Which tidal datum is used for clearances?

Master the Navigation, Seamanship, and Shiphandling (NSS) – Officer of the Deck (OOD) Phase 2 Exam with engaging flashcards and multiple choice questions, complete with hints and explanations. Prepare comprehensively for your test!

Multiple Choice

Which tidal datum is used for clearances?

The amount of space available under fixed structures is defined with respect to Mean High Water. This tidal datum represents the average high-water level and provides a consistent reference for the vertical clearance ships must have during typical high-tide conditions. So, the published clearance is effectively the space you must pass under at high water, not at mean sea level or the low-water depths used for chart sounding. Other datums serve different purposes—Mean Lower Low Water for depths on charts, or Mean Sea Level as a general reference—so Mean High Water is the appropriate basis for clearances.

The amount of space available under fixed structures is defined with respect to Mean High Water. This tidal datum represents the average high-water level and provides a consistent reference for the vertical clearance ships must have during typical high-tide conditions. So, the published clearance is effectively the space you must pass under at high water, not at mean sea level or the low-water depths used for chart sounding. Other datums serve different purposes—Mean Lower Low Water for depths on charts, or Mean Sea Level as a general reference—so Mean High Water is the appropriate basis for clearances.

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