Water wheel

For water wheels used to drive boats, see paddle wheel. For wheels used solely to lift water see noria. For factories or industries driven by water wheels see watermill.

An overshot water wheel standing 42 feet high powers the Old Mill at Berry College in Rome, Georgia.

A water wheel is a hydropower system; a machine for extracting power from the flow of water. Water wheels and hydropower was widely used in the Middle Ages, powering most industry in Europe, along with the windmill. The most common use of the water wheel was to mill flour in gristmills, but other uses included foundry work and machining, and pounding linen for use in paper.

A water wheel consists of a large wooden or metal wheel, with a number of blades or buckets arranged on the outside rim forming the driving surface. Most commonly, the wheel is mounted vertically on a horizontal axle, but the tub or Norse wheel is mounted horizontally on a vertical shaft. Vertical wheels can transmit power either through the axle or via a ring gear and typically drive belts or gears; horizontal wheels usually directly drive their load. A channel created for the water to follow after leaving the wheel is commonly referred to as a "tailrace."

History

See also: Watermill and Noria

Since ancient times, waterwheels have been used as tools to power factories in many different counties. The alternatives were the windmill and human and animal power. The most common use of the waterwheel was to mill flour in gristmills, but other uses included foundry work and machining, and pounding linen for use in the manufacture of paper.

Water wheels are known to have appeared at roughly around the same time in several different regions. The horizontal mill appears almost simultaneously in the Middle East and China between 100 BC and 100 AD. The vertical mill also appears at roughly around the same time in the Middle East and China. When and where the water wheel originated thus remains unclear, with various different hypotheses being proposed regarding its origins.

Ancient Near East

According to a number of historians of technology, the water wheel likely originated from somewhere in the ancient Near East during the last few centuries BC. According to Terry S. Reynolds and R. J. Forbes, it may have originated there in the 3rd century BC for use in moving millstones and small-scale corn grinding. Reynolds suggests that the first water wheels were Norias and, by the 2nd century BC, evolved into the vertical watermill in Syria and Asia Minor, from where it spread to ancient Greece and the Roman Empire. S. Avitsur also supports a Near-Eastern origin for the watermill. According to Donald Routledge Hill, water-powered Norias have been used in the Near East since at least 200 BC.

The water wheel appeared in the ancient Near East, specifically ancient Egypt, by the 4th century BC. Elsewhere in the Near East, there is archaeological evidence indicating water wheel usage in West Asia during the 4th century BC. The water wheel was used in the Near East by the 3rd century BC for use in moving millstones and small-scale grain grinding. Terry S. Reynolds suggests that the first water wheels were norias and, by the 2nd century BC, evolved into the vertical watermill in Syria and Asia Minor, from where it spread to Greece and the Roman Empire. Water wheels were used in the Near East during the Hellenistic period between the 3rd and 1st centuries BC, and the sāqiya spread rapidly across the Near East during this period.

Taking indirect evidence into account from the work of technician Apollonius of Perge, the historian of technology M.J.T. Lewis hypothesizes the appearance of the vertical-axle watermill to the early 3rd century BC, and the horizontal-axle watermill to around 240 BC, assigning Byzantium (in Asia Minor) and Alexandria (in Ptolemaic Egypt) as the places of origin. However, Örjan Wikander notes the hypothesis is open to scholarly discussion. A watermill is reported by the geographer Strabon (c. 64 BC – c. AD 24) to have existed sometime before 71 BC in the palace of the Pontian king Mithradates VI Eupator, but its exact construction cannot be gleaned from the text (XII, 3, 30 C 556).

