Air draft: overhead clearance as an operational limit
Air draft, the height from the waterline to a vessel's highest point, and the bridge and canal clearances that constrain it on major trade routes.
Air draft is the vertical distance from the waterline to the highest point of a vessel, and it is the dimension that decides whether a ship can pass beneath a bridge, a power line or a loading gantry. Unlike draft, it increases as a ship discharges, so the constraining condition is usually the ballast passage rather than the laden one.
The full article will cover the measurement convention, the tidal and meteorological allowances that turn a published clearance into an operating limit, and the way authorities state the figure. The Panama Canal Authority, for example, sets 57.91 m at any state of the tide as an operational limit rather than publishing a bridge clearance, and allows up to 62.5 m case by case at low water. It will also treat the air draft constraints on the principal container and bulk trades.