In the century that has passed since the diesel engine first showed its promise as a power source for ships, it has come to dominate the marine scene. Steam power once dominant, is reduced to niches such as LNG carriers and nuclear warships. Gas turbines have penetrated the marine propulsion market to a limited extent, primarily in warships and some cruise vessels and fast ferries. Natural gas is starting to make inroads in marine propulsion while petrol and LPG are restricted to small craft. Thus in terms of fuel used and installed power, the fuels and emissions question centres on the diesel engine.