Ancient Mesopotamia

Ancient Mesopotamia has also been suggested as another possible place of origin. Irrigation machines are referred to in Babylonian inscriptions, but without details on their construction, suggesting that water power had been harnessed for irrigation purposes. According to Hugh P. Vowles, the primitive use of water-rotated wheels may date back to Sumerian times, with references to a "Month for raising the Water Wheels", though it is not known whether these wheels were turned by the flow of a river. According to Faruk El-Yussif, the utilization of water power by means of simple paddle wheels for irrigation and drainage purposes appears to have also been developed in Mesopotamia. It has been suggested that the water wheel may have been used in Upper Mesopotamia as early as the 7th or 6th centuries BC, based on an interpretation of an Akkadian cuneiform tablet dating back to the Neo-Assyrian Empire. The tablet was found at Harran and describes various irrigation apparatus, followed by a description of a water channel resembling a sluice (or alternatively a qanat). According to J. Laessoe, it implies the use of an undershot water wheel, specifically a hydraulic Noria, powered by the water as it flows through the passage under the lock-gates behind which it was stored up. The tablet is not entirely preserved, thus any Akkadian terms for a sluice or water-wheel cannot be determined.

Persian Empire

The invention of the watermill is a question open to scholarly discussion. According to historian Helaine Selin, there is evidence indicating that the watermill originated from the Persian Empire before 350 BC, likely in what are today Iran or Iraq, originally for the purpose of grinding corn. There were quarries known for their millstones in Iran and on the upper Tigris in what is today Turkey. The mills invented at this date had horizontal, propeller-like water wheels that drove the millstones directly. However, there is a lack of literary evidence of water wheels being used in Mesopotamia at the time.

Ancient Egypt

The water wheel was traditionally dated to the last century BC in the eastern Mediterranean, particularly in Asia Minor, but recent scholarship assigns the appearance of the water wheel to an earlier date in ancient Egypt, where it appeared by the 3rd century BC. This is seen as an evolution of the paddle-driven water-lifting wheels that had been known in Egypt a century earlier. According to John Peter Oleson, both the compartmented wheel and the hydraulic Noria may have been invented in Egypt by the 4th century BC, with the Sakia being invented there a century later. This is supported by archeological finds at Faiyum, Egypt, where the oldest archeological evidence of a water-wheel has been found, in the form of a Sakia dating back to the 3rd century BC. A papyrus dating to the 2nd century BC also found in Faiyum mentions a water wheel used for irrigation, a 2nd-century BC fresco found at Alexandria depicts a compartmented Sakia, and the writings of Callixenus of Rhodes mention the use of a Sakia in Ptolemaic Egypt during the reign of Ptolemy IV in the late 3rd century BC.

The compartmented water wheel comes in two basic forms, the wheel with compartmented body (Latin tympanum) and the wheel with compartmented rim or a rim with separate, attached containers. The wheels could be either turned by men treading on its outside or by animals by means of a sakia gear. While the tympanum had a large discharge capacity, it could lift the water only to less than the height of its own radius and required a large torque for rotating. These constructional deficiencies were overcome by the wheel with a compartmented rim which was a less heavy design with a higher lift.

Paddle-driven water-lifting wheels had appeared in ancient Egypt by the 4th century BC. The Egyptians are credited with inventing the water wheel with attached pots, a water wheel with water compartments and a bucket chain, which ran over a pulley with buckets attached to it. The invention of the compartmentalized water wheel occurred in ancient Egypt around the 4th century BC, in a rural context, away from the metropolis of Hellenistic Alexandria, and then spread to other parts of North Africa.

According to John Peter Oleson, both the compartmented wheel and the hydraulic noria appeared in Egypt by the 4th century BC, with the Sakia being invented there a century later. This is supported by archeological finds at Faiyum, where the oldest archeological evidence of a water-wheel has been found, in the form of a Sakia dating back to the 3rd century BC. A papyrus dating to the 2nd century BC also found in Faiyum mentions a water wheel used for irrigation, a 2nd-century BC fresco found at Alexandria depicts a compartmented Sakia, and the writings of Callixenus of Rhodes mention the use of a Sakia in Ptolemaic Egypt during the reign of Ptolemy IV in the late 3rd century BC.

