ship structure
The level is forty centimetres above prediction and the masthead is still rolling
Mast
Masts and rigging transfer sail loads into the hull.
Deck
Deck beams help the hull resist transverse deformation.
Keel
The main longitudinal member forms the structural baseline of many wooden sailing ships.
Air draft measures upward; water draft measures downward
Water draft or draught runs from the waterline down to the keel or another specified lowest point and supports depth and under-keel-clearance work. Air draft runs in the opposite direction, from the waterline to the highest point, for bridges, overhead cables, dock gates and other overhead restrictions. The shared word draft does not put them in one clearance equation.
The highest point may not be the timber masthead. VHF antennas, lightning conductors, wind instruments, radar, lights and flagstaffs can project higher. Folding equipment counts as lowered only after its safe configuration has been confirmed and secured. A defensible air-draft table records loading condition, configuration, date and the exact limiting point.
- Water draft: immersion below the waterline
- Air draft: highest point above the waterline
- Vertical clearance: overhead limit above a stated water level
Bridge clearance must be read with its water-level datum and usable width
A chart or Coast Pilot normally ties vertical clearance to a stated water level, such as mean high water. When the actual surface is above that datum, available clearance decreases; when below it, clearance may increase subject to local publication rules. Tide, rainfall, river flow, surge, wind setup and control structures can all change the surface under the bridge.
An arch may provide its greatest height only across a narrow central span, while pier spacing limits whether the vessel can align with it. Opening bridges have separate closed and open states. Overhead cables may publish a safe clearance against electrical discharge rather than mere physical contact. A charted number cannot be separated from its note and current local directions.
Loading, fuel and water change the waterline and therefore air draft
Adding cargo, fuel, fresh water or ballast normally increases water draft and reduces the height of a fixed masthead above the surface. Discharging or consuming weight can make the vessel float higher and increase air draft. Trim matters when the limiting point lies forward or aft, so a midships waterline alone may not describe its height.
Heel, pitch, roll and heave create dynamic high points. Slowing before a bridge does not remove surge, wind pressure or another vessel's wake, and moving people or suspended loads can alter attitude. Planning should use the expected transit condition and carry separate motion and measurement allowances instead of treating a static berth reading as an absolute maximum.
A clearance ledger aligns datums before subtracting uncertainty
Assume published bridge clearance is 12.0 metres at its datum and predicted water level during transit is 0.4 metre above that datum, leaving 11.6 metres static clearance. The vessel's air draft in its current loading and equipment configuration is 10.7 metres, a static difference of 0.9 metre. Reserving 0.3 metre for motion and 0.2 metre for water-level and measurement uncertainty leaves a planning margin of 0.4 metre.
The example shows the ledger, not a universal permission to proceed. Bridge managers, ports, pilots, operators or regulation may demand a larger fixed or percentage margin or close the route under particular conditions. If an input lacks a traceable source, real-time level cannot be confirmed or equipment state is uncertain, the remaining difference is not a clearance authorization.
Before transit, verify gauges, notices, communications and an escape plan
Check current charts, sailing directions, bridge notices and local water levels, including opening times, call channels, current, speed and meeting rules. A bridge gauge can be closer to the present level than an old screenshot, but it may be damaged, wave-obscured or valid only at a stated location, so it still needs confirmation against official information.
The approach must preserve enough room to slow, turn or wait in safe water. Do not first test a folding antenna in strong current at the bridge opening, and do not estimate centimetres by eye. If air draft, the limiting bridge point or water level cannot be established, wait, reroute or contact the responsible authority rather than compress an unknown margin.
Questions
Continue exploring this subject
What is the difference between air draft and ship draft?
Air draft runs from the waterline to the vessel's highest point. Ship draft runs from the waterline to the keel or specified lowest point. One checks overhead clearance; the other checks water depth.
If a bridge says 12 metres and air draft is 11.8 metres, can the boat pass?
Not on those figures alone. Check the clearance datum, actual water level, usable span, vessel motion, measurement error and the locally required safety margin.
Does loading a boat reduce air draft?
Usually added weight makes the vessel sit lower and reduces fixed-point air draft, but trim, tank distribution and the location of the highest point affect the result.
Can an antenna be excluded from air draft?
Only when it can be safely lowered, has actually been lowered and secured, and no other fixed item becomes the highest point.
Why can safe cable clearance exceed the distance needed to avoid touching?
High-voltage cables can arc. Published safe clearance may prevent electrical discharge as well as physical contact.
Sources
Continue the research
- United States Coast PilotNOAA Office of Coast Survey
- U.S. Chart No. 1NOAA Office of Coast Survey
- U.S. Coast Pilot 9, Chapter 1NOAA Office of Coast Survey
- Bridge heights and openingsBroads Authority
- Navigation through the entrances to the Baltic SeaInternational Maritime Organization
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