Chart Navigation & Plotting · Tides
On a standard semidiurnal coast, the 'rule of twelfths' estimates that during the third hour after low water the tide rises by what fraction of the range?
- A1/12
- B2/12
- C3/12✓ Correct
- D4/12
Explanation
The rule of twelfths distributes the range over six hours as 1, 2, 3, 3, 2, 1 twelfths. The third hour (and the fourth) each contribute 3/12 — the fastest rate, around mid-tide.
Authority: NOAA Tide Tables; Bowditch (Pub. 9)
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