A marine steam engine is a steam engine that is used to power a ship or boat. A reciprocating type of a marine steam engine was used since the inception of the steamboat in the early 19th century to their last years of large-scale manufacture during World Was II. Reciprocating steam engines were progressively replaced in marine applications during the 20th century by steam turbines and marine diesel engines.
Near the end of the 18th century, compound engines came into widespread use. Compound engines exhausted steam into successively larger cylinders to accommodate the higher volumes at reduced pressures, giving improved efficiency. These stages were called expansions, with double-and -triple expansions engines being common, especially in shipping where efficiency was important to reduce the weight of coal carried. Steam engines remained the dominant source of power until the early 20th century.