In Ptolemaic Egypt, water wheels were used between the 3rd and 1st centuries BC. The earliest literary reference to a water-driven, compartmented wheel appears in a medieval Arabic translation of Pneumatica (chap. 61) by Philo of Byzantium (c. 280 – c. 220 BC), describing its use in Egyptian irrigation. In his Parasceuastica (91.43−44), Philo advises the use of such wheels for submerging siege mines as a defensive measure against enemy sapping. Unlike other water-lifting devices and pumps of the period, the invention of the compartmented wheel cannot be traced to any particular Hellenistic engineer and may have been made in the late 4th century BC in a rural Egyptian context away from the Hellenistic metropolis of Alexandria. The origins of water power is attributed to the reality of rural Egyptian life along the Nile, rather than the intellectual capital of Alexandria which had no stream suitable for driving a paddle wheel. Compartmented wheels later appear to have been the means of choice for draining dry docks in Alexandria under the reign of Ptolemy IV (221−205 BC). Several Greek papyri from Ptolemaic Egypt dated to the 3rd and 2nd centuries BC mention the use of these wheels, but do not give further details.

The earliest depiction of a compartmented wheel is from a tomb painting in Ptolemaic Egypt which dates to the 2nd century BC. It shows a pair of yoked oxen driving the wheel via a sakia gear, which is here for the first time attested, too. The sakia gear system is already shown fully developed to the point that "modern Egyptian devices are virtually identical". It is believed that scientists or technicians from the Museum of Alexandria may have been involved in its development. An episode from the Alexandrian War in 48 BC tells of how Caesar's enemies employed geared waterwheels to pour sea water from elevated places on the position of the trapped Romans.

Late antiquity

Scheme of the 3rd century Hierapolis sawmill, Asia Minor, powered by a breastshot wheel.

Around 300 AD, the noria was introduced when the wooden compartments were replaced with inexpensive ceramic pots that were tied to the outside of an open-framed wheel.

In North Africa, several installations from around 300 AD were found where vertical-axle waterwheels fitted with angled blades were installed at the bottom of a water-filled, circular shaft. The water from the mill-race which entered tangentially the pit created a swirling water column that made the fully submerged wheel act like true water turbines, the earliest known to date.

Ancient China

Two types of hydraulic-powered chain pumps from the Tiangong Kaiwu of 1637, written by the Ming Dynasty encyclopedist, Song Yingxing (1587–1666).

The earliest waterwheel working like a lever was described by Zhuangzi in the late Warring States period (476-221 BC). It says that the waterwheel was invented by Zigong, a disciple of Confucius in the 5th century BC. By at least the 1st century AD, the Chinese of the Eastern Han Dynasty were using water wheels to crush grain in mills and to power the piston-bellows in forging iron ore into cast iron.

In the text known as the Xin Lun written by Huan Tan about 20 AD (during the usurpation of Wang Mang), it states that the legendary mythological king known as Fu Xi was the one responsible for the pestle and mortar, which evolved into the tilt-hammer and then trip hammer device (see trip hammer). Although the author speaks of the mythological Fu Xi, a passage of his writing gives hint that the water wheel was in widespread use by the 1st century AD in China (Wade-Giles spelling):

Fu Hsi invented the pestle and mortar, which is so useful, and later on it was cleverly improved in such a way that the whole weight of the body could be used for treading on the tilt-hammer (tui), thus increasing the efficiency ten times. Afterwards the power of animals—donkeys, mules, oxen, and horses—was applied by means of machinery, and water-power too used for pounding, so that the benefit was increased a hundredfold.

