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

Contents 1 Jib sails 2 Common sails 2.1 Dutch sail types 3 Spring sails 4 Roller reefing sails 5 Patent sails 5.1 Berton sails 6 Annular sails 7 Spring patent sails 8 References

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 one windmill is known to have had jib sails.^{[citation needed]}

Jib sails

More fully spread

St Mary's, Isles of Scilly

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.^[1]

Furled	First Reef (Medieval style sail)	Sword point	Dagger point
Full reef	Full reef (Medieval style sail)

Dutch sail types

In the Netherlands, 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 Weert produced a similar system, with an airfoil on the leading edge.^[1]

Ten Have / Van Riet system

Invented by Dutch millwright Ten Have^[2] of Vorden, 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 Goes.^[1]

Ten Have sail open, Dekkerised Common sails.

Ten Have sail closed.

Van Bussel system on leading edge.

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.^[1]

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 Yorkshire, although the only remaining mill with roller reefing sails intact is Ballycopeland Windmill in Northern Ireland.

Furled

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.^[1]

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.^[1]

Open

Closed

Berton sails

In France 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.

Berton sails closed

Berton sails open

Annular sails

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 Feltwell, Norfolk.^[3] Others are known to have existed at Haverhill, Suffolk,^[4] Boxford, Suffolk and Roxwell, Essex.^[5]

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.

References