Topcliffe Mill Wheel
Topcliffe Mill is believed to have been powered by both undershot and overshot wheels during its life as a working mill.
An undershot wheel (figure 1) is the simplest type of waterwheel and is wholly reliant on the energy of flowing water. It consists of a paddle wheel that is placed so that it is about a quarter submerged in the river. The wheel is turned by the force of the water current against the submerged paddles. During the dry season, the level of the water falls and the river flows more slowly, supplying less power to the water wheel.
An overshot wheel (figure 2) is more efficient because it harnesses the force of gravity as well as the water energy. An overshot wheel is positioned so that the water flows onto the top of the wheel. Futhermore in a given location, an overshot wheel is smaller and less costly than an undershot wheel.
The first detailed experiments to test the relative efficiency of undershot and overshot waterwheels were carried out by John Smeaton in 1754.
Topcliffe Mill Wheel
The most recent water wheel was removed in 1942 and unfortunately there are no known photographs of the wheel.
Mill Wheel Type
Examination of the wear on the cogs shows that the wheel was overshot when last used. The overshot feed pentrough (see the photograph to the right) also remains in place.
Mill Wheel Dimensions
The vertical distance from the feed pentrough to the lower water level (at the existing tail race) is 2.38m. The inner width of the feed pentrough is 2.05m. A reasonable estimate of the most recent wheel dimensions is 2.2m diameter by 2.2m wide.
There is an Environment Agency gauging station immediately upstream of the Mill. EA figures for the period 1/1/1981 to 1/3/2010 show an average flow of 0.124m3/s = 124kg/s.
Power = Energy lost per second = mgh/t
Using m/t = 124kg/s, g = 9.81N/kg and h = 2.2m (the wheel diameter),
Power = 124 x 9.81 x 2.2 = 2676W = 2.676kW
For grinding grain, this is reduced by the efficiency of the wheel (estimate 80%), the efficiency of the machinery (cogs, bearings etc – estimate 30%) giving 0.64kW mechanical power at the grinding stones.
For generating electricity using the same wheel (estimated efficiency 80%) and using a modern and efficient gearbox (estimated efficiency 80%) and generator (estimated efficiency 80%) this suggests around 1.37kW electrical power could be generated.