Editing Water wheel (section)

== Types ==
{{more citations needed section|date=December 2024}}
[[File:Waterwheel that used to crush grain into flour.jpg|thumb|A vertical water wheel]]

Water wheels come in two basic designs:<ref>{{Cite web|url=http://ffden-2.phys.uaf.edu/211_fall2010.web.dir/Brooks/types-of-water-wheels.html |title=Types of Water Wheels – The Physics of a Water Wheel|website=ffden-2.phys.uaf.edu |language=en|access-date=2017-07-10}}</ref>
* a horizontal wheel with a vertical axle; or
* a vertical wheel with a horizontal axle.
The latter can be subdivided according to where the water hits the wheel into backshot (pitch-back<ref>[https://web.archive.org/web/20170815063910/https://en.oxforddictionaries.com/definition/pitch-back pitch-back]</ref>), overshot, breastshot, undershot, and stream-wheels.<ref name="Stream wheel term and specifics">{{Cite web |url=http://www.energy.soton.ac.uk/hydro/waterwheels.html |title=Stream wheel term and specifics |access-date=2009-04-07 |archive-date=2011-10-07 |archive-url=https://web.archive.org/web/20111007081641/http://www.energy.soton.ac.uk/hydro/waterwheels.html |url-status=dead }}</ref><ref>[https://www.merriam-webster.com/dictionary/undershot%20wheel Merriam Webster]</ref><ref>[http://www.powerinthelandscape.co.uk/water/water_wheels.html Power in the Landscape]</ref>
The term ''undershot'' can refer to any wheel where the water passes under the wheel<ref>[https://www.collinsdictionary.com/dictionary/english/undershot Collins English Dictionary]</ref> but it usually implies that the water entry is low on the wheel.

Overshot and backshot water wheels are typically used where the available height difference is more than a couple of meters. Breastshot wheels are more suited to large flows with a moderate [[Hydrostatic head|head]]. Undershot and stream wheel use large flows at little or no head.

There is often an associated [[millpond]], a reservoir for storing water and hence energy until it is needed. Larger heads store more [[Gravitational energy|gravitational potential energy]] for the same amount of water so the reservoirs for overshot and backshot wheels tend to be smaller than for breast shot wheels.

Overshot and pitchback water wheels are suitable where there is a small stream with a height difference of more than {{Convert|2|m|ft|abbr=|round=0.5}}, often in association with a small reservoir. Breastshot and undershot wheels can be used on rivers or high volume flows with large reservoirs.

=== Summary of types ===
{| class="wikitable"
|'''Vertical axis''' also known as tub or Norse mills.
* Horizontal wheel with a vertical axis
* A jet of water strikes blades mounted on the axle
* Driving surfaces – blades
* Water – low volume, high [[Hydrostatic head|head]]
* Efficiency – poor
|| [[File:Vertical waterwheel simple.svg|150px|Diagram of vertical axis water mill]]
|-
|'''Stream''' (also known as [[free surface]]). Ship wheels are a type of stream wheel.
* Vertical wheel with horizontal axle
* The bottom of the wheel is placed into flowing water
* Driving surfaces – blades – flat prior to 18th century, curved thereafter
* Water – very large volume, no head
* Efficiency – about 20% prior to 18th century and later 50 to 60%
|[[File:Stream waterwheel simple.svg|150px|Diagram of stream shot waterwheel]]
|-
|'''Undershot'''
* Vertical wheel with horizontal axle
* The water hits the wheel low down, typically in the bottom quarter
* Driving surfaces – blades – flat prior to 18th century, curved thereafter
* Water – large volume, low head
* Efficiency – about 20% prior to 18th century and later 50 to 60%
| [[File:Undershot waterwheel simple.svg|150px|Diagram of undershot waterwheel showing headrace, tailrace, and water]]
|-
|'''Breastshot'''
* Vertical wheel with horizontal axle
* The water hits the wheel roughly central, typically between one quarter and three quarters of the height.
* Driving surfaces – buckets – carefully shaped to ensure that the water enters smoothly
* Water – large volume, moderate head
* Efficiency – 50 to 60%
| [[File:Breastshot waterwheel simple.svg|150px|Diagram of breastshot waterwheel showing headrace, tailrace, and water]]
|-
|'''Overshot'''
* Vertical wheel with horizontal axle
* The water hits near the top of the wheel and in front of the axle so that it turns away from the head race
* Driving surfaces – buckets
* Water – low volume, large head
* Efficiency – 80 to 90%
|[[File:Overshot waterwheel simple.svg|150px|Diagram of overshot waterwheel showing headrace, tailrace, water, and spillage]]
|-
|'''Backshot''' (also known as pitchback)
* Vertical wheel with horizontal axle
* The water hits near the top of the wheel and before the axle so that it turns back towards the head race
* Driving surfaces – buckets
* Water – low volume, large head
* Efficiency – 80 to 90%
| [[File:Backshot waterwheel simple.svg|150px|Diagram of backshot waterwheel showing headrace, tailrace, water, and spillage]]
|}

=== Vertical axis ===
[[File:Vertical waterwheel simple.svg|thumb|150px|Vertical axis water mill]]

A horizontal wheel with a vertical axle.

