Are they practical or is alternative production of AC power on boats preferable? Generators and shore supply are the main methods of obtaining AC power on board, but there are other methods. You could run a high voltage alternator from the main engine by belt drive, but this is not entirely satisfactory because the alternator needs a constant speed to maintain the correct number of cycles per second. Special drive systems are available which keep the alternator at constant speed when engine speed varies, but the cost of these can only be justified on larger vessels.
Another option to drive an alternator is a hydraulic drive. This system uses a hydraulic pump driven by the main engine, with the alternator driven by a hydraulic motor. This system allows the alternator to be mounted remote from the engine and makes the hydraulic power available for other uses, but in harbour it does mean keeping one of the main engines running to obtain the AC current. It can be a useful system at sea on a motor boat where shore power can be connected when the boat is in harbour.
The other two alternative systems convert power from the battery from DC into AC. This immediately limits their use to power equipment with a comparatively low consumption, otherwise the drain on the battery would be too high. The rotary converter is a unit in which a DC-powered electric motor drives an AC alternator. In practice the motor and generator are combined on the same shaft, sometimes using the same armature to improve efficiency.
Power inverters are solid state appliances which make the conversion from DC to AC electronically. Whilst the power output of a rotary converter is limited only by its size and the tolerance of the battery drain, inverters tend to be more limited in capacity; probably the maximum output you could hope to achieve is around 1 kW, although most are much less.
Neither rotary converter nor power inverters are particularly efficient. Both produce heat which is wasted energy, and the maximum efficiency is likely to be around 80 per cent. This would be achievable with a square wave inverter; a sine wave unit would have a lower efficiency. The low outputs and comparatively low efficiency thus make power inverters suitable only as a temporary means of making AC available, perhaps to power an electric drill or for a television set, but not for any permanent long term use such as powering a refrigerator.
Finally, there are the portable types of marine generators using an air cooled petrol or diesel engine. These are very useful units to have on board, but they do need adequate ventilation, so finding a suitable location where boat generators can be kept dry is not always easy. Most have a plug-in socket for the power output and they can usually produce mains voltage AC or battery charging DC from separate windings on the generator. They are not particularly quiet, and their main purpose is for repair work when using electric tools or for temporary battery charging rather than for constant use.
Obtaining power through any of these alternative systems is not cheap. The low efficiency of power inverters raises the cost of engine generated DC power being used, whilst the fuel costs of a portable petrol-engined charger can be up to ten times the cost of similar power from the mains supply. Even diesel generators can be three or four times the cost of shore power, even without considering the capital costs involved, so the shore cable connection is the option to go for if possible.
