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. However, air draft is expressed as a height (positive upward), while deep draft is expressed as a depth (positive downward).[1][2]
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.
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 the chart datumMean 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 investigated in the recent past were caused by vessels attempting to pass under structures with insufficient clearance resulting in bridge strikes.[1]
Examplesedit
Bridge of the Americas
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, 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]
^ ab"Marine Safety Alert 090-14: AIR DRAFT IS CRITICAL!" (PDF) (Press release). United States Coast Guard Inspections and Compliance Directorate. 2014-09-09. (PDF) from the original on 2022-05-16. Retrieved 2023-09-05.
^2104 Connecticut Boater's Guide(PDF). State of Connecticut Department of Energy and Environmental Protection. p. 60. Retrieved 2015-02-15.
^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"
^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
January 10, 2024
draft, other, uses, draft, disambiguation, draught, distance, from, surface, water, highest, point, vessel, this, similar, deep, draft, vessel, which, measured, from, surface, water, deepest, part, hull, below, surface, however, draft, expressed, height, posit. For other uses see Draft disambiguation 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 However air draft is expressed as a height positive upward while deep draft is expressed as a depth positive downward 1 2 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 Contents 1 Clearance below 2 Examples 3 See also 4 ReferencesClearance below editMain article 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 under side 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 investigated in the recent past were caused by vessels attempting to pass under structures with insufficient clearance resulting in bridge strikes 1 Examples edit nbsp Bridge of the AmericasThe 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 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 editStructural clearance Structure gauge Tower Bridge Cargo ship size categories Chart datum Bridge strikeReferences edit a b 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 2104 Connecticut Boater s Guide PDF State of Connecticut Department of Energy and Environmental Protection p 60 Retrieved 2015 02 15 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 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 Retrieved from https en wikipedia org w index php title Air draft amp oldid 1186658281, wikipedia, wiki, book, books, library,
