Horizontal Tide Mill at Nendrum

The process

Sample of grain recovered from a trench:

So how did it work then?

The upper part of a rising tide filled into the millpond through an open sluice gate.
Then the sluice gate was closed and the tide level receded until it was below the level of the paddlewheel.
The orifice was opened so that its jet impinged on the top of the paddle wheel, turning it and its vertical drive shaft.The shaft passed through the lower millstone which was seated on the floor of the grinding room, above high tide level.
The shaft supported and turned the upper stone, to which it was connected by a metal cross-piece.There was no cog gearing.
The fine gap between the stones was adjusted by raising or lowering the whole shaft and its lower bearing.
The water falling from the wheel flowed away to the sea through the tail race channel.
The sketch below illustrates the principle:

The bed of the mill pond is not clearly defined, but calculatons show that there would have been no gain in having the bed below +0.9 OD.
The key figures are as follows:
Surface area of mill pond 2,000 Square metres
Pond bed, assumed level +1.00 Ordinance Datum
Level of the orifice - 0.12 OD
Spring tide, MHWS +1.90 OD
Neap tide, MHWN +1.40 OD
Exit channel mud, modern - 0.08 OD
Tail race floor - 0.68 OD

It is apparent that the present mud level, downstream of the tail race exit, must have been lower by about a metre when the mill was in use. In confirmation of this, probing has located a hard bottom approximately one metre below the present surface.
A reasonable estimate for power at the millstone is seven eighths of a horse power at the start of discharge from a spring tide which reduces to less than half a horse power as the mill pond is emptied.Further study of the paddles might allow this estimate to be refined.
A much harder problem to solve is how much grain the mill might have processed.To begin with, the horsepower estimate quoted above depends upon the grain feed being maintained at the rate necessary to achieve the optimum rate of rotation to extract the most power.For what proportion of the shift time would this have happened?Then, how efficiently were all the available tides retained in the pond?Again, would all or any of the night time shifts have been worked?Much depends upon how the mill was operated, but taking into account the number of useful tides in the monthly cycle and other known factors and taking an optimistic view of others, there could have been sufficient power to mill about one tonne of grain in a month, to yield a fairly coarse meal.It is possible however, that such an output was never either achieved or approached.

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Early Irish Christian monastic settlement Nendrum Mahee Island