SkipperThe rule of twelfths is a quick way to estimate how much the tide has risen or fallen at any hour between low and high water. It is part of the Day Skipper tidal syllabus.
What is the rule of twelfths?
Take the tidal range: the height of high water minus the height of low water. Assume the tide takes about six hours to rise or fall. Then, in each hour, it moves:
| Hour | Share of the range | Total so far |
|---|---|---|
| 1st | 1/12 | 1/12 |
| 2nd | 2/12 | 3/12 |
| 3rd | 3/12 | 6/12 |
| 4th | 3/12 | 9/12 |
| 5th | 2/12 | 11/12 |
| 6th | 1/12 | 12/12 |
The pattern is 1, 2, 3, 3, 2, 1. The tide moves slowly near high and low water and fastest in the middle two hours.
Worked example
Low water is 0.8 m at 0600. High water is 5.6 m at 1200. The range is 5.6 minus 0.8, which is 4.8 m. One twelfth is 0.4 m.
- 0700: 0.8 + 0.4 = 1.2 m
- 0800: 1.2 + 0.8 = 2.0 m
- 0900: 2.0 + 1.2 = 3.2 m
- 1000: 3.2 + 1.2 = 4.4 m
- 1100: 4.4 + 0.8 = 5.2 m
- 1200: 5.2 + 0.4 = 5.6 m
So if you need 3 m of tide to cross a bar, you can expect it from about 0900.
When not to trust it
The rule assumes a smooth, even tide of about six hours. Many places do not have one. Where the tidal curve is lopsided, such as parts of the Solent with its double high water, use the tidal curve for the standard port in the almanac instead. For the exam, use the method the question asks for.
The rule gives height of tide above chart datum. To get depth, add the charted depth, or subtract a drying height.
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