Windmill Sail - Dutch Sail Types

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 and operation easier in an effort to keep the traditional windmill economically viable in competition with factories and electric pumping stations.

Dekker / Van Bussel system

The Dutch millwright A. J. Dekker improved on the efficiency of the common sail. The stock is given an airfoil shape by completely covering it with galvanised steel plates. Dekkerised sails provide enough surface area to be able to work the mill with no sailcloth spread if the wind is strong enough. As with the common sail, they are not adjustable except by adding more sailcloth. Some disadvantages of the Dekker system led millwright Van Bussel of Weert to invent a similar system, though with just a more rounded airfoil replacing the leading boards and not covering the stock itself.

Ten Have

Invented by Dutch millwright Ten Have of Vorden, Ten Have sails have a small number of wide longitudinal shutters, operating by centrifugal weights and often also by the miller at the tail of the mill similar to patent sails. This way the sail can be adjusted without stopping the mill. The leading edge is commonly streamlined by the Van Bussel system. Ten Have shutters are normally only used on one stock as the wide Ten Have shutters standing open on a vertical stock would leave this stock vulnerable to side winds during a storm.

Fauël or Fok system

The Fok system, invented by engineer P.L. Fauël, was inspired by the jib on a sailing boat ('fok' is the Dutch word for jib). In this design the leading boards are replaced by a rounded profile of wooden slats in the form of a foresail leaving a small slot between this profile and the stock. Its working principle can be compared to a leading edge slot on an aircraft wing. It enables the mill to work in a lower windspeed but in variable windspeeds tends to make it difficult to have the mill run at a steady pace. For this reason it is often equipped with air brakes operating by centrifugal force. The Fauël system is used in addition to common sails (see photo).

Bilau Ventisails/ Van Riet system

The Bilau system uses sails with a streamline covering of the stock, coupled with a full length air brake on the trailing edge, together forming an airfoil. The airbrake is opened by centrifugal weights in the sails but can also be operated by the miller similar to the patent system. When opening the airbrake disturbs the airfoil shape thereby slowing the mill. It was invented by German airplane engineer Kurt Bilau early in the twentieth century and became quite popular in Germany where it was fitted to over 140 mills. A similar system was invented by a millwright by the name of Van Riet of Goes where the leading edge and the airbrake together form a more complete airfoil.

• Ten Have sails open on one stock, common sails on the other, both with van Bussel system.

• Dekkerised sails with air brake and patent system

• Ten Have sail closed with Van Bussel streamline leading edge.

• Close-up of Fok system with air brake

• Close-up of Fok system with spread sailcloth

• Mill equipped with Bilau sails