 The German UB II design improved upon the design of the UB I boats , which had been ordered in September 1914 . In service , the UB I boats were found to be too small and too slow . A major problem was that , because they had a single propeller shaft / engine combo , if either component failed , the U @-@ boat became almost totally disabled . To rectify this flaw , the UB II boats featured twin propeller shafts and twin engines ( one shaft for each engine ) , which also increased the U @-@ boat 's top speed . The new design also included more powerful batteries , larger torpedo tubes , and a deck gun . As a UB II boat , U @-@ 47 could also carry twice the torpedo load of her UB I counterparts , and nearly ten times as much fuel . To contain all of these changes the hull was larger , and the surface and submerged displacement was more than double that of the UB I boats . 
