What Is Draught?
Draught (also spelled Draft in American English) is the vertical distance between the waterline and the lowest point of a vessel's hull, usually the bottom of the keel. It indicates how deeply a ship sits in the water and is one of the most important measurements in ship design, navigation, and port operations.
A vessel's draught changes as cargo, fuel, ballast water, and supplies are loaded or discharged. The deeper a ship sits in the water, the greater its draught.
In vessel tracking and maritime databases, draught is commonly included as part of a vessel's technical or voyage information.
Why Is Draught Important?
Knowing a vessel's draught is essential for safe navigation and operational planning.
Safe Navigation
A ship's draught must always be less than the available water depth to avoid grounding. Before entering ports, rivers, canals, or shallow coastal waters, ship operators verify that sufficient under-keel clearance is available.
Port and Waterway Access
Many ports, terminals, canals, and inland waterways impose maximum allowable draught limits. A vessel exceeding these limits may be unable to enter or transit safely.
Cargo Loading
As cargo is loaded, the vessel sinks deeper into the water and its draught increases. Monitoring draught helps ensure the vessel remains within permitted loading limits and complies with applicable safety requirements.
Voyage Planning
Masters, operators, and voyage planners consider draught when selecting routes, scheduling port calls, and planning ballast operations.
How Is Draught Measured?
Draught is typically measured in meters or feet, depending on local practice.
Measurements are taken from the waterline to the lowest point of the keel.
Most ships display draught marks on both the bow and stern, allowing crews and port personnel to determine the vessel's draught visually.
For operational purposes, ships may record:
- Forward Draught – Measured near the bow
- Aft Draught – Measured near the stern
- Mean Draught – The average of the forward and aft draughts
Why Does a Ship's Draught Change?
A vessel's draught is not fixed. It changes throughout a voyage due to several factors.
Cargo Operations
Loading cargo increases the vessel's displacement, causing it to sit deeper in the water. Discharging cargo reduces draught.
Ballast Water
Ships adjust ballast water to maintain stability, trim, and safe operating conditions. Ballasting and deballasting directly affect draught.
Fuel and Fresh Water Consumption
As fuel, freshwater, and other consumables are used during a voyage, the vessel becomes lighter, which may slightly reduce its draught.
Water Density
A vessel generally floats slightly deeper in fresh water than in salt water because freshwater is less dense. This difference is considered during voyage planning and loading operations.
Draught in AIS and Vessel Tracking
Many AIS-based vessel tracking platforms display a vessel's reported draught as part of its voyage information.
Depending on the vessel and AIS transmission, users may see data such as:
- Current draught
- Destination
- Estimated Time of Arrival (ETA)
- Navigation status
- Speed Over Ground (SOG)
- Course Over Ground (COG)
It is important to note that draught information transmitted via AIS is manually entered by the vessel's crew. As a result, it may not always reflect the vessel's actual draught at a given moment and should be interpreted accordingly.
Draught vs Air Draught
These two terms are often confused but refer to different measurements.
| Term | Definition |
|---|---|
| Draught (Draft) | Distance from the waterline to the bottom of the keel |
| Air Draught | Distance from the waterline to the highest fixed point of the vessel |
Draught is used to determine whether there is sufficient water depth, while air draught is important when passing beneath bridges, overhead cables, or other height restrictions.
Draught vs Depth
Another common source of confusion is the difference between draught and depth.
| Term | Definition |
|---|---|
| Draught | How deeply the vessel is submerged in the water |
| Depth | The vertical distance from the keel to the highest continuous deck (a structural measurement of the vessel) |
Depth is a design characteristic of the ship, while draught varies depending on loading and operating conditions.
Conclusion
Draught is one of the most important measurements in maritime navigation, influencing vessel safety, cargo loading, voyage planning, and port accessibility. Whether monitoring ships through AIS or planning port operations, understanding draught helps maritime professionals make informed operational decisions.
Together with navigation data such as SOG, COG, ETA, and vessel position, draught provides valuable insight into a vessel's operational condition and voyage status.
Frequently Asked Questions
What does draught mean in shipping?
Draught is the vertical distance between the waterline and the bottom of a vessel's keel. It indicates how deeply the ship is submerged.
Why is draught important?
Draught helps determine whether a vessel can safely navigate through ports, channels, rivers, and other waterways without grounding.
Does draught change during a voyage?
Yes. Draught changes as cargo, ballast water, fuel, and other onboard weights change.
Is draught information in AIS always accurate?
AIS may display a vessel's reported draught, but because it is manually entered by the crew, it may not always represent the vessel's current actual draught.
What is the difference between draught and air draught?
Draught measures the portion of the vessel below the waterline, while air draught measures the height of the vessel above the waterline.