Chart Navigation & Plotting · Tides — Height Interpolation
From the Tide Tables, low water is 1.0 ft at 0900 and high water is 7.0 ft at 1512 (a 6h 12m rise). Using the Rule of Twelfths, approximately what is the height of tide at 1100 (two hours after low water)?
- A4.0 feet
- B2.5 feet✓ Correct
- C1.5 feet
- D3.5 feet
Explanation
Range = 7.0 − 1.0 = 6.0 ft. By the Rule of Twelfths the first two hours after low water raise the tide by 1/12 + 2/12 = 3/12 = 1/4 of the range = 1.5 ft. Height ≈ 1.0 + 1.5 = 2.5 ft.
Authority: NOAA Tide Tables; Bowditch (Pub. No. 9), Rule of Twelfths
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