In the year 31 AD, the engineer and Prefect of Nanyang, Du Shi (d. 38), applied a complex use of the water wheel and machinery to power the bellows of the blast furnace to create cast iron. Du Shi is mentioned briefly in the Book of Later Han (Hou Han Shu) as follows (in Wade-Giles spelling):

In the seventh year of the Chien-Wu reign period (31 AD) Tu Shih was posted to be Prefect of Nanyang. He was a generous man and his policies were peaceful; he destroyed evil-doers and established the dignity (of his office). Good at planning, he loved the common people and wished to save their labor. He invented a water-power reciprocator (shui phai) for the casting of (iron) agricultural implements. Those who smelted and cast already had the push-bellows to blow up their charcoal fires, and now they were instructed to use the rushing of the water (chi shui) to operate it ... Thus the people got great benefit for little labor. They found the 'water(-powered) bellows' convenient and adopted it widely.

Water wheels in China found practical uses such as this, as well as extraordinary use. The Chinese inventor Zhang Heng (78–139) was the first in history to apply motive power in rotating the astronomical instrument of an armillary sphere, by use of a water wheel. The mechanical engineer Ma Jun (c. 200–265) from Cao Wei once used a water wheel to power and operate a large mechanical puppet theater for the Emperor Ming of Wei (r. 226–239).

Indian subcontinent

The early history of the watermill in the Indian subcontinent is obscure. Ancient Indian texts dating back to the 4th century BC refer to the term cakkavattaka (turning wheel), which commentaries explain as arahatta-ghati-yanta (machine with wheel-pots attached). On this basis, Joseph Needham suggested that the machine was a noria. Terry S. Reynolds, however, argues that the "term used in Indian texts is ambiguous and does not clearly indicate a water-powered device." Thorkild Schiøler argued that it is "more likely that these passages refer to some type of tread- or hand-operated water-lifting device, instead of a water-powered water-lifting wheel."

According to Greek historical tradition, India received water-mills from the Roman Empire in the early 4th century AD when a certain Metrodoros introduced "water-mills and baths, unknown among them [the Brahmans] till then". Irrigation water for crops was provided by using water raising wheels, some driven by the force of the current in the river from which the water was being raised. This kind of water raising device was used in ancient India, predating, according to Pacey, its use in the later Roman Empire or China.

Around 1150, the astronomer Bhaskara Achārya observed water-raising wheels and imagined such a wheel lifting enough water to replenish the stream driving it, effectively, a perpetual motion machine. The construction of water works and aspects of water technology in India is described in Arabic and Persian works. During medieval times, the diffusion of Indian and Persian irrigation technologies gave rise to an advanced irrigation system which bought about economic growth and also helped in the growth of material culture.

Ismail al-Jazari's Book of Knowledge of Ingenious Mechanical Devices (1206) uses the term "Sindhi wheel" to describe a sāqiya. This indicates it may have originated in the northwestern Indian subcontinent (modern Pakistan). During the Delhi Sultanate (1206–1526), geared water-raising wheels were introduced from the Islamic world to India.

Roman Empire

See also: List of ancient watermills

Romans water wheel devices are described by Vitruvius, including the reverse overshot water-wheel and the Archimedean screw. Many were found during modern mining at the copper mines at Rio Tinto in Spain, one system involving 16 such wheels stacked above one another so as to lift water about 80 feet from the mine sump. Part of such a wheel was found at Dolaucothi, a Roman gold mine in south Wales in the 1930s when the mine was briefly re-opened. It was found about 160 feet below the surface, so must have been part of a similar sequence as that discovered at Rio Tinto. It has recently been carbon dated to about 90 AD, and since the wood from which it was made is much older than the deep mine, it is likely that the deep workings were in operation perhaps 30–50 years after. It is clear from these examples of drainage wheels found in sealed underground galleries in widely separated locations that building water wheels was well within their capabilities, and such verticals water wheels commonly used for industrial purposes.

