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Greek - Roman Water Power #4

PostPosted: Tue Sep 27, 2016 7:51 pm
by Marcus Minucius Audens
>>>> Greek - Roman Water Power #4 <<<<

>>> Overshot Wheel <<<

The third wheel in consideration is the overshot-wheel, which is the most efficient of the three types. Now, in the discussion of the creation of this type of water-wheel, it is normally supposed that the development came from the undershot-wheel, and a middle stage which was a “breastshot-wheel” where the water supply was brought in by a sluice, which was placed the end of the water-flow, at the level of the wheel axle. However, a second view, is that the whole idea of the overshot-wheel was developed with the simple action of reversing the bucket-wheel. A simple determination on the part of the user would probably be, that if one could put power into a machine and get water out of the top, why not put water in the top of th machine and get power out of it? Support for this possibility, is that we know that the bucket-wheel was in use during Vitruvius’ time and probably before.

Although the time and idea of creation for the overshot-wheel may well be in question, the fact is, that it is much more efficient and has a greater power output than the other types. Compared to the undershot-wheel, the overshot-wheel can be made 65%, or even 70%, more efficient than the undershot-wheel. This necessitates that the overshot-wheel revolvea fast enough and the boxes are large enough to carry all the water, as it comes from the source, the potential energy can easily be worked out from the rate of water-flow and the depth of the fall.

However the reaction of the overshot-wheel compared to the reaction of the undershot-wheel under an increased load is quite different. In the overshot-wheel operation, any slowing of the wheel's minimum rotation speed, will cause the overflow of water flowing into the boxes and water spillage into the pit below the wheel, thus suffering a reduction in both speed, efficiency, and power output. In cost, the overshot wheel is quite costly, requiring the construction of a pit below the wheel and a run-off channel. In regard to the water source, if the ground gradient is fairly shallow, the cost of raising the water level above the top of the water wheel, and waterproofing it ,can be quite costly. In the case of an aqueduct of 200-300 yards in length, to supply water, can also be costly.

In the case of the undershot wheel, any river or stream could be used and the flow of water was free even though the efficiency and power derived would be somewhat less. So the choice between the utilization of either an undershot or an overshot wheel would factually depend upon the source of water. If it were a a choice between an undershot-wheel which would just turn two millstones, or an overshot-wheel which would turn them faster and cost four to five times as much, and might have to be miles some distance away, the undershot was the preferable choice. Where the water source was too small for anything but an overshot-wheel, there was no choice.


>> J. G. Landels, “Engineering In the Ancient World,” (Univ. of Calif. Press, Berkeley, 1981);
>> Henry Hodges, J. Newcomer (Dwgs.), “ Technology In the Ancient World,” (Alfred A. Knopf, N.Y., 1970).

Respectfully Submitted;
Marcus Audens