Navigation General · Tides
A vessel drawing 14 feet needs to cross a bar charted at 10 feet (MLLW). Low water is 0.5 feet above MLLW. High water will be 5.5 feet above MLLW. How much additional height of tide above low water is needed before the vessel can safely cross with 2 feet of under-keel clearance?
- A4.5 feet
- B5.5 feet✓ Correct
- C5.0 feet
- D3.5 feet
Explanation
Depth needed = draft + clearance = 14 + 2 = 16 feet. Depth at LW = charted depth + LW height above MLLW = 10 + 0.5 = 10.5 feet. Additional tide rise needed above LW = 16 − 10.5 = 5.5 feet. Therefore the tide must rise 5.5 feet above low water before the vessel can cross safely.
Authority: NOAA Tide Tables; Bowditch (Pub. 9), Ch. 9
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