About the same time, the overshot wheel appears for the first time in a poem by Antipater of Thessalonica, which praises it as a labour-saving device (IX, 418.4–6). The motif is also taken up by Lucretius (ca. 99–55 BC) who likens the rotation of the waterwheel to the motion of the stars on the firmament (V 516). The third horizontal-axled type, the breastshot waterwheel, comes into archaeological evidence by the late 2nd century AD context in central Gaul. Most excavated Roman watermills were equipped with one of these wheels which, although more complex to construct, were much more efficient than the vertical-axle waterwheel. In the 2nd century AD Barbegal watermill complex a series of sixteen overshot wheels was fed by an artificial aqueduct, a proto-industrial grain factory which has been referred to as "the greatest known concentration of mechanical power in the ancient world".

Medieval Islamic world

See also: Muslim Agricultural Revolution

The norias of Hama on the Orontes River

Arab engineers took over the water technology of the hydraulic societies of the ancient Near East; they adopted water wheels as early as the 7th century, excavation of a canal in the Basra region discovered remains of a water wheel dating from this period. Hama in Syria still preserves one of its large wheels, on the river Orontes, although they are no longer in use. One of the largest had a diameter of about 20 metres and its rim was divided into 120 compartments. Another wheel that is still in operation is found at Murcia in Spain, La Nora, and although the original wheel has been replaced by a steel one, the Moorish system during al-Andalus is otherwise virtually unchanged. Some medieval Islamic compartmented water wheels could lift water as high as 30 meters. Muhammad ibn Zakariya al-Razi's Kitab al-Hawi in the 10th century described a noria in Iraq that could lift as much as 153,000 litres per hour, or 2550 litres per minute. This is comparable to the output of modern Norias in East Asia which can lift up to 288,000 litres per hour, or 4800 litres per minute.

The industrial uses of watermills in the Islamic world date back to the 7th century, while horizontal-wheeled and vertical-wheeled water mills were both in widespread use by the 9th century. A variety of industrial watermills were used in the Islamic world, including gristmills, hullers, paper mills, sawmills, shipmills, stamp mills, steel mills, sugar mills, and tide mills. By the 11th century, every province throughout the Islamic world had these industrial watermills in operation, from al-Andalus and North Africa to the Middle East and Central Asia. Muslim and Christian engineers also used crankshafts and water turbines, gears in watermills and water-raising machines, and dams as a source of water, used to provide additional power to watermills and water-raising machines. Fulling mills, paper mills and steel mills may have spread from Islamic Spain to Christian Spain in the 12th century. Industrial water mills were also employed in large factory complexes built in al-Andalus between the 11th and 13th centuries.

Waterwheel in Djambi, Sumatra c. 1918

The engineers of the Islamic world developed several solutions to achieve the maximum output from a water wheel. One solution was to mount them to piers of bridges to take advantage of the increased flow. Another solution was the shipmill, a type of water mill powered by water wheels mounted on the sides of ships moored in midstream. This technique was employed along the Tigris and Euphrates rivers in 10th century Iraq, where large shipmills made of teak and iron could produce 10 tons of flour from corn every day for the granary in Baghdad. The flywheel mechanism, which is used to smooth out the delivery of power from a driving device to a driven machine, was invented by Ibn Bassal (fl. 1038-1075) of Al-Andalus; he pioneered the use of the flywheel in the saqiya (chain pump) and noria. The engineers Al-Jazari in the 13th century and Taqi al-Din in the 16th century described many inventive water-raising machines in their technological treatises. They also employed water wheels to power a variety of devices, including various water clocks and automata.

Late Medieval Europe

By the 11th century there were parts of Europe where the exploitation of water was commonplace. The water wheel is understood to have actively shaped and forever changed the outlook of Westerners. Europe began to transit from human and animal muscle labor towards mechanical labor with the advent of the water wheel. Medievalist Lynn White Jr. contended that the spread of inanimate power sources was eloquent testimony to the emergence of the West of a new attitude toward, power, work, nature, and above all else technology.

