 During their trials the Type <unk> Is were found to be too small and too slow and had a reputation for being <unk> ; one commander compared his Type <unk> I to a " <unk> machine " . According to authors R. H. Gibson and Maurice <unk> in their 1931 book The German <unk> War , 1914 – 1918 , the <unk> did not have enough power to chase down <unk> while surfaced and lacked the endurance to spend any extended amount of time underwater , exhausting their batteries after little over an hour 's running . In @-@ service use revealed another problem : with a single propeller shaft / engine combination , if either component failed , the U @-@ boat was almost totally disabled . 
