The Rule of Twelfths is a rule of thumb for estimating the height of the tide at any given time. The rate of flow in a tide increases smoothly to a maximum halfway point between high and low tide, before smoothly decreasing to zero again.
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.
If a tide table gave us the information that tomorrow’s low water would be at noon and that the water level at this time would be two meters above chart datum and further, that at the
following high tide the water level would be 14 meters. We could work out
the height of water at 3 p.m. as follows:
* The total increase in water level between low and high tide would
be: 14 – 2 = 12 meters.
* In the first hour the water level would rise by 1 twelfth of the
total (12 m) or: 1 m
* In the second hour the water level would rise by another 2 twelfths
of the total (12 m) or: 2 m
* In the third hour the water level would rise by another 3 twelfths
of the total (12 m) or: 3 m
* This gives us the increase in the water level by 3:00 p.m. as 6
This represents only the increase – the total depth of the water (relative to chart datum) will include the 2 m depth at low tide: 6 m + 2 m = 8 meters. Obviously the calculation can be simplified by adding twelfths together and
reducing the fraction beforehand.
However, there is warning about when using the rule. The rule is a rough approximation only and should be applied with great caution when used for navigational purposes. Officially produced tide tables should be used in preference whenever possible.
The rule assumes that all tides behave in a regular manner, this is not true
of some geographical locations, such as Poole Harbour or the Solent where there are “double” high waters or Weymouth Bay where there is a double low water.
The rule assumes that the period between high and low tides is six hours but
this is an underestimate and can vary anyway.