Harnessing water-power enabled gains in agricultural productivity, food surpluses and the large scale urbanization starting in the 11th century. The usefulness of water power motivated European experiments with other power sources, such as wind and tidal mills. Waterwheels influenced the construction of cities, more specifically canals. The techniques that developed during this early period such as stream jamming and the building of canals, put Europe on a hydraulically focused path, for instance water supply and irrigation technology was combined to modify supply power of the wheel. Illustrating the extent to which there was a great degree of technological innovation that met the growing needs of the feudal state.

The assembly convened by William of Normandy, commonly referred to as the "Domesday" or Doomsday survey (circa 1086), took an inventory of all potentially taxable property in England, which included over six thousand mills spread across three thousand different locations, up from less than a hundred in the previous century.

The water mill was used for grinding grain, producing flour for bread, malt for beer, or coarse meal for porridge. Hammermills used the wheel to operate hammers. One type was fulling mill, which was used for cloth making. The trip hammer was also used for making wrought iron and for working iron into useful shapes, an activity that was otherwise labour-intensive. The water wheel was also used in papermaking, beating material to a pulp. In the 13th century water mills used for hammering throughout Europe improved the productivity of early steel manufacturing. Along with the mastery of gunpowder, waterpower provided European countries worldwide military leadership from the 15th century.

Early modern West

Early modern Europe

Millwrights distinguished between the two forces, impulse and weight, at work in water wheels long before 18th-century Europe. Fitzherbert, a 16th-century agricultural writer, wrote "druieth the wheel as well as with the weight of the water as with strengthe [impulse]". Leonardo da Vinci also discussed water power, noting "the blow [of the water] is not weight, but excites a power of weight, almost equal to its own power". However, even realisation of the two forces, weight and impulse, confusion remained over the advantages and disadvantages of the two, and there was no clear understanding of the superior efficiency of weight. Prior to 1750 it was unsure as to which force was dominant and was widely understood that both forces were operating with equal inspiration amongst one another. The waterwheel sparked questions of the laws of nature, specifically the laws of force. Evangelista Torricelli's work on water wheels used an analysis of Galileo's work on falling bodies, that the velocity of a water sprouting from an orifice under its head was exactly equivalent to the velocity a drop of water acquired in falling freely from the same height.

Industrial Europe

The water wheel was a driving force behind the earliest stages of industrialization in Britain. Water-powered reciprocating devices were used in trip hammers and blast furnace bellows. Richard Arkwright's water frame was powered by a water wheel.

The most powerful water wheel built in the United Kingdom was the 100 hp Quarry Bank Mill water wheel near Manchester. A high breastshot design, it was retired in 1904 and replaced with several turbines. It has now been restored and is a museum open to the public.

The biggest working water wheel in mainland Britain has a diameter of 15.4 m (51 ft) and was built by the De Winton company of Caernarfon. It is located within the Dinorwic workshops of the National Slate Museum in Llanberis, North Wales.

The largest working water wheel in the world is the Laxey Wheel (also known as Lady Isabella) in the village of Laxey, Isle of Man. It is 72 feet 6 inches (22.10 m) in diameter and 6 feet (1.83 m) wide and is maintained by Manx National Heritage.

During the Industrial Revolution, in the first half of the 19th century engineers started to design better wheels. In 1823 Jean-Victor Poncelet invented a very efficient undershot wheel design that could work on very low heads, which was commercialized and became popular by late 1830s. Other designs, as the Sagebien wheel, followed later. At the same time Claude Burdin was working on a radically different machine which he called turbine, and his pupil Benoît Fourneyron designed the first commercial one in the 1830s.

Development of water turbines led to decreased popularity of water wheels. The main advantage of turbines is that its ability to harness head is much greater than the diameter of the turbine, whereas a water wheel cannot effectively harness head greater than its diameter. The migration from water wheels to modern turbines took about one hundred years.

North America

Water wheels were used to power sawmills, grist mills and for other purposes during development of the United States. The 40 feet (12 m) diameter water wheel at McCoy, Colorado, built in 1922, is a surviving one out of many which lifted water for irrigation out of the Colorado River.

