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 .