 In early 1916 , UB @-@ 7 and UB @-@ 8 were still cruising in the Black Sea out of Varna . The Germans did not have good luck in the Black Sea , which was not a priority for them . The Bulgarians , who saw the value of the submarines in repelling Russian attacks , began negotiations to purchase UB @-@ 7 and UB @-@ 8 . Bulgarian sailors practiced in the pair of boats and technicians were sent to Kiel for training at the German submarine school there . The transfer of UB @-@ 8 to the Bulgarian Navy took place on 25 May 1916 , but for reasons unreported in sources , UB @-@ 7 remained under the German flag . 
