Chart Navigation & Plotting · Tides — Rule of Twelfths
The tidal range is 10 feet (6-hour rise). By the Rule of Twelfths, how much has the tide risen 4 hours after low water?
- A6.0 feet
- B5.0 feet
- C9.0 feet
- D7.5 feet✓ Correct
Explanation
After 4 hours the cumulative twelfths are (1+2+3+3)/12 = 9/12 = 0.75 of the range. 0.75 × 10 ft = 7.5 ft.
Authority: Bowditch (Pub. No. 9); NOAA Tide Tables
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