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P.O. Box 775
Medfield, MA 02052

The indispensable resource for East Coast boaters, the ELDRIDGE TIDE AND PILOT BOOK has tides and currents for ports from Nova Scotia to Key West.


The Rule of Twelfths

Peter Kuliesis

Since the average interval between high and low tide is just over six hours, we can divide the cycle into six segments of one hour each. On average the tide rises or falls approximately according to the following:

1st hour - 112
2nd hour - 212
3rd hour - 312
4th hour - 312
5th hour - 212
6th hour - 112

Vineyard Haven Harbor, for example, has an average tide of 19 inches (1.6 feet.) 1 hour after low water, the tide will have risen approximately 1.6 inches (112 × 19 inches;) 2 hours after low tide, it will have increased an additional 3.2 inches (212 × 19 inches,) for a total increase of 4.8 inches from low tide; and so on.