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Liquefied Natural Gas (LNG), compared to the traditional marine fossil fuels, has important key advantages as high efficiency, environmental friendliness and economic competitiveness. Unfortunately, up to now, the use of LNG as marine fuel is mainly hampered by the lack of appropriate infrastructures for ships bunkering. During the last ten years, LNG, as a bunker fuel, has been introduced in Norway, a true pioneer in marine gas propulsion. Presently it could be also available for ship bunkering in Italy and in several European countries, usually by a tank truck.

In this paper the preliminary design of an ASD (Azimuth Stern Drive) tug, for towing and escorting operations (including fire fighting and oil recovery duties) is illustrated The vessel is characterised by a hybrid propulsion configuration, combining electric drive and gas engines. In fact, each of the two shaftlines is driven by a single LNG engine (Rolls Royce Bergen 2430 kW@1000 rpm) and an electric motor/generator (PTI/PTO) coupled through a dedicated gearbox. In full LNG mode, the maximum bollard pull and the maximum speed of the tug are respectively 75 tons and 14 knots, while only the two electric motors, fed by the electrical power supplied by two Diesel Gensets, allow a cruise speed of at least 8.5 knots. It will be possible to adopt the MDO fuelled propulsion when the LNG system may be unavailable for any reason or whenever it will appear necessary to save LNG for the heavy duty phases of a service.