Commonly called a '''tub wheel''', '''Norse mill''' or '''Greek mill''',<ref>{{cite book|last1=Denny|first1=Mark |title=Ingenium: Five Machines That Changed the World|date=2007|publisher=Johns Hopkins University|isbn=9780801885860 |url=https://archive.org/details/ingeniumfivemach0000denn |url-access=registration|access-date=19 January 2018}}</ref><ref>{{cite web| title=Waterwheel|url=https://www.britannica.com/technology/waterwheel-engineering |publisher=Encyclopædia Britannica, Inc.|access-date=19 January 2018}}</ref> the horizontal wheel is a primitive and inefficient form of the modern turbine. However, if it delivers the required power then the efficiency is of secondary importance. It is usually mounted inside a mill building below the working floor. A jet of water is directed on to the paddles of the water wheel, causing them to turn. This is a simple system usually without gearing so that the vertical axle of the water wheel becomes the drive spindle of the mill.

=== Stream ===
[[File:Stream waterwheel simple.svg|thumb|150px|Stream shot waterwheel]]

A stream wheel<ref name="Stream wheel term and specifics"/><ref name="PITLtypes"/> is a vertically mounted water wheel that is rotated by the water in a water course striking paddles or blades at the bottom of the wheel. This type of water wheel is the oldest type of horizontal axis wheel.{{citation needed|date=April 2017}} They are also known as [[free surface]] wheels because the water is not constrained by millraces or wheel pits. {{citation needed|date=April 2017}}

Stream wheels are cheaper and simpler to build and have less of an environmental impact than other types of wheels. They do not constitute a major change of the river. Their disadvantages are their low efficiency, which means that they generate less power and can only be used where the flow rate is sufficient. A typical flat board undershot wheel uses about 20 percent of the energy in the flow of water striking the wheel as measured by English civil engineer [[John Smeaton]] in the 18th century.<ref>The History of Science and Technology by Bryan Bunch with Alexander Hellmans p.&nbsp;114</ref> More modern wheels have higher efficiencies.

Stream wheels gain little or no advantage from the head, a difference in water level.

Stream wheels mounted on floating platforms are often referred to as hip wheels and the mill as a [[ship mill]]. They were sometimes mounted immediately downstream from [[bridges]] where the flow restriction of the bridge piers increased the speed of the current.{{citation needed|date=April 2017}}

Historically they were very inefficient but major advances were made in the eighteenth century.<ref name="ASMEnoria">{{cite web
|url=https://www.asme.org/wwwasmeorg/media/resourcefiles/aboutasme/who%20we%20are/engineering%20history/landmarks/241-noria-al-muhammadiyya.pdf
|title=Noria al-Muhammadiyya
|publisher=The American Society of Mechanical Engineers
|author=The American Society of Mechanical Engineers
|access-date=12 Feb 2017
|date=December 2006
}}</ref>

=== Undershot wheel ===
[[File:Undershot waterwheel simple.svg|thumb|150px|Undershot waterwheel showing headrace, tailrace, and water]]

An undershot wheel is a vertically mounted water wheel with a horizontal axle that is rotated by the water from a low weir striking the wheel in the bottom quarter. Most of the energy gain is from the movement of the water and comparatively little from the head. They are similar in operation and design to stream wheels.

The term undershot is sometimes used with related but different meanings:
* all wheels where the water passes under the wheel<ref>{{cite web
|url=https://www.collinsdictionary.com/dictionary/english/undershot
|author=Collins English Dictionary
|access-date=12 February 2017
|title=undershot
}}</ref>
* wheels where the water enters in the bottom quarter.
* wheels where paddles are placed into the flow of a stream. See the stream above.<ref>{{cite web
|url=https://www.merriam-webster.com/dictionary/undershot%20wheel
|author=Merriam Webster
|title=stream wheel
}}</ref><ref name="PITLtypes">{{cite web
|url=http://www.powerinthelandscape.co.uk/water/water_wheels.html
|author=Power in the landscape
|access-date=12 February 2017
|title=Types of water wheels
}}</ref>

This is the oldest type of vertical water wheel.

=== Breastshot wheel ===
[[File:Breastshot waterwheel simple.svg|thumb|150px|Breastshot waterwheel showing headrace, tailrace, and water]]

The word '''breastshot''' is used in a variety of ways. Some authors restrict the term to wheels where the water enters at about the 10 o’clock position, others 9 o’clock, and others for a range of heights.<ref>{{Cite journal |last1=Müller |first1=G. |last2=Wolter |first2=C. |date=2004 |title=The breastshot waterwheel: design and model tests |url=https://hmf.enseeiht.fr/travaux/CD0708/beiere/3/html/bi/3/fichiers/Muller_exp.pdf |journal=Proceedings of the Institution of Civil Engineers - Engineering Sustainability |language=en |volume=157 |issue=4 |pages=203–211 |doi=10.1680/ensu.2004.157.4.203 |issn=1478-4629 |via=Semantic Scholar |archive-date=2021-02-11 |access-date=2022-03-04 |archive-url=https://web.archive.org/web/20210211215637/http://hmf.enseeiht.fr/travaux/CD0708/beiere/3/html/bi/3/fichiers/Muller_exp.pdf |url-status=dead }}</ref> In this article it is used for wheels where the water entry is significantly above the bottom and significantly below the top, typically the middle half.

