Calculating the height of the tide between High and Low water.
The Rule of Twelfths is a rule of thumb method of estimating the height of the tide at any time between high and low water, given only the TIME and HEIGHT of HIGH and LOW water.
This is important when navigating a boat or a ship in shallow water and when launching and retrieving boats on slipways on a tidal shore. The rule assumes that the rate of flow of a tide increases smoothly to a maximum halfway between high and low tide before smoothly decreasing to zero again and that the interval between low and high tides is approximately six hours. The rule states that in the first hour after low tide the water level will rise by one twelfth of the range, in the second hour two twelfths, and so on according to the sequence 1:2:3:3:2:1.
You can use the tide height calculator (below) to give you an indication of the height of the tide at certain times in the tidal phase.
The rule is a method for giving a quick, rough approximation only and should be used with great caution when entering or leaving port.
The tide tables themselves are predictions based on previous experience and the figures for times and heights can vary according to weather conditions and atmospheric pressure.
Navigating safely when entering or leaving port should be an accumulation of input from as many sources as possible, particularly input from your very own eyeballs!
Tide Height Calculator
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