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.

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 A bridge is a structure built to span a physical obstacle (such as a body of water, valley, road, or rail) without blocking the way underneath. It is constructed for the purpose of providing passage over the obstacle, which is usually somethi ...

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 chart datum is the water level surface serving as origin of depths displayed on a nautical chart. A chart datum is generally derived from some tidal phase, in which case it is also known as a tidal datum. Common chart datums are ''lowest ast ...

(MHW), a less restrictive clearance than

Mean Higher High Water A chart datum is the water level surface serving as origin (mathematics), origin of depth (coordinate), depths displayed on a nautical chart. A chart datum is generally derived from some tidal phase, in which case it is also known as a tidal datu ...

(MHHW). In 2014, the

United States Coast Guard The United States Coast Guard (USCG) is the maritime security, search and rescue, and law enforcement service branch of the United States Armed Forces and one of the country's eight uniformed services. The service is a maritime, military, mult ...

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

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 (the two first ships are , but do have lowerable funnels, enabling them to pass the Great Belt Bridge in Denmark). New vessels are rarely built not clearing , a height which accommodates all but the largest cruise and

container ship A container ship (also called boxship or spelled containership) is a cargo ship that carries all of its load in truck-size intermodal containers, in a technique called containerization. Container ships are a common means of commercial intermodal ...

s. 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 $1.7 billion modification to raise its roadbed to .Bayonne Bridge rededication ceremony marks end of $1.7 billion project
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References

{{DEFAULTSORT:Air Draught Ship measurements

Clearance below

Examples

See also

References