Rule of Twelfths
By Tom Neale, 7/13/2004
by Mel Neale
When you are cruising most tidal areas of the coast, it is possible to quickly determine approximately how fast the tide is rising or falling and the velocity of the accompanying current by using the Rule of Twelfths. 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 nautical almanacs like Reeds or Eldridges. Use your own observation of the shoreline if no other information is available.
The Rule of Twelfths is a general guide for semi-diurnal tides where there are two nearly identical complete tidal cycles a day (high, low, high, low, all within approximately 24 hours). It usually takes about six hours for the tide to completely rise (flood) or fall (ebb). The slack period when the tide is reversing directions varies in duration depending upon the location, the lunar stage, wind and other factors. You should consult current tables for this exact information, and observe local conditions. Slack tide may last only a few minutes or much longer, and does not necessarily correspond to the exact time of high and low tide.
In some parts of the world (for example: most areas of US Gulf Coastal States including the western part of the Florida Panhandle, Eastern Mexico, some Caribbean Islands) tide cycles are diurnal, with only one twelve-hour rise and fall a day. 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 can use the Rule of Twelfths to help determine how long you might have to sit and wait for the tide to rise enough to re-float your boat if you are aground, or to determine how fast you must act to get off. You can use it to approximate the time the current will slow down and become slack (the less rise or fall, the slower the current will flow). It is important to note that one half of the total rise and fall (and therefore the strongest current) occurs during the third and fourth hour.
For simplicity, well use a six-foot tidal range (range = difference between high and low tide heights) in the example below. The range should be divided into twelve parts: 6 divided by 12 = one-half foot. During each hour of the cycle, the tide will rise or fall approximately the following amounts:
Hour 1: One twelfth of range (6 ft) = one-half foot
Hour 2: Two twelfths of range = one foot
Hour 3: Three twelfths of range = one and one-half foot
Hour 4: Three twelfths of range = one and one-half foot
Hour 5: Two twelfths of range = one foot
Hour 6: One twelfth of range = one-half foot
Copyright 2004-2010 Tom Neale