A mariner's shortcut that splits the range into twelfths and assumes the tide moves 1, 2, 3, 3, 2 and 1 twelfths in successive hours between low and high water. It reflects the roughly sinusoidal shape of a regular semidiurnal tide, with the fastest change mid-tide. It breaks down in mixed, diurnal and shallow-water tides, and anywhere with a long stand or double high water.
Range is 4.8 on this tide, so by the rule of twelfths we'll get 1.2 more in the third hour alone.
In mixed-tide waters the rule can be badly wrong; use the tide curve or a prediction instead.
Also heard as
- twelfths rule
- 1-2-3-3-2-1
Related terms
Semidiurnal tide
A tide with two high waters and two low waters of similar height in each lunar day.
Tidal range
The vertical difference between a high water and the neighbouring low water.
Stand
The pause at the top or bottom of the tide, when the level hangs nearly still before turning.
Agger
A double high water or double low water, where the tide peaks or bottoms out twice in quick succession.