The tjasker (West Frisian: jasker, German: Fluttermühle) is a small type of windmill used solely for drainage purposes. It is distinctive for its simple construction, featuring only a single inclined shaft that carries the sails on one end and an Archimedes' screw on the other, in this way avoiding the need for any gearing. [1] The tjasker is commonly known as a typical Frisian windmill [2] though it is also found in other Dutch provinces and in north Germany.
The tjasker seems to have been developed somewhere towards the end of the 16th century, though there is no conclusive date for its invention. [2] The total number of tjaskers at their peak is also unclear. The mills were used to drain small plots of land but also found their use at peat digging sites as they could be easily moved. [3] Millwright R.W. Dijksma of Giethoorn was well known for his tjasker building. He constructed 400 to 500 tjaskers between 1910 and 1945, though these numbers include hand powered Archimedes' screws. [2] In the first half of the 20th century tjaskers almost completely disappeared as they couldn't compete with the metal windpump which was more practical as it could turn itself to face the wind and didn't require any supervision. The last tjasker was erected in 1935 and by 1963 there were only three left in the Netherlands. In that year emerging views on nature preservation and cultural history resulted in the installation of a brand new tjasker in De Weerribben. [2] Since then, several other new tjaskers have been built, to pump water into wetlands and as cultural monuments. Currently, there are 28 tjaskers in the Netherlands [4] and six tjaskers in Germany.
The tjasker has only one wooden millshaft on which all machinery is carried. This shaft is inclined at an angle of approximately 30° and supported in a wooden frame. On the lower end it carries the Archimedean screw which is partly submerged in water. The screw doesn't rotate in a pipe but has a fixed encasing which simplifies the design further. The top end of the shaft carries the Common sails with a span of approximately 5 to 6 meters. In some cases wooden slats are used instead of sail cloth. The mill can be stopped using a band brake on the brake disk which is fitted on the shaft behind the top bearing.
The entire mill has to be rotated to face the wind. To accomplish this two types of tjasker have developed. In the first type, known as paaltjasker (English: post tjasker), the mill is supported on and turned around a central post. The Archimedes' screw pumps water from the outer circular ditch to a central circular ditch or pool. The screw moves in a circle when the mill is turned so these circular ditches enable the mill to function in whatever direction it is pointing. Only the outlet from the central pool taking the raised water across the outer circular ditch prevent the use of the mill facing opposite that particular direction as the screw would collide with the water outlet.
The second type of tjasker, the boktjasker (English: trestle tjasker), is not balanced on a central post but supported on a trestle. The lower submerged support is fixed, the mill is rotated around this point, while the other end is movable. Here the trestle is commonly equipped with rollers or wheels on a fixed ring to make winding easier. In this method of construction the Archimedes' screw takes water from a central pool and deposits it in a circular ditch that surrounds the central pool. Water can be taken to the central pool by a pipe or a gap in the circular ditch. Besides the structural differences, a typical paaltjasker is designed so it can relatively easily be dismantled to be moved to another location or stored indoors though nowadays this feature is rarely used. A boktjasker is more or less fixed to its location.
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A third type of tjasker has been rediscovered from old drawings. It has a tail keeping the mill facing in the wind and a scoop wheel instead of an Archimedes' screw. After having built a functional model the reconstruction of a full sized staarttjasker (English: tail tjasker) is now planned. [5]
A windmill is a structure that converts wind power into rotational energy by means of vanes called sails or blades, specifically to mill grain (gristmills), but the term is also extended to windpumps, wind turbines and other applications. The term wind engine is sometimes used to describe such devices.
The post mill is the earliest type of European windmill. Its defining feature is that the whole body of the mill that houses the machinery is mounted on a single vertical post, around which it can be turned to bring the sails into the wind. All post mills have an arm projecting from them on the side opposite the sails and reaching down to near ground level. With some, as at Saxtead Green, the arm carries a fantail to turn the mill automatically. With the others the arm serves to rotate the mill into the wind by hand.
