Air draft
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Air draft (or air draught) is the vertical 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. However, air draft is expressed as a height (positive upward), while deep draft is expressed as a depth (positive downward).[1][2]
Clearance below
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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 underside of the bridge at the chart datum Mean High Water (MHW),[3][4] a less restrictive clearance than Mean Higher High Water (MHHW).
In 2014, the United States Coast Guard reported that 1.2% of the collisions that it had investigated in the recent past were bridge strikes caused by vessels attempting to pass under structures with insufficient clearance.[1]
Examples
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, Template:MS, Template:MS and the Template:MS 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 72 m (236 ft), but do have lowerable funnels, enabling them to pass the 65-metre (213 ft) Great Belt Bridge in Denmark). New vessels are rarely built not clearing 65 m (213 ft), a height which accommodates all but the largest cruise and container ships.
The Suez Canal Bridge has a 70-metre (230 ft) 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 66 m (217 ft).[5]
See also
- Structural clearance
- Structure gauge
- Tower Bridge
- Cargo ship size categories
- Chart datum
- Bridge strike
References
- ^ a b Page Module:Citation/CS1/styles.css has no content."Marine Safety Alert 090-14: AIR DRAFT IS CRITICAL!" (PDF) (Press release). United States Coast Guard Inspections and Compliance Directorate. 2014-09-09. Archived (PDF) from the original on 2022-05-16. Retrieved 2023-09-05.
- ^ Page Module:Citation/CS1/styles.css has no content.2104 Connecticut Boater's Guide (PDF). State of Connecticut Department of Energy and Environmental Protection. p. 60. Archived from the original (PDF) on May 20, 2013. Retrieved 2015-02-15.
- ^ Page Module:Citation/CS1/styles.css has no content."See: NOAA Navigation Chart #12335, Hudson and East Rivers, Governors Island to 67th Street, Revised October 1, 2019, "HEIGHTS: Heights in feet above Mean High Water"" (PDF). Archived from the original (PDF) on October 22, 2015. Retrieved December 22, 2019.
- ^ See: U.S. Coast Pilot 5, Chapter 8, p. 354, Structures across the Inner Harbor Navigation Canal, New Orleans, 15 December, 2019, "Vertical clearance measured at Mean High Water"
- ^ Bayonne Bridge rededication ceremony marks end of $1.7 billion project
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