A paddle steamer is a steamship or riverboat powered by a steam engine that drives paddle wheels to propel the craft through the water. In antiquity, paddle wheelers followed the development of poles, oars and sails, where the first uses were wheelers driven by animals or humans. Modern paddle wheelers may be powered by diesel engines. Save for tourism and small pleasure boats (paddle boats), paddle propulsion is largely superseded by the screw propeller and other marine propulsion systems that have a higher efficiency, especially in rough or open water. The paddle wheel is a large steel framework wheel. The outer edge of the wheel is fitted with numerous, regularly-spaced paddle blades (called floats or buckets). The bottom quarter or so of the wheel travels underwater. An engine rotates the paddle wheel in the water to produce thrust, forward or backward as required. More advanced paddle wheel designs feature feathering methods that keep each paddle blade closer to vertical while in the water to increase efficiency. The upper part of a paddle wheel is normally enclosed in a paddlebox to minimise splashing. There are two basic ways to mount paddle wheels on a ship; either a single wheel on the rear, known as a sternwheeler, or a paddle wheel on each side, known as a sidewheeler. Both sternwheelers and sidewheelers were used as riverboats in the United States. Some still operate for tourists, for example on the Mississippi River. Sidewheelers are used as riverboats and as coastal craft. Though the side wheels and enclosing sponsons make them wider than sternwheelers, they may be more maneuverable, since they can sometimes move the paddles at different speeds, and even in opposite directions. This extra maneuverability makes sidewheelers popular on the narrower, winding rivers of the Murray-Darling system in Australia, where a number still operate. European sidewheelers, such as the PS Waverley, connect the wheels with solid drive shafts that limit maneuverability and give the craft a wide turning radius. Some were built with paddle clutches that disengage one or both paddles so they can turn independently. However, wisdom gained from early experience with sidewheelers deemed that they be operated with clutches out, or as solid shaft vessels. Crews noticed that as ships approached the dock, passengers moved to the side of the ship ready to disembark. The shift in weight, added to independent movements of the paddles, could lead to imbalance and potential capsizing. Paddle tugs were frequently operated with clutches in, as the lack of passengers aboard meant that independent paddle movement could be used safely and the added maneuverability exploited to the full. In a simple paddle wheel, where the paddles are fixed around the periphery, power is lost due to churning of the water as the paddles enter and leave the water surface. Ideally, the paddles should remain vertical while under water. This ideal can be approximated by use of levers and linkages connected to a fixed eccentric. The eccentric is fixed slightly forward of the main wheel centre. It is coupled to each paddle via a rod and lever. The geometry is designed such that the paddles are kept almost vertical for the short duration that they are in the water.
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