Air draught

Air draught (also known as air draft or air clearance) is the distance from the surface of the water to the highest point on a vessel. This is a critical measurement for both navigation and bridge design.

Overview

Air draught is used to ensure that marine vessels can safely pass under bridges or power lines, clear overhanging trees, or enter low-lying harbours. For ships, the term air draught is synonymous with the term "height of mast" or "highest fixed point". A bridge's "air draught" is determined by considering the maximum height of vessels that will pass under it, plus an additional margin for safety.

Measurement

Air draught is generally measured from the waterline to the highest point on the vessel that cannot be lowered. On a sailing vessel, this is typically the top of the mast. On a vessel with a fixed superstructure, it may be the top of the highest point of that superstructure.

Impact on Bridge Design

Air draught is a critical factor in the design of bridges over navigable waters. The bridge must be high enough to allow the passage of vessels expected to use the waterway, but not so high as to be impractical or excessively expensive. The air draught of a bridge is usually published on nautical charts, so that mariners can ensure their vessels can safely pass underneath.

See Also

• Draught (hull)
• Bridge clearance
• Navigation
• Ship measurements