These levers extended, on the cylinder side, to each side of the bottom of the vertical engine cylinder. A piston rod, connected vertically to the piston, extended out of the top of the cylinder. This rod attached to a horizontal crosshead, connected at each end to vertical rods known as side-rods.
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These rods connected down to the levers on each side of the cylinder. This formed the connection of the levers to the piston on the cylinder side of the engine. The other side of the levers the opposite end of the lever pivot to the cylinder were connected to each other with a horizontal crosstail.
This crosstail in turn connected to and operated a single connecting rod , which turned the crankshaft. The rotation of the crankshaft was driven by the levers—which, at the cylinder side, were driven by the piston's vertical oscillation. The main disadvantage of the side-lever engine was that it was large and heavy. 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.
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From the first Royal Navy steam vessel in until , 70 steam vessels entered service, the majority with side-lever engines, using boilers set to 4psi maximum pressure. The side-lever engine was a paddlewheel engine and was not suitable for driving screw propellers.
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.
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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.
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. Some crosshead engines had more than one cylinder, in which case the piston rods were usually all connected to the same crosshead. An unusual feature of early examples of this type of engine was the installation of flywheels —geared to the crankshafts—which were thought necessary to ensure smooth operation. These gears were often noisy in operation.
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. The name of this engine can cause confusion, as "crosshead" is also an alternative name for the steeple engine below.
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. Model of a crosshead or "square" engine, showing location of engine cylinder above the crankshaft; also piston rod, crosshead, connecting rods and paddlewheels. The paddle steamer New York. Between the paddlewheels is the tall square or "A-frame" engine, within which can be seen the long piston rod, near the top of its stroke, making a "T" with the horizontal crosshead.
The walking beam, also known as a "vertical beam", "overhead beam", or simply "beam", was another early adaptation of the beam engine, but its use was confined almost entirely to the United States. 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" was applied because the beam, which rose high above the ship's deck, could be seen operating, and its rocking motion was somewhat fancifully likened to a walking motion.
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.
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.
The vessel's diamond shaped "walking beam" can clearly be seen amidships. The steeple engine, sometimes referred to as a "crosshead" engine, was an early attempt to break away from the beam concept common to both the walking beam and side-lever types, and come up with a smaller, lighter, more efficient design. In a steeple engine, the vertical oscillation of the piston is not converted to a horizontal rocking motion as in a beam engine, but is instead used to move an assembly, composed of a crosshead and two rods, through a vertical guide at the top of the engine, which in turn rotates the crankshaft connecting rod below.
The triangular assembly above the engine cylinder gives the engine its characteristic "steeple" shape, hence the name. Steeple engines were tall like walking beam engines, but much narrower laterally, saving both space and weight. Because of their height and high centre of gravity, they were, like walking beams, considered less appropriate for oceangoing service, but they remained highly popular for several decades, especially in Europe, for inland waterway and coastal vessels.
Steeple engines began to appear in steamships in the s and the type was perfected in the early s by the Scottish shipbuilder David Napier. The Siamese engine, also referred to as the "double cylinder" or "twin cylinder" engine, was another early alternative to the beam or side-lever engine. This type of engine had two identical, vertical engine cylinders arranged side-by-side, whose piston rods were attached to a common, T-shaped crosshead.
The vertical arm of the crosshead extended down between the two cylinders and was attached at the bottom to both the crankshaft connecting rod and to a guide block that slid between the vertical sides of the cylinders, enabling the assembly to maintain the correct path as it moved. The Siamese engine was invented by British engineer Joseph Maudslay son of Henry , but although he invented it after his oscillating engine see below , it failed to achieve the same widespread acceptance, as it was only marginally smaller and lighter than the side-lever engines it was designed to replace.
There are two definitions of a direct-acting engine encountered in 19th-century literature. The earlier definition applies the term "direct-acting" to any type of engine other than a beam i. Unlike the side-lever or beam engine, a direct-acting engine could be readily adapted to power either paddlewheels or a propeller.
As well as offering a lower profile, direct-acting engines had the advantage of being smaller and weighing considerably less than beam or side-lever engines. One disadvantage of such engines is that they were more prone to wear and tear and thus required more maintenance. An oscillating engine was a type of direct-acting engine that was designed to achieve further reductions in engine size and weight.
Oscillating engines had the piston rods connected directly to the crankshaft, dispensing with the need for connecting rods. To achieve this, the engine cylinders were not immobile as in most engines, but secured in the middle by trunnions that let the cylinders themselves pivot back and forth as the crankshaft rotated—hence the term, oscillating.
The oscillating motion of the cylinder was usually used to line up ports in the trunnions to direct the steam feed and exhaust to the cylinder at the correct times. However, separate valves were often provided, controlled by the oscillating motion.
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This let the timing be varied to enable expansive working as in the engine in the paddle ship PD Krippen. This provides simplicity but still retains the advantages of compactness. The first patented oscillating engine was built by Joseph Maudslay in , but the type is considered to have been perfected by John Penn. Oscillating engines remained a popular type of marine engine for much of the 19th century. Oscillating engines could be used to drive either paddlewheels or propellers.
Oscillating engine built in by J. Blyth of London for the Austrian paddle steamer Orsova. The trunk engine, another type of direct-acting engine, was originally developed as a means of reducing an engine's height while retaining a long stroke.
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A long stroke was considered important at this time because it reduced the strain on components. A trunk engine locates the connecting rod within a large-diameter hollow piston rod. This "trunk" carries almost no load.