The Rule of Twelfths
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Approximating the Sinusoidal nature of Tidal flows Example for a 6m tide
1st hour   1/12 of the tidal range 0.5 m
2nd hour a further 2/12 of the tidal range 1.0 m
3rd hour a further 3/12 of the tidal range 1.5 m
4th hour a further 3/12 of the tidal range 1.5 m
5th hour a further 2/12 of the tidal range 1.0 m
6th hour a final 1/12 of the tidal range 0.5 m

Half the total amount of water flows during the middle 2 hours of the 6-hour long tidal stream.