Marine steam engine
A marine steam engine is a steam engine that is used to power a ship or boat. This article deals mainly with marine steam engines of the reciprocating type, which were in use from the inception of the steamboat in the early 19th century to their last years of large-scale manufacture during World War II. Reciprocating steam engines were progressively replaced in marine applications during the 20th century by steam turbines and marine diesel engines.
History
The first commercially successful steam engine was developed by Thomas Newcomen in 1712. The steam engine improvements brought forth by James Watt in the later half of the 18th century greatly improved steam engine efficiency and allowed more compact engine arrangements. Successful adaptation of the steam engine to marine applications in England would have to wait until almost a century after Newcomen, when Scottish engineer William Symington built the world's "first practical steamboat", the Charlotte Dundas, in 1802. Rivaling inventors James Rumsey and John Fitch were the first to build steamboats in the United States. Rumsey exhibited his steamboat design in 1787 on the Potomac River; however, Fitch won the rivalry in 1790 after his successful test resulted in a passenger service on the Delaware River. In 1807, the American Robert Fulton built the world's first commercially successful steamboat, simply known as the North River Steamboat, and powered by a Watt engine.Following Fulton's success, the steamboat technology developed rapidly on both sides of the Atlantic. Steamboats initially had a short range and were not particularly seaworthy due to their weight, low power, and tendency to break down, but they were employed successfully along rivers and canals, and for short journeys along the coast. The first successful transatlantic crossing by a steamship occurred in 1819 when sailed from Savannah, Georgia to Liverpool, England. The first steamship to make regular transatlantic crossings was the sidewheel steamer in 1838.
As the 19th century progressed, marine steam engines and steamship technology developed alongside each other. Paddle propulsion gradually gave way to the screw propeller, and the introduction of iron and later steel hulls to replace the traditional wooden hull allowed ships to grow ever larger, necessitating steam power plants that were increasingly complex and powerful.
Types of marine steam engine
A wide variety of reciprocating marine steam engines were developed over the course of the 19th century. The two main methods of classifying such engines are by connection mechanism and cylinder technology.Most early marine engines had the same cylinder technology but a number of different methods of supplying power to the crankshaft were in use. Thus, early marine engines are classified mostly according to their connection mechanism. Some common connection mechanisms were side-lever, steeple, walking beam and direct-acting.
However, steam engines can also be classified according to cylinder technology. One can therefore find examples of engines classified under both methods. An engine can be a compound walking beam type, compound being the cylinder technology, and walking beam being the connection method. Over time, as most engines became direct-acting but cylinder technologies grew more complex, engines began to be classified solely according to cylinder technology.
More commonly encountered marine steam engine types are listed in the following sections. Note that not all these terms are exclusive to marine applications.
Engines classified by connection mechanism
Side-lever
The side-lever engine was the first type of steam engine widely adopted for marine use in Europe. In the early years of steam navigation, the side-lever was the most common type of marine engine for inland waterway and coastal service in Europe, and it remained for many years the preferred engine for oceangoing service on both sides of the Atlantic.The side-lever was an adaptation of the earliest form of steam engine, the beam engine. The typical side-lever engine had a pair of heavy horizontal iron beams, known as side-levers, each secured in the centre by a pin near the base of the engine, allowing the levers to pivot through a limited arc. The engine cylinder stood vertically between this pair of levers at one end, with the piston rod attached to a horizontal crosshead above, from each end of which a vertical rod, known as a side-rod, extended down each side of the cylinder to connect to the end of the side-lever on the same side. The far ends of the two side-levers were connected to one another by a horizontal crosstail, from which extended a single, common connecting rod which operated the crankshaft as the levers rocked up and down around the central pin.
The main disadvantage of the side-lever engine was that it was large and heavy. For inland waterway and coastal service, lighter and more efficient designs soon replaced it. It remained the dominant engine type for oceangoing service through much of the first half of the 19th century however, due to its relatively low centre of gravity, which gave ships more stability in heavy seas. It was also a common early engine type for warships, since its relatively low height made it less susceptible to battle damage. From the first Royal Navy steam vessel in 1820 until 1840, 70 steam vessels entered service, the majority with side-lever engines, using boilers set to 4psi maximum pressure. The low steam pressures dictated the large cylinder sizes for the side-lever engines, though the effective pressure on the piston was the difference between the boiler pressure and the vacuum in the condenser.
