Windmill Sail - Types of Windmill Sails

Common sails – The most commonly used on European windmills, which could be stringed over sailbars in several different configuration that can catch the precise amount of wind force so that windmill is operating at peak of its efficiency. Those different sail configuration can be Furled, Sword Point, Dagger point, First Reef, and Full Sail.
Jib sails – Very simple type of sail that consists from simple wooden frame and sailcloth in the form of triangle. This triangle can be made in several distinct sizes, and changing of the cloth (as with many other sail types) is possible only when the windmill is stopped. This type of sail was most commonly used in Mediterranean.
Spring sails – Spring sails utilize the concept of dividing of sails into number of bays, which can be opened and closed by shutters. Shutters can be joined together into bars or other forms, with each individual bar having his own set of springs that can cause shutter to open if the wind is too strong. This way, windmill operator can create sail that can automatically reduce the surface of the sail if the wind suddenly becomes too strong. To set springs and shutters in original position, sail needs to be stopped rotating.

Roller reefing sails – Similar in function as Spring sails, except here instead of individual springs that control the open/close state of individual shutters, here one central rod control number of spars with cloth wound around them, enabling windmill operators to control the size of sails while windmill is rotating.
Patent sails – Windmill sail type that combines the technology of both Spring Sails and Roller reefing sails.
Dutch sails – Dutch windmills have become famous for numerous types of sails. Most famous ones are Dekker sails (steel plated sails of airfoil shape that extracted more force from wind), Beckers sails (which used series of centrifugal weights that could manipulate size of the sails on the fly), Fok system (adjustable ends of sails that can increase of decrease their efficiency), Van Riet system and several others.
Berton sails – French-made sail type that utilizes system of longitudinal shutters running the length of the sail that can be adjusted without stopping the windmill.
Annular sails – Windmill that uses array of sails that forms full circle.