Definition of a Tide Mill
Perhaps the following quotation sums up most succinctly the essence of a tide mill.
“A tide mill is quite simply a water mill that derives its power from the rise and fall of the tides. It is almost never referred to as a "seawater" or "saltwater" mill because the chemical composition of the water driving the mill wheel is not important. What counts is that the water impounded behind a mill dam can only be put to work after the water level outside of the dam has sufficiently dropped during the ebb tide.” [The Tinkham Brothers’ Tide-Mill by J.T. Trowbridge. Edited and with Commentary by Richard A. Duffy. Arlington, MA: Arlington Historical Society, 1999]
The following sources offer other descriptions:
The following description comes from the exhibit brochure moinhos de maré do ociente europeu 2005:
“A tide mill comprises the building which houses the milling machinery, the mill-pond, where the tidal water is retained, and the dam or causeway which confine and controls it.
Sea water enters the pond through large sluices or “sea gates” which open under pressure from the rising tide, and close automatically after high water. By opening the internal sluice gate, the pressure of the water from the mill pond can be controlled as it flows through one or more narrow channels before hitting the paddle blades and setting the water wheel and the milling machinery in motion.”
These illustrations come from a brochure about the Eling Mill in Hampshire, England.
“… Whatever the dam, pond and waterwheel configuration of any particular mill complex, the means of obtaining and releasing water was fairly universal. The power of the incoming tide would force open one or more pairs of tide gates (or valve flaps) to admit the flow of water into the millpond. These gates would automatically close by natural force of the water current turning at ebb tide. This impounded both the tidewater and any incoming fresh-water from behind. As mentioned earlier, once the tide dropped below the level of the water inside the dam, the trapped water could be released through a sluice to fall on the wheel and set it in motion. …
“The most efficient operation of the mill occurred when the tide fell to a point below the level of the entire waterwheel, allowing it to "run clear." The wheel would continue to turn until either the water behind the dam fell below the level of the sluice, or more frequently, until the water level in front of the dam rose above the sluice at high tide.
“Even in coastal areas with freshwater falls nearby, tide mills offered the unique advantage of a water supply that was entirely dependable. They were typically free from the risks of drought or upstream diversions into manmade reservoirs, canals or irrigation flow of water into ditches. Compared to windmills, tide mills had the obvious advantage of not depending on the strength or direction of the wind on a given day. Indeed, too-strong winds were a hazard to the sails of windmills. They were often cheaper to locate and build than water mills because no dam was needed; however, they were usually more expensive to keep in good working order over the long term.
“Of course, tide mills had the major disadvantage that the tides, while predictable, occurred at different times of the day. Humans naturally follow the sun to determine their activities, but tide millers worked according to a tidal calendar chiefly determined by the moon. The lunar day being 24 hours and 50 minutes long placed the tide miller among the earliest categories of rotating-shift workers. At harvest and other peak times, all-night duties were common. A tide miller would need to split his "full-night's sleep" into nap periods during the twice-daily incoming tides.” [The Tinkham Brothers’ Tide-Mill by J.T. Trowbridge. Edited and with Commentary by Richard A. Duffy. Arlington, MA: Arlington Historical Society, 1999]
The rising tide flows through the gates into the mill pond. When the tide turns and begins to flow out, the gates close automatically, trapping a large volume of water that may be used to power a water wheel.
Charlier and Finkl:
“A tide mill is part of a unit that comprises the mill proper, outbuildings, a dyke and a pond. The workings of a classic mill are simple. At high tide seawater flows into a pond, protected by a dyke, through a sluice gate which closes automatically, under pressure from the water accumulated in the pond, as the tide begins to withdraw. The water flows out of the pond through one or several narrower gates and, in so doing, turns the hydraulic wheels, which may be overshot, undershot or midshot. The energy provided by tides, though intermittent, is regular and inexhaustible because it is constantly renewed. A mill functions from three hours before to three hours after low tide. Since wind can influence the speed of the incoming tide there could be marginal differences in the times, but these rarely vary by more than half an hour each way. Thus, the miller, who had to wait for the ebb tide before he could set his grinding stones in motion, had to adapt his working hours to the rhythm of the cycles of the tide, though on average he could expect to work a total of 12.4 h in every 24.” [Roger H. Charlier and Charles W. Finkl. Ocean Energy: Tide and Tidal Power. Heidelberg, 2009. p. 39-40.]
