Chart Navigation & Plotting · Tides — Rule of Twelfths
The tidal range today is 12 feet. Using the Rule of Twelfths, how much has the tide risen 3 hours after low water (assuming a 6-hour rise)?
- A3 feet
- B6 feet✓ Correct
- C9 feet
- D4 feet
Explanation
The Rule of Twelfths apportions the rise as 1/12, 2/12, 3/12, 3/12, 2/12, 1/12 per successive hour. After 3 hours: (1+2+3)/12 = 6/12 of the range. Half of a 12-foot range is 6 feet.
Authority: Bowditch (Pub. No. 9); NOAA Tide Tables
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