Two early improvements were suspension wheels and rim gearing. Suspension wheels are constructed in the same manner as a bicycle wheel, the rim being supported under tension from the hub- this led to larger lighter wheels than the former design where the heavy spokes were under compression. Rim-gearing entailed adding a notched wheel to the rim or shroud of the wheel. A stub gear engaged the rim-gear and took the power into the mill using an independent line shaft. This removed the rotative stress from the axle which could thus be lighter, and also allowed more flexibility in the location of the power train. The shaft rotation was geared up from that of the wheel which led to less power loss. An example of this design pioneered by Thomas Hewes and refined by William Armstrong Fairburn can be seen at the 1849 restored wheel at the Portland Basin Canal Warehouse.

Somewhat related were fish wheels used in the American Northwest and Alaska, which lifted salmon out of the flow of rivers.

Australia

Australia has a relatively dry climate, nonetheless, where suitable water resources were available, water wheels were constructed in 19th-century Australia. These were used to power sawmills, flour mills, and stamper batteries used to crush gold-bearing ore. Notable examples of water wheels used in gold recovery operations were the large Garfield water wheel near Chewton—one of at least seven water wheels in the surrounding area—and the two water wheels at Adelong Falls; some remnants exist at both sites. The mining area at Walhalla once had at least two water wheels, one of which was rolled to its site from Port Albert, on its rim using a novel trolley arrangement, taking nearly 90 days. A water wheel at Jindabyne, constructed in 1847, was the first machine used to extract energy—for flour milling—from the Snowy River.

Compact water wheels, known as Dethridge wheels, were used not as sources of power but to measure water flows to irrigated land.

New Zealand

Water wheels were used extensively in New Zealand. The well-preserved remains of the Young Australian mine's overshot water wheel exist near the ghost town of Carricktown, and those of the Phoenix flour mill's water wheel are near Oamaru.

Modern developments

A recent development of the breastshot wheel is a hydraulic wheel which effectively incorporates automatic regulation systems. The Aqualienne is one example. It generates between 37 kW and 200 kW of electricity from a 20 m³ (710 cu ft) waterflow with a head of 1 to 3.5 m (3 to 11 ft). It is designed to produce electricity at the sites of former watermills.

Types

Undershot wheel

Undershot water wheel

A vertically-mounted water wheel that is rotated by water striking paddles or blades at the bottom of the wheel is said to be undershot. This is generally the least efficient, oldest type of wheel (with the exception of the poncelet wheel). It has the advantage of being cheaper and simpler to build, but is less powerful and can only be used where the flow rate is sufficient to provide torque.

Undershot wheels gain no advantage from head. They are most suited to shallow streams in flat country.

The Anderson Mill is undershot, backshot, and overshot using two sources of water. This allows the speed of the wheel to be controlled

Undershot wheels are also well suited to installation on floating platforms. The earliest were probably constructed by the Roman general Belisarius during the siege of Rome in 537. Later they were sometimes mounted immediately downstream from bridges where the flow restriction of arched bridge piers increased the speed of the current.

Breastshot wheel

This is a view inside of the largest water wheel in the UK, situated at the Quarry Bank cotton mill in the UK. It is still working today and powers the looms at Quarry Bank Mill.

A vertically-mounted water wheel that is rotated by falling water striking buckets near the center of the wheel's edge, or just above it, is said to be breastshot. Breastshot wheels are the most common type in the United States of America and are said to have powered the American industrial revolution.

Breastshot wheels are less efficient than overshot wheels (see below), more efficient than undershot wheels, and are not backshot (see below). The individual blades of a breastshot wheel are actually buckets, as are those of most overshot wheels, and not simple paddles like those of most undershot wheels (the Poncelet design being a notable exception). A breastshot wheel requires a good trash rake and typically has a masonry "apron" closely conforming to the wheel face, which helps contain the water in the buckets as they progress downwards. Breastshot wheels are preferred for steady, high-volume flows such as are found on the fall line of the North American East Coast.

