Below the surface the U-boat is driven by electric motors using electricity drawn from the giant internal batteries. When these are depleted, they must be recharged. This is done by traveling on the surface under diesel engine power. The diesel engines are used to turn the electric motors so that they act as dynamos (generators) to recharge the depleted batteries. For rapid recharge, both diesel engines would be clutched onto the electric motors. For maximum range, one diesel engine would be clutched to an electric motor – which in turn acted as a generator to drive the other electric motor. Both propellers were turning in this way with only one diesel engine running.