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Windmills are powered by their sails. These sails are found in different designs, from the primitive common sails to the advanced patent sails.

Jib sails

The jib sail is found in Mediterranean countries, consisting of a simple triangle of cloth wound around a spar. The mill needs to be stopped in order to adjust the reefing of the sail. Although rare in the UK, at least two windmills are known to have had jib sails (St Mary's, Isle of Scilly and Cann Mills, Melbury Abbas).

Image:Windmill Antimahia Kos.jpg|Jib sailsImage:Sobreiro.jpg|More fully spreadImage:Spanish Mill, St Mary's.jpg|St Mary's, Isles of ScillyFile:Cann Mill, Melbury Abbas.jpg|Cann Mills, Melbury Abbas

Common sails

The simplest form of sail. In medieval mills the sailcloth was wound in and out of a ladder type arrangement of sails. Medieval sails could be with or without outer sailbars. Post-medieval mill sails have a lattice framework that the sailcloth is spread over. There are various "reefs" for the different spread of sails. These are full reef, dagger point, sword point and first reef. The mill needs to be stopped in order to adjust the reefing of the sail.

Image:Ellezelles JPG02.jpg|FurledImage:Jard moulin a vent.JPG|First Reef (Medieval style sail)Image:Windmuehle.JPG|Sword pointImage:Wissel.jpg|Dagger pointImage:Cassel Nord (moulin).jpg|Full reefImage:Moulin-montfuron-2.JPG|Full reef (Medieval style sail)

Spring sails

Spring sails were invented by the Scottish millwright Andrew Meikle in 1772. The sail is divided into a number of bays with each bay having a number of shutters in it. All of the shutters are joined together by the shutter bar, and the force required for the wind to open the shutters is adjusted by a separate spring on each individual sail. Although automatic in operation, the mill needs to be stopped in order to adjust the reefing of the sail.

Image:Outwood Windmill.jpg|Open

Roller reefing sails

Roller reefing sails were invented by Stephen Hooper in 1789. As with spring sails, the sail is divided into a number of bays. Each bay has a number of spars, with cloth wound around it. The cloth is extended or retracted by a rod and lever system, and connected with a shutter bar on each sail. Adjustment of the roller reefing sail can be made without stopping the mill. This type of sail was popular in Yorkshiremarker, although the only remaining mill with roller reefing sails intact is Ballycopeland Windmill in Northern Irelandmarker.

Image:279350930 a90dcc8d8c b.jpg|FurledImage:Haigh windpump.jpg|Unfurled

Patent sails

Patent sails were invented by William Cubitt in 1813. They combine the shutters of the spring sail with the automatic adjustment of the roller reefing sail. Their construction is similar to that of the spring sail. Adjustment of patent sails can be made without stopping the mill.

Air brakes
In 1860, the English millwright Catchpole fitted an automatic air brake to the end patent sails. These were longitudinal shutters at the tip of each sail, which opened up if the wind got too strong, thus slowing the sail.

Image:Sarre mill.jpg|OpenImage:Maud Foster Windmill.JPG|Closed

Spring patent sails

Spring patent sails have a spring to enable each sail to be adjusted individually, with the patent sail system allowing all sails to be adjusted without stopping the mill. The system was not a common one.

Dutch sail types

In the Netherlandsmarker, the common sail predominates. During the nineteenth and twentieth centuries, Dutch millwrights developed the windmill sail to make it more efficient aerodynamically.

Dekker / Van Bussel system
The Dutch millwright A J Dekker improved on the design of the common sail by streamlining the leading edge. Dekkerised sails can work the mill with no sailcloth spread if the wind is strong enough. They are not adjustable except by adding more sailcloth as with a common sail. The millwright Van Bussel of Weertmarker produced a similar system, with an airfoil on the leading edge.

Ten Have / Van Riet system


Invented by Dutch millwright Ten Have of Vordenmarker, Ten Have sails have a number of longitudinal shutters, operated in a similar method to patent sails. The sail can be adjusted without stopping the mill. A similar system was invented by a millwright by the name of Van Riet of Goesmarker.

Fauël system


The Fauël system is similar in principle to Krueger flaps on an aircraft wing. It enables the mill to work in a lower windspeed. The Fauël system is used in addition to Common sails (see photo).

Fok system
The Fok system is similar to the Fauël system. (see photo).

Bilau system
The Bilau system uses sails with an aerofoil shape on the leading edge, coupled with an air brake on the trailing edge. They were invented by Germanmarker engineer Kurt Bilau early in the twentieth century. Photo of Bilau sail fully open

Image:Bredevoort 005.jpg|Ten Have sail open, Dekkerised Common sails.Image:Molen Venemansmolen Ten Have-klep.jpg|Ten Have sail closed.Image:Molen Venemansmolen Busselneus.jpg|Van Bussel system on leading edge.File:Molen het Hert Putten Fauelfokwiek.jpg|Close-up of Fauël systemFile:Molen d'Admiraal fokwiek.jpg|Close-up of Fok systemfile:Muehle donsbrüggen.JPG|Mill equipped with Bilau sails

Berton sails

In Francemarker some mills have a system with longitudinal shutters running the length of the sail. These sails can be adjusted without stopping the mill. The system is called Ailes Berton, which translates as Berton sails.

Image:Moulin de la batie.jpg|Berton sails closedImage:Moulin de la batie deploye.jpg.JPG|Berton sails open

Annular sails

Crux Easton wind engine
A few mills had annular sails, forming a circle. These sails utilised the patent system, enabling adjustment to be made without stopping the mill. One example of a mill with annular sails was at Feltwellmarker, Norfolk. Others are known to have existed at Haverhillmarker, Suffolk, Boxford, Suffolk and Roxwell, Essex. Annular sails were also employed on large wind engines, such as the Titt engine at Crux Easton, Hampshire.

Notes

Sails are colloquially known as sweeps in Kentmarker and Sussex, primarily due to the physical movement they perform and also that their construction does not necessarily involve sailcloth or canvas. Hence, patent sails are referred to as patent sweeps.

References

  1. Pronounced "Ten Halve"



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