Overshot wheel

Overshot water wheel

A vertically-mounted water wheel that is rotated by falling water striking paddles, blades or buckets near the top of the wheel is said to be overshot. In true overshot wheels the water passes over the top of the wheel, but the term is sometimes applied to backshot or pitchback wheels where the water goes down behind the waterwheel.

A typical overshot wheel has the water channeled to the wheel at the top and slightly to one side in the direction of rotation. The water collects in the buckets on that side of the wheel, making it heavier than the other "empty" side. The weight turns the wheel, and the water flows out into the tail-water when the wheel rotates enough to invert the buckets. The overshot design can use all of the water flow for power (unless there is a leak) and does not require rapid flow.

Unlike undershot wheels, overshot wheels gain a double advantage from gravity. Not only is the force of the flowing water partially transferred to the wheel, the weight of the water descending in the wheel's buckets also imparts additional energy. The mechanical power derived from an overshot wheel is determined by the wheel's physical size and the available head, so they are ideally suited to hilly or mountainous country.

Overshot wheels demand exact engineering and significant head, which usually means significant investment in constructing a dam, millpond and waterways. Sometimes the final approach of the water to the wheel is along a lengthy flume or penstock.

Backshot wheel

A backshot wheel (also called pitchback) is a variety of overshot wheel where the water is introduced just behind the summit of the wheel. It combines the advantages from breastshot and overshot systems, since the full amount of the potential energy released by the falling water is harnessed as the water descends the back of the wheel.

A backshot wheel continues to function until the water in the wheel pit rises well above the height of the axle, when any other overshot wheel will be stopped or even destroyed. This makes the technique particularly suitable for streams that experience extreme seasonal variations in flow, and reduces the need for complex sluice and tail race configurations. A backshot wheel may also gain power from the water's current past the bottom of the wheel, and not just the weight of the water falling in the wheel's buckets.

Materials for construction

Although traditionally water wheels have been made mostly from wood, the use of iron or steel in overshot (and pitchback) wheels allows faster rotation (possibly reducing the need for gearing) without extreme reductions in available torque. A wooden wheel with a wooden axle that can easily turn low-speed, high-torque loads such as a run of millstones cannot necessarily sustain high speeds such as are needed for hydroelectric power generation.

Overshot (and particularly backshot) wheels are the most efficient type; a backshot steel wheel can be more efficient than all but the most advanced and well-constructed turbines. Nevertheless, in some situations an overshot wheel is vastly preferable to any turbine.

See also

• Hydraulic ram
• Cable railway

Example applications

The following installations use a water wheel as the prime mover:

• Watermills in the United Kingdom
• Claverton Pumping Station – canal water pumping station
• Derby Industrial Museum – originally a silk mill
• Laxey Wheel – pumping water from a mine
• Snaefell Wheel – pumping water from a mine

Water turbines

• Water turbine
• Pelton wheel
• Banki turbine
• Francis turbine
• Kaplan turbine
• Turgo turbine
• Tyson turbine

Bibliopgrahy

• Morton, W. Scott and Charlton M. Lewis (2005). China: It's History and Culture. New York: McGraw-Hill, Inc.
• Needham, Joseph (1986). Science and Civilization in China: Volume 4, Part 2. Taipei: Caves Books, Ltd.
• Pacey, Arnold, Technology in World Civilization: A Thousand-year History, The MIT Press; Reprint edition (July 1, 1991). ISBN 0262660725.
• Reynolds, Terry S., Stronger Than a Hundred Men: A History of the Vertical Water Wheel, (1983), Johns Hopkins University Press. ISBN 0801872480.

External links

• Esay/audio clip
• Glossary of water wheel terms
• Persian Wheel in India, 1814-1815 painting with explanatory text, at British Library website.
• Water wheel history
• Waterwheel Factory, with pictures of water wheels
• WaterHistory.org - Several articles concerning water wheels