Windmill sail

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

The jib sail is found in Mediterranean countries and consists of a simple triangle of cloth wound round a spar. The mill must be stopped in order to adjust the reefing of the sail. Though 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).

Jib sails

More fully spread

St Mary's, Isles of Scilly

Cann Mills, Melbury Abbas

The common sail is 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 constructed with or without outer sailbars. Post-medieval mill sails have a lattice framework over which the sailcloth is spread. There are various "reefs" for the different spread of sails; these are full sail, dagger point, sword point and first reef. The mill must be stopped in order to adjust the reefing of the sail.

Furled

First reef (Medieval style sail)

Sword point

Dagger point

Full sail

Full sail (Medieval style sail)

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

Open

Roller reefing sails were invented by Stephen Hooper in 1789. As with spring sails, the sail is divided into a number of bays, each with a number of spars with cloth wound around them. 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 were invented by William Cubitt in 1807. They combine the shutters of the spring sail with 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.

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Wikipedia