Air Draft

Air Draft, also spelled Air Draught, is the vertical distance from a vessel's waterline to its highest point. That point may be the top of a mast, antenna, funnel, crane or another fixed structure, depending on the ship's design and operational configuration.

Air Draft indicates how much of the vessel extends above the water. It is essential when determining whether a ship can safely pass beneath bridges, overhead cables, loading structures and other height restrictions.

Unlike Length Overall or maximum beam, a vessel's Air Draft can change with its loading condition. When a ship is loaded more deeply, it sits lower in the water and its Air Draft generally decreases. When weight is discharged, the vessel rises and its Air Draft increases.

What Does Air Draft Mean in Shipping?

The IMO Compendium defines ship air draught as the height from the waterline to the highest point of the ship.

In practice, determining that highest point may require more detail than the basic definition suggests. Potential limiting points include:

• Mainmast or foremast

• Radar mast

• Communication antennas

• Funnel or exhaust structure

• Navigation lights

• Cargo cranes or derricks

• Retractable or folding equipment

A published vessel Air Draft may refer to a particular loading condition and equipment configuration. If an antenna, mast or crane can be raised, lowered or folded, operators must establish which configuration applies during the planned passage.

For safety-critical clearance calculations, Air Draft should come from approved vessel plans, an Air Draft diagram, verified ship particulars or measurements confirmed by the vessel’s master or operator.

How Is Air Draft Calculated?

If the vessel's height above the keel and its current draught use the same reference point, Air Draft can be expressed in simplified form as:

Air Draft = Height from keel to highest point − Current draught

For example, increasing the vessel's draught through cargo loading or ballasting normally lowers its highest point relative to the waterline. This reduces Air Draft by approximately the corresponding amount, assuming the vessel remains upright and has no material change in trim.

The actual calculation may be more complex because of:

• Forward or aft trim

• Vessel list or heel

• Different locations of draught marks

• Multiple possible highest points

• Retractable equipment

• Hull deflection

• Measurement uncertainty

The limiting point may also change with trim. If the highest structure is located near the bow or stern, using only the vessel’s mean draught may not accurately represent its height above the water at that location.

What Affects a Vessel's Air Draft?

Cargo and Ballast

Loading cargo, bunkers, water or ballast increases the vessel's displacement and normally increases its draught. As the hull sits deeper, the ship's Air Draft decreases.

Discharging weight has the opposite effect. The vessel rises in the water, increasing the height of its uppermost point above the waterline.

Ballasting may sometimes be considered as a way to reduce Air Draft before passing beneath an overhead obstruction. However, additional ballast also increases underwater draught and must remain within the vessel's stability, structural, load-line and under-keel-clearance limits.

Trim and List

Air Draft is affected not only by mean draught but also by the vessel's attitude.

Trim changes the relative height of structures located forward or aft. List or heel can cause an off-centre mast, antenna or crane to move vertically. Operators should therefore evaluate the actual loading condition rather than relying solely on a single standard Air Draft figure.

Masts, Antennas and Cargo Gear

Some vessels have folding masts, telescopic antennas, adjustable funnels or movable cargo equipment. Their minimum Air Draft may be substantially different from their normal operating Air Draft.

Any equipment that must be lowered should be secured in the correct position before the vessel enters the restricted area. Its status should be confirmed as part of the passage plan.

Tides and Water Levels

A change in tide does not, by itself, change the vessel's Air Draft for the same loading condition because the ship rises and falls with the surrounding water.

Tide does change the available vertical clearance beneath a fixed bridge or overhead structure. At a higher water level, the distance between the water surface and the obstruction becomes smaller. At a lower water level, more vertical clearance may be available.

River levels, lock levels, reservoir conditions, storm surge and other water-level changes can have the same effect.

Air Draft vs Related Ship Measurements

Air Draft vs Draught

Draught measures the part of the vessel below the waterline. Air Draft measures the part above it.

They often move in opposite directions:

• More loading generally increases draught and decreases Air Draft.

• Less loading generally decreases draught and increases Air Draft.

This relationship does not mean that cargo or ballast can be adjusted without restriction. Both underwater and overhead clearances must remain safe.

Air Draft vs Overall Height

Overall height is usually measured from a fixed point such as the keel or baseline to the vessel's highest point. It is primarily a structural dimension.

Air Draft uses the waterline as its lower reference, so it varies with loading. When the same keel reference is used:

Overall height ≈ Draught + Air Draft

The relationship may require corrections if the measurements use different reference points or if trim is significant.

Air Draft vs Freeboard

Freeboard is the vertical distance from the waterline to the freeboard deck at the vessel's side. It is an important loading and reserve-buoyancy measurement.

Air Draft extends to the ship's highest point and is therefore normally much greater than freeboard. The two terms should not be used interchangeably.

Air Draft vs Bridge Clearance

Air Draft is a vessel measurement. Bridge clearance is an infrastructure and water-level measurement.

A safe transit requires the available bridge clearance to exceed the vessel's maximum effective Air Draft, with an appropriate safety margin for uncertainty and expected operating conditions.

Why Is Air Draft Important?

Bridge and Overhead-Cable Transit

Air Draft is critical when planning a voyage through waterways with fixed bridges, power cables, pipelines or other overhead restrictions. Striking an overhead structure can damage the vessel, the obstruction and equipment carried above deck.

Port, Canal and Lock Planning

Ports, canals and inland waterways may impose maximum Air Draft limits. Operators must check the current or forecast water level, the controlling overhead clearance and any local operating requirements.

Length, beam and underwater draught must be assessed separately because a vessel may satisfy the height limit while exceeding another dimensional restriction.

Voyage and Passage Planning

The bridge-clearance assessment should identify:

• The vessel's verified maximum Air Draft

• Its expected loading and trim

• The controlling overhead obstruction

• Predicted tide or water level

• Charted or officially published clearance

• Required safety allowance

• Procedures for lowering equipment, if applicable

Changes in loading or water-level forecasts may require the calculation to be repeated before transit.

Vessel Design and Conversion

Naval architects consider Air Draft when designing vessels intended for routes with overhead restrictions. Folding masts, retractable antennas and other arrangements may help vessels operate beneath bridges.

Structural modifications or newly installed communication equipment can increase Air Draft. Updated measurements and documentation may therefore be required after a conversion.

FAQ

What is the Air Draft of a ship?

Air Draft is the vertical distance from the vessel's current waterline to its highest point, such as a mast, antenna or funnel.

Does Air Draft change when a ship is loaded?

Yes. Loading the ship normally increases its draught and reduces its Air Draft. Discharging weight normally causes the vessel to rise, increasing its Air Draft.

Does high tide increase a ship's Air Draft?

No. The ship rises with the water, so tide alone does not change its Air Draft. High tide reduces the available clearance beneath a fixed bridge or overhead structure.

What is the difference between draft and Air Draft?

Draft or draught measures from the waterline downward to the keel. Air Draft measures from the waterline upward to the ship's highest point.

Is Air Draft transmitted through AIS?

No. Air Draft is not a standard AIS static-data field. It should be obtained from verified vessel plans, ship particulars or the vessel's operator for clearance planning.