They are characterized by:
* buckets carefully shaped to minimize turbulence as water enters
* buckets ventilated with holes in the side to allow air to escape as the water enters
* a masonry "apron" closely conforming to the wheel face, which helps contain the water in the buckets as they progress downwards

Both [[kinetic energy|kinetic]] (movement) and [[potential energy|potential]] (height and weight) energy are utilised.

The small clearance between the wheel and the masonry requires that a breastshot wheel has a good [[trash rack]] ('screen' in British English) to prevent debris from jamming between the wheel and the apron and potentially causing serious damage.

Breastshot wheels are less efficient than overshot and backshot wheels but they can handle high flow rates and consequently high power. They are preferred for steady, high-volume flows such as are found on the [[Atlantic Seaboard fall line|Fall Line]] of the North American East Coast. Breastshot wheels are the most common type in the United States of America{{Citation needed|date=February 2010}} and are said to have powered the industrial revolution.<ref name="ASMEnoria"/>

=== Overshot wheel ===
[[File:Overshot waterwheel simple.svg|thumb|150px|Overshot waterwheel showing headrace, tailrace, water, and spillage]]

A vertically mounted water wheel that is rotated by water entering buckets just past the top of the wheel is said to be overshot. The term is sometimes, erroneously, applied to backshot wheels, where the water goes down behind the wheel.

A typical overshot wheel has the water channeled to the wheel at the top and slightly beyond the axle. 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 is very efficient, it can achieve 90%,<ref>{{Cite web|date=2021-04-12|title=What type of water wheel is most efficient?|url=https://faq-ans.com/en/Q%26A/page=6b333be1485a8633ba94ae74c9b8bf70 |access-date=2021-11-23|website=faq-ans.com|language=en}}</ref> and does not require rapid flow.

Nearly all of the energy is gained from the weight of water lowered to the tailrace although a small contribution may be made by the kinetic energy of the water entering the wheel. They are suited to larger heads than the other type of wheel so they are ideally suited to hilly countries. However even the largest water wheel, the [[Laxey Wheel]] in the [[Isle of Man]], only utilises a head of around {{Convert|30|m||abbr=on|sigfig=1}}. The world's largest head turbines, [[Bieudron Hydroelectric Power Station]] in [[Switzerland]], utilise about {{Convert|1869|m||abbr=on}}.

Overshot wheels require a large head compared to other types of wheel which usually means significant investment in constructing the headrace. Sometimes the final approach of the water to the wheel is along a [[flume]] or [[penstock]], which can be lengthy.

=== Backshot wheel ===
[[File:Backshot waterwheel simple.svg|thumb|150px|Backshot waterwheel showing headrace, tailrace, water, and spillage]]

A backshot wheel (also called '''pitchback''') is a variety of overshot wheel where the water is introduced just before the summit of the wheel. In many situations, it has the advantage that the bottom of the wheel is moving in the same direction as the water in the tailrace which makes it more efficient. It also performs better than an overshot wheel in flood conditions when the water level may submerge the bottom of the wheel. It will continue to rotate until the water in the wheel pit rises quite high on the wheel. This makes the technique particularly suitable for streams that experience significant variations in flow and reduces the size, complexity, and hence cost of the tailrace.

The direction of rotation of a backshot wheel is the same as that of a breastshot wheel but in other respects, it is very similar to the overshot wheel. See below.

=== Hybrid ===
==== Overshot and backshot ====
[[File:Finch Foundary Water Wheel, Devon, UK - Diliff.jpg|thumb|left|One of Finch Foundry's water wheels.]]

Some wheels are overshot at the top and backshot at the bottom thereby potentially combining the best features of both types. The photograph shows an example at [[Finch Foundry]] in Devon, UK. The head race is the overhead timber structure and a branch to the left supplies water to the wheel. The water exits from under the wheel back into the stream.

==== Reversible ====
[[File:AndersonMill.jpg|thumb|The '''Anderson Mill''' of [[Anderson Mill, Texas|Texas]] is undershot, backshot, and overshot using two sources of water. This allows the direction of the wheel to be reversed.]]

A special type of overshot/backshot wheel is the reversible water wheel. This has two sets of blades or buckets running in opposite directions so that it can turn in either direction depending on which side the water is directed. Reversible wheels were used in the [[mining]] industry in order to power various means of ore conveyance. By changing the direction of the wheel, barrels or baskets of ore could be lifted up or lowered down a shaft or inclined plane. There was usually a cable drum or a chain basket on the axle of the wheel. It is essential that the wheel have braking equipment to be able to stop the wheel (known as a braking wheel). The oldest known drawing of a reversible water wheel was by [[Georgius Agricola]] and dates to 1556.