Electric propulsion of marine vessels has been in use for over a century on aircraft carriers, all submarines and ships such as the Queen Mary. It is universally recognized as being more efficient than a direct mechanical drive and easier to service. The system becomes "hybrid" when a bank of batteries is combined to provide power to the drive motors when the engine is not operating. World War II submarines were among the first true hybrid electric ships.
In a modern hybrid electric vehicle, a large bank of lead-acid or lithium batteries stores power for silent running, while a diesel or gasoline generator provides electric power to charge the battery bank.
The benefits of hybrid electric propulsion include better fuel efficiency because the generator can always operate at peak efficiency, even though the required motor power is low. Also hybrid propulsion provides flexibility in positioning the generator which can ease cabin layout constraints and provide a better balanced boat. When running under electric power alone, operation is silent and response is immediate, and there's much better control at low speeds for maneuvering.
Electric hybrid propulsion is particular desirable for cruising sailboats. While under sail, the propulsion motor can be driven by the water moving over the propeller. Up to 25% of the waterline power can be recovered and used to recharge the batteries. A fifty foot cruising sailboat may generate 20 kW of power while under full sail on a long passage. Anchored in a fast moving river or tidal current, enough power may be generated to keep the batteries well charged when you are not aboard. Also, while motorsailing, boat speed can be increased dramatically and apparent wind moved aft by a running the electric motor silently and efficiently at modest speed.
hybrid electric vehicle, a large bank of lead-acid or lithium batteries stores power for silent running, while a diesel or gasoline generator provides electric power to charge the battery bank. 