Germany and Bulgaria negotiated the purchase of two UB I boats for the Bulgarian Navy , UB @-@ 7 and UB @-@ 8 , in 1916 . Two crews of Bulgarian sailors were sent to Kiel for training . Before the purchase could be completed , UB @-@ 7 was sunk , leaving only one boat for Bulgaria . On 25 May 1916 , UB @-@ 8 was officially transferred to Bulgaria for the remainder of the war .