The Type UB I was a class of small coastal submarines ( U @-@ boats ) built in Germany at the beginning of the First World War . 20 boats were constructed , most of which went into service with the German Imperial Navy . Boats of this design were also operated by the Austro @-@ Hungarian Navy ( Kaiserliche und Königliche Kriegsmarine or K.u.K. Kriegsmarine ) and the Bulgarian Navy . The group is sometimes known as the UB @-@ 1 class after SM UB @-@ 1 , the class leader . In the Austro @-@ Hungarian Navy , it was called the U @-@ 10 class .