 During their trials, the UB I boats were found to be too small and too slow, but in-service use revealed another problem.The UB I boats had a single propeller shaft/engine combo such that if either component failed, the U-boat became almost totally disabled.The UB II boats featured twin propeller shafts and twin engines (one shaft for each engine) which not only alleviated this problem, but also had the added benefit of increasing the top speed.