De Onrust, Stelling van Amsterdam

De Onrust or Meermolen is an octagonal ground-sailer, also known as an octagonal windmill, near Muiderberg on the Boezemkade, dating from 1809. The mill was built as a polder mill to drain the Naardermeer. Nowadays it drains the Naardermeer, after the reclamation was undone. Since June 4, 1973 De Onrust has the status of a national monument.

The mill is 15 meters high and has a flight of 26.10/26.20 meters. The airfoil is old Dutch angled and has double radial sails with brake flaps. The gap, measured in the middle behind the rod is 15 cm at the shaft head and 12 cm at the tip.

The iron scoop wheel has a diameter of 5.6 meters. The mill has a drainage area of 680 hectares. The water is raised between 60 and 100 centimetres from the polder level of the Naardermeer to the storage basin level. From the windmill a canal, Uitwatering van het Naardermeer, runs to the west, which flows into the Utrechtse Vecht just south of Muiden.

The mill can also draw in water from the Vecht into the Naardermeer. The scoop wheel can be disconnected from the mechanism in the mill and then rotate freely with the incoming water. At the beginning of the driveway to the windmill there is a small purification installation. Water from the IJmeer is here stripped of phosphates and the nutrient-poor water is discharged into the Naardermeer. In the wet winter months the excess water is pumped out and in dry summers water is let into the Naardermeer.

A thorough restoration of the mill was completed in 1983. Ten years later, the sails were blown off in a severe storm and the gears in the hood became dislocated. All this damage has been repaired. For the Naardermeer, the mill is the only way to get rid of excess water. The scoop wheel moves about 10 m3 of water per revolution and in order to get a 1 centimeter drop in water level, the mill has to work for about 16 hours with a good wind.

The mill is owned by Vereniging Natuurmonumenten and volunteer millers keep the mill running. The mill is open to the public when the sails are turning or when the blue millwale is flying. On Sundays, the mill is always open.