Chart Navigation & Plotting · Tides — Height Calculation
If low water is 2.0 ft and high water is 14.0 ft (range 12 ft), the rule of twelfths predicts the height about 3 hours after low water to be approximately:
- A8.0 ft✓ Correct
- B5.0 ft
- C11.0 ft
- D2.0 ft
Explanation
In the first 3 hours the tide rises 1+2+3 = 6/12 = half the 12-ft range = 6 ft. Height = LW 2.0 ft + 6 ft = 8.0 ft.
Authority: NOAA Tide Tables; rule of twelfths
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