Tide mills could be constructed in many ways. The earliest may have been merely anchored or moored boat mills. When the tide came in, their wheels would be pushed in one direction; when the tide went out, in the other. But the form of the tide mill that came to most widely applied was more complex. It required impounding the water of incoming tides by means of dams and gates placed across the mouth of a river, creek, or bay. The gates would be open to an incoming tide, but closed when the tide went out, creating large tidal reservoirs. The water from these reservoirs would then be released through a mill race onto the blades of a water wheel. Tide mills were not as convenient as conventional watermills. They could be used only six to ten hours a day, and the hours when they could be operated varied from day to day as the tides changed. But where conventional mills could not be used, they were applied, and they did have the advantage of never freezing over. [Stronger than a Hundred Men: A History of the Vertical Water Wheel by Terry S. Reynolds Baltimore: The Johns Hopkins University Press, 1983.]
John Morse’s father of Bath, Maine, whose family owned a tidal mill used for sawing lumber at Winnegance said that at his mill they used to saw “small stuff” like light pine boards or cedar at the beginning of the tide, when the head of water was not much, and then they would save the thicker beams or oak to saw later in the ebb tide cycle when there was a goodly head of water. [John Morse statement]
I was born in a one-story building of wood on the southwestern side of Mill Street in Dorchester, (now Boston), Massachusetts, on the fifth day of June, 1838. On the opposite side of the street was a grist mill of the old style, owned by Ebenezer Tileston and my father. … This was an ancient privilege, granted by the Massachusetts Colony in 1645; the grantee and his successors were authorized to construct and maintain a dam across an arm of the sea from Massachusetts Bay. The power was furnished by the incoming tide which flowed into the pond and closed the gate when it receded. My father was the miller and worked in the night or daytime according to the ebb and flow of the tide. [Memoirs of a Many-Sided Man: The Personal Record of a Civil War Veteran, Montana Territorial Editor, Attorney, Jurist. Memoirs of Henry N. Blake written in 1916. Edited by Vivian A. Paladin and published in Montana, the Magazine of Western History, Autumn, 1964.]
“Hayfords came to Hudson Wisconsin before the rebellion [Civil War] then moved to Minnesota. Politically all the Hayfords were Republicans. Asa Hayford and Ed returned to Maine many years ago. Asa doesn’t know the town there. However, he has heard Ed speak many times of a tide mill which they operated on the coast of Maine, The rising of the tide pushed the lock of their gates inward and filled the pond. When the tide went down, the gates closed, and with the water they obtained the power to operate a saw mill, shingle mill and lath hill. He had heard Ed complain many times they had of the hard life they led at this work, for the tide going out later each day, their time for opening work often commenced at most untimely hours, and as they worked in all seasons, they often commenced work at eleven o’clock at night or two in the morning with equal promptness, as every moment that their power lasted must be improved.” [History of the Hayford Family. 1100-1900. By Otis Hayford. Canton, ME, 1901. p. 87]
“The time of working will be limited. … Since the time of high tides moves each day, the times of working are somewhat complicatedfor the miller. No two days together will be the same for him and depending on the type of mill and the amount of work he has on hand, he may have to star or finish his work at the oddest hour. At Birdham in Sussex, work normally started 3 ½ hours after high tide and continued for 5 ½ hours, work being limited to two periods of this length per day. This mill had three pairs of wheat stones and one pair for oats, and was working until 1935. … Eventually it became possible at some tide mills to work ‘round the clock’. This was done by having either two wheels or two ‘gates’ to one wheel. When the mill pond was at its fullest and the force owater greatest, the wheel could work as a breast wheel. As the level of the water fell, the flow of water could be switched to an undershot wheel or alternatively, by opening a second sluice or gate at the bottom, the flow water could be lowered to operate the same wheel as an undershot. [British Water-Mills by Leslie Syson. London, 1965.
The working cycle of Mill 2 at Nendrum (in chapter 8 of Harnessing the Tides).
At high tide, when the millpond had filled, the intake sluice gate and penstock sluice gate were closed.
Waiting for tide to fall below wheel.
When the water level in the wheelhouse and tailrace had fallen below the water wheel (about four hours later), the penstock sluice gate was opened, allowing the jet to commence driving the waterwheel and continue until the rising tide in the wheelhouse reached the wheel once more. This working period would, again, have been about four hours.
Waiting, with the millpond intake sluice gate open, until the rising tide had filled the millpond again.