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 .