Air draft

Air draft is the vertical distance from the waterline to the highest fixed point of the ship, normally the masthead, radar, or funnel top. It governs clearance under bridges, power lines, and cranes and is the complement of draft in any height-restricted transit. Air draft changes with loading: a light ship floats higher and presents a larger air draft than the same ship loaded to its summer marks. Pilots and masters compute it against charted vertical clearance at the relevant tidal height, then apply a safety margin. For ballast-condition passages under fixed bridges, air draft, not draft, is usually the binding constraint. It is not a class-defined molded dimension but an operational figure that varies with displacement and trim.