-9 was initially assigned to the Baltic Flotilla, and was joined by UB-2 and UB-5 in early 1916. All three became training boats at Kiel in 1916, joining UB-11 in that duty. Little information is available about the Type UB I boats operating in the Baltic. Constantinople Flotilla Four of the German Imperial Navy boats—UB-3, UB-7, UB-8, and UB-14—were selected for service with the Constantinople Flotilla. All were sent to Pola for assembly and trials there as part of the Pola Flotilla before sailing on to join the Constantinople Flotilla. UB-3 disappeared en route to Constantinople in May 1915, but the other three arrived there by mid-June. The three Type UB I boats of the Constantinople Flotilla seem to have patrolled primarily in the Black Sea.