Chart Navigation & Plotting · Tides — Clearance Under Keel
Charted depth at a spot is 18 ft (referenced to the chart datum), the predicted height of tide is +4 ft, and your draft is 16 ft. What is your clearance under the keel?
- A4 feet
- B2 feet
- C8 feet
- D6 feet✓ Correct
Explanation
Depth of water = charted depth + height of tide = 18 + 4 = 22 ft. Under-keel clearance = 22 − 16 (draft) = 6 ft. Charted depths are reductions to chart datum, so the tidal height is added.
Authority: NOAA Tide Tables; Bowditch (Pub. No. 9), depth of water and tide
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