Air draft

{{Short description|Distance from water to the highest point on a vessel or lowest point on a bridge span}} {{Other uses|Draft (disambiguation)}}

[[Image:Allanburg Bridge.jpg|thumb|The deck of the Allanburg Bridge on Canada's Welland Canal typically rests only a few metres above the water level. When a ship approaches, the deck is raised to provide sufficient air draft (or draught) for the vessel to pass through. This bridge was involved in a collision with a lake freighter in 2001 as a result of lowering the span before the ship fully cleared the bridge.]]

'''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).<ref name="uscg" /><ref>{{cite book|url=http://www.ct.gov/deep/lib/deep/boating/boating_guide/boaterguide.pdf|archive-url=https://web.archive.org/web/20130520230317/http://www.ct.gov/deep/lib/deep/boating/boating_guide/boaterguide.pdf|url-status=dead|archive-date=May 20, 2013|page=60|title=2104 Connecticut Boater's Guide|publisher=State of Connecticut Department of Energy and Environmental Protection|access-date=2015-02-15}}</ref>

== Clearance below == {{Main|Clearance (civil engineering)#Waterways}}

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),<ref>{{Cite web |url=https://charts.noaa.gov/PDFs/12335.pdf |title=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" |access-date=December 22, 2019 |archive-date=October 22, 2015 |archive-url=https://web.archive.org/web/20151022232202/https://www.charts.noaa.gov/PDFs/12335.pdf |url-status=dead }}</ref><ref>[https://nauticalcharts.noaa.gov/publications/coast-pilot/files/cp5/CPB5_C08_WEB.pdf 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"]</ref> 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.<ref name="uscg" />

==Examples== thumb|right|''Bridge of the Americas'' The Bridge of the Americas in Panama limits which ships can traverse the Panama Canal due to its height at {{convert|61.3|m|ft|0|abbr=on}} above the water. The world's largest cruise ships, {{MS|Oasis of the Seas||2}}, {{MS|Allure of the Seas||2}} and the {{MS|Harmony of the Seas||2}} 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 {{convert|72|m|ft|0|abbr=on}}, but do have lowerable funnels, enabling them to pass the {{convert|65|m|ft|0|adj=on}} Great Belt Bridge in Denmark). New vessels are rarely built not clearing {{convert|65|m|ft|0|abbr=on}}, a height which accommodates all but the largest cruise and container ships.

The Suez Canal Bridge has a {{convert|70|m|ft|0|adj=on}} 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 {{convert|66|m|ft|0|abbr=on}}.<ref>[https://www.silive.com/news/2019/06/bayonne-bridge-rededication-ceremony-marks-end-of-17-billion-project.html Bayonne Bridge rededication ceremony marks end of $1.7 billion project]</ref>

==References== <references> <ref name="uscg">{{cite press release |url=https://www.dco.uscg.mil/Portals/9/TVNCOE/Documents/SafetyAlerts/AirDraft.pdf?ver=2017-08-11-142836-740 |publisher=United States Coast Guard Inspections and Compliance Directorate |title=Marine Safety Alert 090-14: AIR DRAFT IS CRITICAL! |date=2014-09-09 |archive-url=https://web.archive.org/web/20220516210520/https://www.dco.uscg.mil/Portals/9/TVNCOE/Documents/SafetyAlerts/AirDraft.pdf?ver=2017-08-11-142836-740 |archive-date=2022-05-16 |url-status=live |access-date=2023-09-05}}</ref> </references>

{{DEFAULTSORT:Air Draught}} Category:Ship measurements Category:Vertical extent