Rule of 12ths, Rule of twelths, Twelth's Rule12TH RULE of Tides (Not for Navigation) 1:2:3:3:2:1
The height of the tide will change approximately at the following rates between HW-LW and LW-HW in a six hour period :
Hour 1: One Twelfth of the tidal range.(= 1/12)
Hour 2: Two Twelfths of the tidal range.(= 3/12)
Hour 3: Three Twelfths of the tidal range.(= 6/12)
Hour 4: Three Twelfths of the tidal range.(= 9/12)
Hour 5: Two Twelfths of the tidal range.(= 11/12)
Hour 6: One Twelfth of the tidal range.(= 12/12)
.......and then six hours of the reverse as the tide turns.
The Twelfths Rule is an approximate rule of thumb for calculating the height of the tide, and the flow of the tide, between Low and High water, when you know the tidal range.
There are many factors that can affect the height of the tide, including the shape of the surrounding land, the rise and set of the sun and moon, ocean currents, winds, and the natural flow of nearby rivers. The proper way to calculate the height of the tide is to use a tide table for that particular area, especially in critical conditions.
If you know the tidal range between high and low water and you don't have tide tables handy, the 12th Rule can give you some indication of what to expect. It can also give you a good indication of what sort of tidal current to expect as the current flow is strongest 3 hours before or after high or low tide, and slack at high or low water.
The following example shows the change of height of the tide with a tidal range of 6 feet.
Of course, tidal currents run at maximum between the 2nd and 4th hour and are slack at low and high water.
Howard [ Just Imagine ] 07-Jan-2009