PracticalBoater
Skills

Reading A Tide Table

By Tim Murphy
Published: August/September 2014

Knowing how to read a tide table can mean the difference between a good day and a bad one.

Illustration of the Rule of Twelves

If you've ever waited anxiously for the twing of your antenna against the underside of a highway bridge, you know that playing with tides can be a game of inches. To pass safely under that bridge or over the bar that lies between here and home, we need to understand all the components of the tides. Along most of the coast, tides rise twice and fall twice each day. These are called semidiurnal tides. In some places, the tides cycle only once per day; these are called diurnal tides. And in still other places, one daily high tide is much higher than the day's second high tide; these are called mixed tides. Tide tables, provided by the U.S. National Oceanic and Atmospheric Administration (NOAA) at tidesandcurrents.noaa.gov, tell you three important things for any given place: the time of high tide, the time of low tide, and the heights of each. But what about the times in between? For that, you'll need the Rule of Twelfths (see chart at right).

Illustration of tide timetable
Illustration: Marcus Floro
Click on image to enlarge

Here's an example: It's August 15, 2014. We're in a sailboat with a 52-foot-high mast, and we'd like to pass under the Hood Canal Bridge, off Washington's Puget Sound, where the east span has a published vertical clearance of 50 feet at Mean High Water (MHW). The published datum for Hood Canal tells us that MHW is 9.82 feet above the charted depths in this area's Mean Lower Low Water (MLLW). Low tide today is 2.7 feet above charted depths. The difference between MHW and today's actual low is 7.14 feet. Add that to the bridge's charted vertical clearance (50 feet plus 7.14 feet equals 57.14 feet), and we get an estimated clearance of 5.14 feet at low tide (57.14 feet minus 52 feet equals 5.14 feet).

Now let's apply our Rule of Twelfths. With a high of 10.65 feet (9:10 p.m.) and a low of 2.68 feet (2:57 p.m.), this tide cycle has a total range of 7.97 feet. Divide that total by 12, and we get 0.664 feet. So at Hour 1 (approx. 4 p.m.) we must subtract 0.664 feet (1/12th) from our clearance to get 4.48 feet. At Hour 2, we must subtract an additional 2/12ths, or 1.33 feet, giving us just 3.15 feet of clearance, and so on. See chart at left for the complete table.

Of course, such weather conditions as barometric pressure, prolonged winds, and storm surges can all affect the actual clearance. And be ready for a passing wake! Moral of the story: Leave a nice margin between you and that twing (or worse).End of story marker


Tim Murphy is an independent writer, editor, musician, and the coauthor of Fundamentals of Marine Service Technology (ABYC, 2012).

 

 Recommended Articles
Gray rule

Illustration of a hand clockOn The Water

Tips for maneuvering and anchoring your boat



Thumbnail photo of Tux Turkel piloting a 16-foot StarcraftTime And Tide

Passamaquoddy Bay, a protected bay on the western edge of the Bay of Fundy, is a wild, wind-swept place of remote islands


Thumbnail photo of Power Pole at workWhen Water Does More Than Float Your Boat

Tips for launching your boat in water with currents or tide changes

 


BoatUS Magazine Is A Benefit Of BoatUS Membership

Membership Also Provides:

  • Subscription to the print version of BoatUS Magazine
  • 4% back on purchases from West Marine stores or online at WestMarine.com
  • Discounts on fuel, transient slips, repairs and more at over 1,000 businesses
  • Deals on cruises, charters, car rentals, hotel stays and much more ...
  • All For Only $24 A Year!


Join Today!