Air draft

Clearance below

Main 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 underside of the bridge at the chart datum Mean High Water (MHW), 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.

Examples

The Bridge of the Americas in Panama limits which ships can traverse the Panama Canal due to its height at 61.3 m 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 72 m, but do have lowerable funnels, enabling them to pass the 65 m Great Belt Bridge in Denmark). New vessels are rarely built not clearing 65 m, a height which accommodates all but the largest cruise and container ships.

The Suez Canal Bridge has a 70 m 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.

References

References

1. "2104 Connecticut Boater's Guide". State of Connecticut Department of Energy and Environmental Protection.
2. "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"".
3. [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"]
4. [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]
5. (2014-09-09). "Marine Safety Alert 090-14: AIR DRAFT IS CRITICAL!". United States Coast Guard Inspections and Compliance Directorate.