Reading A Tide Table
By Tim Murphy
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).
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).
Tim Murphy is an independent writer, editor, musician, and the coauthor of Fundamentals of Marine Service Technology (ABYC, 2012).
— Published: August/September 2014
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