Energy and environmental conservation are two major issues of national and international concern due to the conversion and limited reserves of fossil fuels. Natural gas and vegetable oils have long been established as alternative energy sources suitable for road transport vehicles and power generation using conventional engines. However, the use of these fuels for commercially operated high-speed marine craft in inland waterways and harbours is not common. In this investigation, the performance and emission levels in the operation of a single-cylinder spark-ignition engine using natural gas are compared with data for diesel, gasoline and emulsified vegetable oil under simulated propeller-load and near lean-misfire limit conditions for high-speed craft. The brake specific fuel consumption with natural gas operation was found to be higher when compared with those of gasoline and emulsified vegetable oil. The NOx exhaust emissions from the natural gas operation were lower than the limit set in Annex VI of International Maritime Organisation (IMO) MAEPOL 73/78: Regulations for prevention of air pollution from ships. An accelerated demand for natural gas for use in the maritime industry will encourage technological development in ship and power plant design for further improvement of ship operational efficiency and economy as well as environmental conservation and maritime safety. Concerns over the unknown hazards associated with the use of natural gas in its conpressed form in the propulsion of marine craft may retard the widespread adoption in the short term. Experience and demonstrated safe operating practice could encourage use of the gas to gain wider acceptance in marine transportation in the longer term.