Air draft (or air draught) 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.
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 Mean Highest High Water (MHHW) which is the most restrictive clearance. The height of the tide at any time below its highest point at MHHW will then increase the clearance under the bridge.
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.
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 61.3 m (201 ft) above the water. The world's largest cruise ships, Oasis of the Seas, Allure of the Seas and the Harmony of the Seas will fit within the canal's new widened locks, but they are too tall to pass under the Bridge of the Americas, even at low tide, unless the Bridge of the Americas is raised or replaced in the future (the two first ships are 72 m but do have lowerable chimneys, enabling them to pass the 65 m Great Belt Bridge). New ships are rarely built not clearing 65 m.
The Suez Canal Bridge has a 70-metre (230 ft) clearance over the canal, which is 8.7 m (29 ft) higher than Panama.
The Bayonne Bridge is an arch bridge connecting New Jersey with New York City, the roadbed was raised to 66 m, a height suitable for larger container ships to pass, the modification cost $1.32 billion.
- "Marine Safety Alert 090-14: AIR DRAFT IS CRITICAL!" (PDF) (Press release). United States Coast Guard Inspections and Compliance Directorate. 2014-09-09. Retrieved 2015-02-15.
- 2104 Connecticut Boater's Guide (PDF). State of Connecticut Department of Energy and Environmental Protection. p. 60. Retrieved 2015-02-15.