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.[4] On 25 May 1916, UB-8 was officially transferred to Bulgaria for the remainder of the war.[2]