The Cornwerdermolen is a smock mill in Cornwerd, Friesland, Netherlands which has been restored to working order. The mill is listed as Rijksmonument number 39329.
De Mearmin, formerly also known as Geestermermeermolen is a smock mill in Damwâld, Friesland, Netherlands which was built in 1968 at Dokkum. The mill is listed as a Rijksmonument, number 467708. Between 2014 and 2016, the mill was dismantled, restored and moved to a new site in Damwâld.
Kramersmolen is a hollow post mill in Goutum, Friesland, Netherlands which was built in 2002. The mill is listed as a Rijksmonument, number 24508.
The Hegebeintumer Mûne is a smock mill in Hegebeintum, Friesland, Netherlands which was built in 1860. The mill has been restored to working order. It is listed as a Rijksmonument, number 15629.
De Huinsermolen is a smock mill in Húns, Friesland, Netherlands which was built in 1829. The mill has been restored to working order. It is listed as a Rijksmonument, number 8530.
De Edensermolen is a smock mill in Iens, Friesland, Netherlands which was built in 1847. The mill has been restored to working order. It is listed as a Rijksmonument, number 21575.
De Volharding is a smock mill in Jislum, Friesland, Netherlands which was built in 1872. The mill has been restored to working order and designated as being held in reserve in times of emergency. It is listed as a Rijksmonument, number 15632.
De Eendracht is a smock mill in Kimswerd, Friesland, Netherlands which was built in 1872. The mill has been restored to working order. The mill is officially designated as being held in reserve for use in times of emergency. It is listed as a Rijksmonument, number 39358.
De Bullemolen English: The Executioner's Mill is a smock mill in Lekkum, Friesland, Netherlands which was built in 1825. The mill has been restored to working order and is listed as a Rijksmonument, number 24505.
Wijnsermolen is a smock mill in Wyns, Friesland, Netherlands which is currently (2011) being restored to working order. The mill is listed as a Rijksmonument, number 35691.
The Victor is a drainage mill in Wanswerd, Friesland, Netherlands. The mill is listed as a Rijksmonument, number 15644.
The Terpensmole is a drainage mill in IJlst, Friesland, Netherlands. It was moved from its earlier location in Sneek in 2011 where it was known as the Himmole. The mill is listed as a Rijksmonument, number 22914. It is fully functional and can be used to help drain the adjacent polder.
Terpzigt is a drainage mill in Marssum, Friesland, Netherlands. It is the smallest spinnenkop, a type of hollow post windmill. The mill is listed as a Rijksmonument, number 8625.
The Geeuwpoldermolen is a drainage mill near the twin village of Oppenhuizen, Friesland, Netherlands. It is a hollow post windmill of the type called spinnenkop by the Dutch. The mill is listed as a Rijksmonument, number 39810 and it is still in use for draining the Geeuwpolder. It was almost completely renewed at a restoration in 1987, earlier restorations were in 1954 and 1965.
The Himriksmole, also known as Groene Ster after the recreational area and nature reserve where it is located, is a drainage mill near the village of Tytsjerk, Friesland, Netherlands. It is a hollow post windmill of the type called spinnenkop by the Dutch. The mill is listed as a Rijksmonument, number 35675, and is used to raise the water level in the nature reserve.
Teetlum, after a nearby terp, also known as Duivenhok 'dove coat' named after the polder it drained, is a drainage mill near the village of Tzum, Friesland, Netherlands. It is a hollow post windmill of the type called spinnenkop by the Dutch. The mill is listed as a Rijksmonument, number 15877 and can be used to drain the adjacent polder.
De Hoop is a smock mill in Readtsjerk, Friesland, Netherlands which was built in 1911. The mill has been restored to working order. It is listed as a Rijksmonument.
De Snip is a smock mill in Workum, Friesland, Netherlands. It has been restored to working order. Designated as being held in reserve, it is listed as a Rijksmonument.
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