Air draft


Air draft is the distance from the surface of the water to the highest point on a vessel. This is similar to the "deep draft" of a vessel which is measured from the surface of the water to the deepest part of the hull below the surface, but air draft is expressed as a height, not a depth.

Clearance below

The vessel's "clearance" is the distance in excess of the air draft which allows a vessel to pass safely under a bridge or obstacle such as power lines, etc. A bridge's "clearance below" is most often noted on charts as measured from the surface of the water to the under side of the bridge at the chart datum Mean High Water, a less restrictive clearance than Mean Higher High Water.
In 2014, the United States Coast Guard reported that 1.2% of the collisions it investigated in the recent past were due to vessels attempting to pass underneath structures with insufficient clearance.

Examples

At several bridges, such as the Gerald Desmond Bridge in Long Beach, California, NOAA has installed an "Air Gap" measuring device that accurately measures the distance from its sensor on the bridge to the water surface and can be accessed by marine pilots and ship's masters to aid them in making real time determination of clearance.
The Bridge of the Americas in Panama limits which ships can traverse the Panama Canal due to its height at above the water. The world's largest cruise ships,, and the will fit within the canal's new widened locks, but they are too tall to pass under the bridge, even at low tide. New vessels are rarely built not clearing, a height which accommodates all but the largest cruise and container ships.
The Suez Canal Bridge has a clearance over the canal.
The Bayonne Bridge, an arch bridge connecting New Jersey with New York City, undertook a $1.32 billion modification to raise its roadbed to.