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.