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When I did my coxswains ticket 20 or so years ago as part of the learning curve I was introduced to “the rule of twelfths”.

Tide charts tell us high and low tide times and water levels and for those in the know the rule of twelfths provides a pretty good estimate of what the water level will be progressively through the run of the tide. The tidal run is influenced by a number of factors including the geography of the area so whilst is a good guideline for both falling and rising tides for falling tides in shallow water it's a good idea to always allow for a safety margin.

The difference between the high and low tide or low and high tide level is the tidal range and the rise or fall of the tide is known as the tidal run. The rule of twealths provides a formula that calculates an approximation of what the water level will be at six times through the tidal run.

This is basically how the rule of twelfths works:

To know the "twelfth" of its tide, we take the tidal range and divide it by 12.

Example :

High Tide at 0924 is 3.73m
Low tide at 1526 is 0.69m

Therefor the tidal range is 3.04m and that divided by 12 is 0.25m per 12th

Hour 1 tide fall 1/12 of 3.04m = 0.25m
Hour 2 tide fall 2/12 of 3.04m = 0.50m
Hour 3 tide fall 3/12 of 3.04m = 0.75m
Hour 4 tide fall 3/12 of 3.04m = 0.75m
Hour 5 tide fall 2/12 of 3.04m = 0.50m
Hour 6 tide fall 1/12 of 3.04m = 0.25m

So the rule of 12ths estimate for the tide depth over say 4 hours is
3.73m less 0.25m + 0.50m +0.75m i.e. 1.5m = 1.54m

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