The side-lever engine was a paddlewheel engine and was not suitable for driving screw propellers. The last ship built for transatlantic service that had a side-lever engine was the Cunard Line's paddle steamer, considered an anachronism when it entered service in 1862.
Grasshopper
The grasshopper or half-lever engine was a variant of the side-lever engine. The grasshopper engine differs from the conventional side-lever in that the location of the lever pivot and connecting rod are more or less reversed, with the pivot located at one end of the lever instead of the centre, while the connecting rod is attached to the lever between the cylinder at one end and the pivot at the other.Chief advantages of the grasshopper engine were cheapness of construction and robustness, with the type said to require less maintenance than any other type of marine steam engine. Another advantage is that the engine could be easily started from any crank position. Like the conventional side-lever engine however, grasshopper engines were disadvantaged by their weight and size. They were mainly used in small watercraft such as riverboats and tugs.
Crosshead (square)
The crosshead engine, also known as a square, sawmill or A-frame engine, was a type of paddlewheel engine used in the United States. It was the most common type of engine in the early years of American steam navigation.The crosshead engine is described as having a vertical cylinder above the crankshaft, with the piston rod secured to a horizontal crosshead, from each end of which, on opposite sides of the cylinder, extended a connecting rod that rotated its own separate crankshaft. The crosshead moved within vertical guides so that the assembly maintained the correct path as it moved. The engine's alternative name—"A-frame"—presumably derived from the shape of the frames that supported these guides. Some crosshead engines had more than one cylinder, in which case the piston rods were usually all connected to the same crosshead.
Because the cylinder was above the crankshaft in this type of engine, it had a high center of gravity, and was therefore deemed unsuitable for oceangoing service. This largely confined it to vessels built for inland waterways. As marine engines grew steadily larger and heavier through the 19th century, the high center of gravity of square crosshead engines became increasingly impractical, and by the 1840s, ship builders abandoned them in favor of the walking beam engine.
The name of this engine can cause confusion, as "crosshead" is also an alternative name for the steeple engine. Many sources thus prefer to refer to it by its informal name of "square" engine to avoid confusion. Additionally, the marine crosshead or square engine described in this section should not be confused with the term "square engine" as applied to internal combustion engines, which in the latter case refers to an engine whose bore is equal to its stroke.
Walking beam
The walking beam, technically known as a vertical beam or overhead beam, and sometimes simply referred as a "beam", was another early adaptation of the beam engine, but its use was confined almost entirely to the United States. After its introduction, the walking beam quickly became the most popular engine type in American waters for inland waterway and coastal service, eventually making its way into American transoceanic steamships as well. The type proved to have remarkable longevity, with walking beam engines still being occasionally manufactured as late as the 1940s. In marine applications, the beam itself was generally reinforced with iron struts that gave it a characteristic diamond shape, although the supports on which the beam rested were often built of wood. The adjective "walking" is believed to have originated from a corruption of the technical term "working beam".Walking beam engines were a type of paddlewheel engine and were rarely used for powering propellers. They were used primarily for ships and boats working in rivers, lakes and along the coastline, but were a less popular choice for seagoing vessels because the great height of the engine made the vessel less stable in heavy seas. They were also of limited use militarily, because the engine was exposed to enemy fire and could thus be easily disabled. Their popularity in the United States was due primarily to the fact that the walking beam engine was well suited for the shallow-draft boats that operated in America's shallow coastal and inland waterways.
Walking beam engines remained popular with American shipping lines and excursion operations right into the early 20th century. Although the walking beam engine was technically obsolete in the later 19th century, it remained popular with excursion steamer passengers who expected to see the "walking beam" in motion. There were also technical reasons for retaining the walking beam engine in America, as it was easier to build, requiring less precision in its construction. Wood could be used for the main frame of the engine, at a much lower cost than typical practice of using iron castings for more modern engine designs. Fuel was also much cheaper in America than in Europe, so the lower efficiency of the walking beam engine was less of a consideration. The Philadelphia shipbuilder Charles H. Cramp blamed America's general lack of competitiveness with the British shipbuilding industry in the mid-to-late 19th century upon the conservatism of American domestic shipbuilders and shipping line owners, who doggedly clung to outdated technologies like the walking beam and its associated paddlewheel long after they had been abandoned in other parts of the world.