The first Type UB I to enter service was UB @-@ 10 , which formed the nucleus of the Flanders Flotilla , on 27 March 1915 . By the end of April five more Type UB I boats had become operational . UB @-@ 10 was eventually joined in the Flanders Flotilla by UB @-@ 2 , UB @-@ 4 , UB @-@ 5 , UB @-@ 6 , UB @-@ 12 , UB @-@ 13 , UB @-@ 16 , and UB @-@ 17 ; of these , only UB @-@ 2 made the journey to Flanders by sea rather than rail .