A water wheel is a hydropower system; a machine for extracting power from the flow of water. Water wheels of various types have been known since Roman times 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'.
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 and are never backshot. They are most suited to shallow streams in flat country.
The Anderson Mill is undershot, backshot, and overshot using two sources of water
Undershot wheels are also well suited to installation on floating platforms, and the Romans mounted them immediately downstream from bridges where the flow restriction of arched bridge supports increased the speed of the current.
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. Overshot wheels are the most efficient type; an overshot steel wheel that is also backshot can be more efficient than all but the most advanced and well-constructed turbines. In some situations an overshot wheel is vastly preferable to any turbine.
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.
Overshot wheels demand exact engineering and significant head, which usually means significant investment in constructing a dam, millpond and waterways.
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.
Although traditionally water wheels have been made mostly from wood, the use of steel in overshot 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.
Most overshot water wheels turn in the opposite direction of the water's flow. This is because the water flows over the top of the wheel, and not under it as in an undershot or breastshot design.
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, more efficient than undershot wheels, and are not backshot. 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.
Water wheel technology
The technology of the water wheel had long been known, but it was not put into widespread use until the Middle Ages when an acute shortage of labour made machines such as the water wheel cost effective. Cistercian monasteries, in particular, made extensive use of water wheels to power watermills of many kinds. An early example of a very large waterwheel is the still extant wheel at the early 13th century Real Monasterio de Nuestra Senora de Rueda, a Cistercian monastery in the Aragon region of Spain. Grist mills (for corn) were undoubtedly the most common, but there were also sawmills, fulling mills and mills to fulfil many other labor-intensive tasks. The water wheel remained competitive with the steam engine well into the Industrial Revolution.
Water wheel technology was developed extensively in England in the 18th century, with notable figures including John Smeaton and James Brindley, following theoretical calculations and practical experiments in France and elsewhere. Smeaton performed experiments in 1754 that conclusively demonstrated the superiority of the overshot system: one of Brindley's water wheels can be seen at the Brindley Mill in Leek, Staffordshire, England. In the 19th century, Jean-Victor Poncelet worked on improving the efficiency of the undershot design using modern hydraulic physics for the first time.
The main difficulty of water wheels was their inseperability from water. This meant that mills often needed to be located far from population centres and away from natural resources. Water mills were still in commercial use well into the twentieth century, however.
Overshot & pitchback waterwheels are suitable where there is a small stream with a height difference of more than 2 metres, often in association with a small reservoir. Breastshot and undershot wheels can be used on rivers or high volume flows with large reservoirs.
The most powerful waterwheel built in the United Kingdom was the 100 hp Quarry Bank Mill Waterwheel 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.
Modern Hydro-electric dams can be viewed as the descendants of the water wheel as they too take advantage of the movement of water downhill.