Types of Waterwheel.

Before dealing briefly with the history of the four rivers as sources of power, it may be of value to show how the power was obtained. The earliest type of Saxon waterwheel was the paddle, set either horizontally, or vertically in the stream. Practicability and ease of maintenance caused the vertical paddlewheel to become usual. (It is interesting to note that the most modern waterwheel, the turbine is set horizontally.)

A stout frame of timber supported the paddle beside the stream, the lower third being in the water. Too much water or too little being usual on so many of our rivers, weirs were built to regularise the flow; an earth dam strengthened by stones and timbers, a sluice on the wheel side to deliver the pounded water, and a floodgate to release excess water, were early improvements, as was the building of a house over the millworks. This not only kept grain and flour dry, and helped sustain the vibrations of the wheel, but made a home for that unpopular man, the miller.

For good reasons it became usual on all but the smallest streams to build the mill on a side channel cut from the main stream above the weir, although in some cases it is the river itself which is diverted through sluices into a side channel.

In each case the pounded water rushes down a ramp to strike the lower paddles of the wheel; it is shot under the wheel, hence the name 'Undershot' for the paddle-type wheel.

The water's momentum turns the wheel, and a plentiful supply is needed. The Undershot Wheel is no more than fifty per cent efficient at best, but this is unimportant in a simple grist mill having abundant water.

Another type of wheel, only less ancient than the paddle, is the Overshot Wheel, which employs not the force of water but its weight. Water falls from above into so-called 'buckets' on the farther side of the vertical axis, the resulting imbalance causing the wheel to turn in the direction of flow.

Water remains in the buckets for about a third of a revolution, and it is claimed that the Overshot Wheel can be as much as 90 per cent efficient. Relatively little water is required, but there must be a good fall or 'head', greater than the wheel's diameter. In hilly country this presents no problem - a stream can be diverted over a cliff edge beneath which a wheel has been placed, and only a short timber flume is needed to being the water over the wheel's axis.

The Overshot Wheel is sparing of water, but produces relatively little power; to obtain adequate drive, the wheel must be large - 12, 15 even 30 feet in diameter. But this necessarily implies that the 'head' be greater still : if 13 feet of 'head' be required, the water must be able to fall 13 feet and continue to fall so that it does not linger at the bottom of the wheel-chamber, thus slowing or stopping the wheel. As much as 15 feet of fall may be needed, most of it concentrated at one point, the wheel. How to obtain this on river whose gradient is less than 15 feet in a mile ?

If, lacking sufficient water for a paddle wheel, a millwright sought to install an Overshot type, he was obliged to concentrate his fall by the use of a long millrace. This was cut from the river well up-stream and practically followed a contour, having only the smallest gradient, so that at the mill-site the river was well below and far enough away to permit the making of a pool which the race fed.

The pool was not dug into the meadow - often the gravel terrace - but was banked up from it, so that the hard-won 'head' was retained.

The mill was built against the end dam of the pool, and water flowed from the bottom of the pool down a short flume to the wheel. Beyond the wheel-chamber, the water might well be actually blow river level, but the tail-race maintained the lesser gradient and was at length able to discharge into the river at a downstream level.

Considering now the gradients of local rivers, and taking only the central reaches of each, where most mills have been concentrated. the following figures are obtained : -

TAME 13 feet lessening to 6 feet in a mile
BLYTHE 16 feet lessening to 5 feet in a mile
COLE 10.5 feet average
REA 26 feet lessening to 10 feet in a mile

It is at once clear that except on the upper Rea a 12-foot Overshot Wheel would require races between one and two miles in length, which would be impracticable : such wheels of any great size could not therefore have been employed. It was possible to compensate for small diameter by increasing width, as was done at Edgbaston, but until the later eighteenth century at least the Undershot Wheel was probably the normal type of installation.

It is certain that whether small Overshot or wasteful Undershot Wheels were in use, they were unsatisfactory, either because of inadequate power or lack of water, and since water power was in greatest demand, and the number of mills sited along every stream was at peak, in the latter half of the eighteenth century, it is not surprising that the needed innovations appeared at that time.

John Smeaton conducted a number of experiments into the design of both windmills and watermills, and his paper on these won him the Royal Society's medal in 1759. He made windmill sails more efficient, and improved both the Overshot and Breast Wheels. The latter term was in use in Tudor times, and referred to a wheel whose buckets received water at breast height, below the vertical axis, on the side nearest the inflow, so that the wheel turned against the flow.

Smeaton's development of this type was of particular value on our rivers, since a smaller 'head' was required ; it was less efficient than the Overshot, because water remained in the buckets for only 1/4 revolution, and still demanded long races, but there was a considerable saving, as the diagram shows. Most of the few mill-buildings that survive are eighteenth century rebuildings and several have Breast Wheel installations ; they were economical of water, produced adequate power, and required not impossibly long leats.