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Do you know this boating rule of thumb?

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Vol. III, No. 7

A strong current rushes past a buoy near Woods Hole, MA. Photo by Allen Clark / PhotoBoat.com.

A strong current rushes past a buoy near Woods Hole, MA. Photo by Allen Clark / PhotoBoat.com.

The rule of twelfths is one of the most useful laws in boating, yet too many recreational skippers don’t know this trick, which is helpful for powerboaters and kayakers alike. It’s a formula for estimating the tide height at any given point in the tide cycle.

Let’s say the tidal range is 6′, which is a bit on the moderate side for Westport waters, but it’s good for math. And let’s round off the tide cycle to an even 6 hours.  So if low tide is at 3pm, high tide will be at 9pm.  Now let’s pretend it’s 6pm and you plan to paddle up Gray’s Creek, which as locals know is dry at low tide.  The tide is going out, so the adverse current makes the trip take longer, and at 7pm, you’re all the way at the “top.” No problem, right? The tide will be with you on the way out.  And you’ve still got 2/6 or 1/3 of the water left?  Actually, no, and that’s where the rule of twelfths comes in.   Here it is:

1-2-3, 3-2-1  or  1/12, 2/12, 3/12, 3/12, 2/12, 1/12

During the first hour of tide change, 1/12 of the water goes out (or comes in if the tide is rising). During hour two, 2/12 (1/6), and in the third hour, 3/12 or 1/4 of the water goes out.  So basically, during the middle of a tide cycle, the water is flowing the fastest.  This is important for understanding and anticipating the magnitude of currents.  And it also means that if you find yourself all the way up Gray’s creek two hours before low tide, you may very well end up high and dry before you get to the mouth.  At 7 pm, only 1/4 of the tide change remains.

For power boaters, understanding the rule of twelfths means knowing that 1 hour after low tide, it’s still basically low tide, as only 1/12 of the tide rise has occurred.  It’s probably best to avoid that sand bar, or avoid going over Cockenoe Reef.

-DC

Categories: WaterViews

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