The basic mechanism
A paddlewheel is a large wheel with boards, called paddles or floats, fixed around its rim. When the wheel turns, the lower boards push backwards against the water and the vessel moves forward. The floats enter the water, push, and leave it again as the wheel rotates. Power comes through a shaft from an engine, a treadmill or an animal.
The simplicity of the idea explains why it appeared early. Anyone who understood that an oar pushes a boat could imagine a rotating set of oars. The difficulty was supplying enough power to turn the wheel, which is why the paddlewheel became practical on a large scale only with the steam engine.
Early ideas
Several early designs are known from written sources and are best treated with care. A late Roman military text, usually called De Rebus Bellicis, describes a warship driven by paddle wheels turned by oxen. Scholars debate whether such a ship was ever built, and the text may describe an idea rather than an actual vessel, though it fits with what is known of Roman navy fleets and their interest in new devices.
In China, there are accounts of paddle-wheel boats from the Tang and Song dynasties, often described as wheel ships. These are said to have been driven by crews treading on a mechanism, and some were reportedly used as warships in river campaigns. Accounts of the Song period are more numerous than those of earlier centuries. Descriptions of Chinese junk fleets typically focus on sailing vessels, but treadmill-powered wheelers were part of the wider picture on rivers and inland waters. The exact sizes and capacities given in later sources should be treated cautiously.
In Europe, paddle designs also appeared in Renaissance technical drawings, often driven by hand cranks or animals. They remained curiosities until engines became available.
Steam and the paddle
When engineers in the late eighteenth century began to experiment with steam boats, the paddle wheel was an obvious way to apply engine power. It could be driven directly from a beam or crank engine, it worked at low rotational speeds well suited to early engines, and it was easy to build and repair. Nearly all the early commercial steamers used it, as described in the article on the first steamboat pioneers.
Side wheels and stern wheels
There are two main arrangements.
Side wheels are mounted one on each side of the hull, usually inside a housing called a paddle box. They allow a vessel to turn on the spot if the two wheels are driven separately or in opposite directions, which made them handy for ferries, tugs and excursion boats in confined waters. They also gave the characteristic wide profile of the classic paddle steamer. Side wheels worked well at sea in calm to moderate conditions, but they were exposed to waves and could be damaged in heavy weather. In a roll, one wheel lifted out of the water while the other was deeply immersed.
Stern wheels are mounted across the back of the hull, as a single wide wheel. They were very common on shallow rivers, especially in North America, where they kept the machinery protected from snags and floating debris and kept the hull narrow. A stern-wheeler could be built with a very shallow draught and a broad deck for cargo. Steering depended on rudders, often several, placed behind the wheel.
Some vessels used other arrangements, including a pair of wheels in tandem or a wheel inside a recess in the hull.
Feathering floats
An important refinement concerned the angle of the boards. In a plain radial wheel, the paddles are fixed rigidly, and they strike the water at an angle, wasting energy and causing shocks. Feathering wheels, introduced in the early to middle nineteenth century, used linkages to keep the boards closer to vertical as they entered and left the water. This improved efficiency, though it added complexity and wear. Many ships used a simple radial wheel and accepted the loss.
Why the screw replaced it
The screw propeller was developed in the 1830s by several inventors, including the Englishman Francis Pettit Smith and the Swedish-born engineer John Ericsson. Its advantages became clear over a period of trials and practical experience.
- A screw sits under the water, protected from waves and from gunfire, whereas a paddle box occupies the ship’s side.
- A screw works at a steady depth and is less affected by rolling or by changes in load.
- A screw needs less deck width, leaving more room for guns, cargo or passengers.
- Screw machinery could be placed low in the hull, which improved stability.
Isambard Kingdom Brunel’s Great Britain, launched in 1843, was designed with paddle propulsion in mind and then changed to a screw, an early sign of the new direction. In 1845 the Royal Navy ran trials between the paddle sloop HMS Alecto and the screw-driven HMS Rattlesnake, including a well-known tow in which Rattlesnake pulled Alecto backwards. The test was not perfectly controlled, and it is best seen as one of several demonstrations rather than a single decisive proof, but it helped persuade naval officers.
As iron hulls and more powerful engines spread, as the history of iron shipbuilding shows, the screw became the standard for ocean-going and naval ships. Paddles survived where their advantages mattered: in shallow rivers, where draught was critical, in some tugs and harbour boats, and in excursion vessels where appearance and a wide deck were appreciated. Some large ships, notably Brunel’s Great Eastern, used both paddles and a screw, which indicates how recently the screw had been proven. A traditional harbour tugboat might also keep its paddles for manoeuvrability, though screw tugs eventually became the norm.
Legacy
The paddlewheel was never a dead end. It provided the first reliable means of applying steam to vessels, it gave river and coastal communities regular services, and it shaped a generation of designs. Its replacement by the screw was gradual rather than abrupt, and in particular places the wheel proved the better tool for many decades.