20 Type UB I submarines were built , 17 for the German Imperial Navy and three for the Austro @-@ Hungarian Navy . Two of the German submarines — UB @-@ 1 and UB @-@ 15 — were sold to Austria @-@ Hungary and commissioned into the Austro @-@ Hungarian Navy as U @-@ 10 and U @-@ 11 , respectively . Those two and a further three built by AG Weser comprised the virtually identical U @-@ 10 class for the Austro @-@ Hungarian Navy . Another of the German submarines , UB @-@ 8 , was sold to Bulgaria in May 1916 , becoming Podvodnik No. 18 .