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The Rule of Twelfths is a handy rule of thumb for estimating tidal heights. You will only need to know the Time and Heights of High and Low Water. It assumes a six hour period from Low Water to High Water and that the rise starts slowly, increases to maximum in the middle two hours and then decreases to nil at High Water.
The example shown here is within about 20cms of the answer derived from a tidal curve. For an explanation of Tidal Curves Click here.
This is a rough and ready system and should always be used with a certain amount of caution. It will work well where the curve is regular, such as the Plymouth, which is shown elsewhere on this site. It will not work at all in places like Poole or The Solent where the tidal curves are far from regular.
As with all other tidal calculations it is never exact as influences such as barometric pressure and the weather can all effect the actual tidal heights and times.
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