Air draft: the vertical clearance of a vessel above the waterline

Measurement and influencing factors

Air draft is measured from the waterline (mean or local) up to the highest point. Several factors change the effective air draft in practice: tidal range, loading and ballast, squat and trim while underway, and seasonal or local water-level changes. For safety, mariners compare a vessel's air draft with published bridge or cable clearances and include appropriate margins.

Operational importance

Knowing the air draft is essential for route planning where bridges, power lines, or lock gates are present. Port authorities, pilots, and ship masters use it to confirm that a transit is possible. Some bridges publish a vertical clearance that varies with tide or river level; planners must ensure the vessel's air draft remains below that clearance throughout the passage.

Design responses and practical adaptations

Ship designers and operators use several strategies to manage air-draft limits: lowering or folding masts and antennas, designing retractable funnels or stacks, and adjusting ballast and cargo stowage. Certain waterways and terminals set maximum air-draft limits that influence the size and type of vessels that can call there.

Examples and notable distinctions

• Air draft differs from "draft" (or "draught"), which is the depth of a vessel below the waterline. Both measurements together define a ship's vertical footprint.
• Published vertical clearances refer to fixed structures; they may be given relative to a datum such as mean high water, so mariners must account for local water levels.
• Some modern ships are built with features specifically to reduce air draft and permit access to constrained routes or terminals.

Accurate assessment of air draft reduces accident risk, prevents costly delays, and shapes both everyday navigation and long-term ship design decisions.