Rule of Twelfths
Some tidal flows work chaotically. But the regular diurnal type - that's to say tides with two high waters and two low waters per day - can be estimated approximately
In areas where there are regular diurnal tides it's possible to estimate the rate at which the tide either floods or ebbs by applying the Rule of Twelfths.
The Rule states simply that the tide will:
Rise or fall one-twelfths of its total range in the first hour and thereafter it will...
...rise or fall two-twelfths in the second hour, and...
...rise or fall three-twelfths in the third hour, and ...
...rise or fall three-twelfths (again) in the fourth hour, and ...
...rise or fall two-twelfths in the fifth hour, and...
...rise or fall one-twelfth in the sixth hour.
The graph (below) shows how the rule looks ... well, graphically. It doesn't comply with the tidal curve we're most familiar with but it can be seen that it's a close enough approximation to provide a useful rule of thumb for navigators.
