The Rule Of Twelfths

By Mel Neale

Tide tables tell you three important things for any given place: time of high tide, time of low tide, and heights of each. Here's how to figure out the times in between.

Rule of 12ths illustrationFor a real-world example of this great technique, see FIG 1 and follow a shallow-water journey. Click image to enlarge. (Illustration: ©2016 Mirto Art Studios)

When you need to know approximately how much water is below your boat for a particular time of day, in a particular place, and you have access to the tide tables, the "Rule of Twelfths" will serve you well. It's an easy-to-use guide for "semi-diurnal" tides, which means there are two nearly identical complete tidal cycles a day (high, low, high, low, all within approximately 24 hours).

Basically, it takes about six hours for this tide to completely rise (flood) or fall (ebb). The "slack" period (when the tide is reversing directions) varies in duration depending upon your location, the stage of the moon, the force of the wind, and other factors. Slack tide may last only a few minutes or much longer, and doesn't necessarily correspond to the exact time of high and low tide.

The times of high and low tides, as well as tidal heights above or below chart datum (the numbers showing depths on your chart) for each day, can be determined from a number of sources, such as weather broadcasts, tide tables, navigation programs, some charts, and books such as Eldridge Tide and Pilot Book, published annually. If you're coastal cruising, keep a print version of the tide tables aboard for times when electricity and Internet connections are unavailable.

OK, Let's Try This Together

To learn how to work through the tide tables, let's take a hypothetical cruise toward Maine. The date is Saturday, June 4. You're crossing Massachusetts Bay on your outbound trek. The chart shows a protected cove to spend the night, just off the Annisquam River, which runs between Gloucester Harbor and Ipswich Bay.

Annisquam River chartFig. 1 (Illustration: ©2016 Mirto Art Studios)

Your chosen anchorage is behind Rust Island to the west of the #36 nun (see Fig. 1 above). The water in the anchorage is 8 to 10 feet at mean low water (red circle), plenty for your boat, which draws 4'6". But you need to cross one spot that draws 2 feet at low tide (blue circle). Approximately what time do you need to arrive at #36 to make it safely into the anchorage? Answer that with tide tables and the Rule of Twelfths.

Here's How To Use A Tide Table

First, locate your place in the tide tables (see Fig. 2). You'll see pages of tide charts, but for only a handful of major ports: Portland, Maine; Boston, Massachusetts; Newport, Rhode Island; and others. NOAA calls these "reference ports;" there are nine on the East Coast between Maine and Miami. In Portland on June 4, high tide occurs at 11:09 a.m.; in Boston at 11:25 a.m.

Tidal tableFig. 2 (Click image to enlarge.)

Time of high water chartFig. 3 (Click image to enlarge.)

Intermediate tidal heights

In addition to the reference ports, there are roughly 350 NOAA substations between Maine and Miami, all pegged to reference stations (see Fig. 3). The port of reference or substation will be marked adjacent to any tidal info on the chart. Your chosen anchorage lies between two substations in the tide tables: "Gloucester Harbor" and "Annisquam, Lobster Cove." The entry for Annisquam says high tide occurs 10 minutes after high tide in Portland. So, 11:09 plus 10 minutes means you can expect high tide at Annisquam at 11:19 a.m. What about Gloucester? Its entry in the tide tables says "same as Boston." High tide in Boston occurs at 11:25 a.m. Notice that for both substations the "rise in feet" is 8.5, or 8'6". What time do you need to arrive at #36? Between about 8 a.m. and 2:30 p.m.

— Tim Murphy

For simplicity, let's use a 6-foot tidal range (range = difference between high and low tide heights). The range should be divided into 12 parts: 6 divided by 12 = half a foot. The tide will rise or fall one-twelfth in the first and sixth hours, two-twelfths in the second and fifth hours, and three-twelfths in the third and fourth hours.

Keep in mind that all this is approximate and can be affected by phenomena such as high winds and storm surges. Also, in some parts of the world (for example, most areas of U.S. Gulf Coastal States, eastern Mexico, and some Caribbean Islands) tide cycles are "diurnal" (only one 12-hour rise and fall in each 24-hour period). Diurnal tidal areas often have weak currents with long periods of slack and little tidal range. Some areas have a mixture, where highs and lows are unequal and irregular. You will see this reflected in the tide charts. There are other exceptions where wind plays a predominant role, and depending on which way it is blowing on the water's surface, can make depths different from what you see in the tide tables. Always combine what the books say with what you observe around you. 

Mel Neale has lived aboard and cruised with her family for 30 years up and down the East Coast to the Caribbean.

— Published: October/November 2016

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