Air draft — the height limit that bridges impose on ships

Air draft is the vertical distance from the waterline to the highest fixed point on a ship — the top of the mast, funnel or, on a container ship, the uppermost stacked box. It is the mirror image of draft: draft is the limit imposed from below by the seabed and canal floor, air draft is the limit imposed from above by bridges and power lines.

It is a real operational constraint, not a footnote. The Panama Canal's Bridge of the Americas sets a 57.91-metre air-draft ceiling; the Suez Canal Bridge allows 68 metres. A megamax container ship fully stacked can approach these limits, and tide matters — a ship that clears a bridge at low water may not at high. Some ships lower masts or top-mounted equipment specifically to make a passage.

Air draft also varies with loading in the opposite direction to draft: unload a ship and she rises, increasing her air draft while decreasing the draft below. A lightly loaded ship can therefore be too tall for a bridge she would clear when fully laden — one of the counterintuitive facts that makes ship handling in constrained waters a specialist skill.