distance from water to the highest point on a vessel or lowest point on a bridge span, etc.
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Air draft (or air draught) is the distance from the surface of the water to the highest point on a vessel (a ship or boat).
The deck of this lift bridge on Canada's Welland Canal typically rests only a few metres above the water level. When a ship approaches, the deck is raised to provide sufficient air draft (or draught) for the vessel to pass through.