A marine steam engine is a type of steam engine designed for use on a ship or other marine vessel. Steam engines first began appearing on ships in the early 19th century and were the primary type of engine used for powered marine travel throughout most of the next 100 years, when diesel engines began to supplant steam engines as the standard for ship propulsion. A huge number of designs, improvements, and configurations appeared during this time, but every marine steam engine operated on the same basic principle of using heated steam to perform work.
The first working steam engine was invented in the late 17th century but was not successfully developed commercially until the early 18th century. It was almost 100 years later, in 1807, that the first commercially successful steam ship design was developed by an American, Robert Fulton. Early marine steam engines were somewhat unreliable and underpowered, but engineers and inventors in the United States and Europe quickly made improvements, and within another 30 years steam ships were making regular transatlantic trips.
Early ships outfitted with these types of engines had boilers that used wood for fuel. Later, many ships used coal for fuel. A boiler, a large sealed tank containing water was heated to produce steam, which created pressure within the closed system. This pressure was used to move one or more pistons in cylinders. The pistons were connected by mechanical means to a drive mechanism, which was used to turn a paddlewheel, or later, on more advanced ships, a propeller screw.
Simple expansion steam engines had cylinders that all operated under the same pressure, and in the early stages of marine steam engine development, all steam engines were designed this way. Later, the compound expansion steam engine was developed, which had cylinders that ran under successively lower pressures as the steam was fed through the cylinders in succession, cooling as it passed through each before returning to the boiler. Engines of this type were were usually designed with two cylinders. Compound engines with three cylinders were known as triple-expansion engines.
The method by which a marine steam engine transferred power from the pistons to the drive mechanism was the second element in how an engine was classified. Many different power transfer designs were developed during the time when steam engines were prevalent. A type of engine known as the side-lever engine, in which the pistons were connected to two large levers mounted on the sides of the engine, was the primary type of marine steam engine during the early years of steam power. Later designs included the steeple, crosshead, oscillating, and walking beam engines.