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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 Electric power transmission is the bulk movement of electrical energy from a generating site, such as a power plant, to an electrical substation. The interconnected lines that facilitate this movement form a ''transmission network''. This is d ...
, 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 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 astr ...
Mean High Water (MHW), a less restrictive clearance than Mean Higher High Water (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, mu ...
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 The Bridge of the Americas ( es, Puente de las Américas; originally known as the Thatcher Ferry Bridge) is a road bridge in Panama which spans the Pacific entrance to the Panama Canal. Designed by Sverdrup & Parcel, it was completed in 1962 at ...
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 The Great Belt Bridge ( da, Storebæltsbroen) or Great Belt fixed link ( da, Storebæltsforbindelsen) is a multi-element fixed link crossing the Great Belt strait between the Danish islands of Zealand and Funen. It consists of a road suspension ...
in Denmark). New vessels are rarely built not clearing , a height which accommodates all but the largest cruise and container ships. The
Suez Canal Bridge The Suez Canal Bridge, also known as the Egyptian–Japanese Friendship Bridge, Al Salam Bridge, Al Salam Peace Bridge or Mubarak Peace Bridge, is a road bridge crossing the Suez Canal at El-Qantara, whose name means "the bridge" in Egyptian Ar ...
has a clearance over the canal. The
Bayonne Bridge Bayonne Bridge is an arch bridge spanning the Kill Van Kull and connecting Bayonne, New Jersey with Staten Island in New York City. It carries New York State Route 440 (NY 440) and New Jersey Route 440. It is the sixth-longes ...
, 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
/ref>


See also

* Structural clearance *
Structure gauge A structure gauge, also called the minimum clearance outline, is a diagram or physical structure that sets limits to the extent that bridges, tunnels and other infrastructure can encroach on rail vehicles. It specifies the height and width of pl ...
* Tower Bridge * Cargo ship size categories *
Chart datum 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 astr ...


References

{{DEFAULTSORT:Air Draught Ship measurements