Draft (ship)

In maritime terminology, the draft (also spelled draught in British English) of a ship refers to the vertical distance between the waterline and the lowest point of the hull (keel). It represents how deeply a vessel sits in the water and is one of the most important measurements in naval architecture and marine operations. The draft determines a ship’s navigational clearance, cargo capacity, and stability, influencing where and how the vessel can safely operate.

Definition and Measurement

The draft of a ship is the depth of water that the vessel requires to float without touching the seabed. It is measured from the bottom of the keel (the ship’s lowest structural point) to the waterline (the level at which the ship floats).
Mathematically:Draft = Vertical distance from waterline to bottom of keel
This measurement changes depending on the load, water density, and distribution of weight aboard the ship.
Draft is usually measured at three main points:

• Forward draft – at the bow (front)
• Aft draft – at the stern (rear)
• Mean draft – the average of forward and aft drafts

If a ship’s forward and aft drafts differ, the ship is said to be trimmed—either by the bow (front deeper) or by the stern (rear deeper).

Importance of Draft

The draft is a critical parameter for safe and efficient navigation. It affects almost every operational aspect of a vessel, including:

1. Navigational Safety – A vessel’s draft determines whether it can safely enter certain harbours, rivers, or channels without grounding. Shallow waters require ships with lighter drafts.
2. Cargo Capacity – The deeper a ship sits in the water (greater draft), the more cargo it can carry. Loading beyond the safe draft limit can endanger stability.
3. Stability and Performance – Draft influences a vessel’s centre of gravity, buoyancy, and hydrodynamic behaviour, affecting manoeuvrability, speed, and resistance.
4. Fuel Efficiency – A ship’s hydrodynamic resistance increases with draft, influencing fuel consumption and overall voyage economics.

Factors Affecting Draft

Several factors can alter a ship’s draft during a voyage:

• Load and Cargo Distribution: Heavier loads increase draft, while uneven distribution affects trim.
• Ballast Water: Ballast tanks are filled or emptied to adjust the ship’s draft and stability.
• Water Density: Ships float deeper in fresh water than in salt water because salt water is denser and provides greater buoyant force.
• Fuel and Provisions: As fuel and supplies are consumed, the ship’s draft decreases slightly.
• Weather and Waves: Heavy seas or uneven wave patterns can temporarily alter the apparent draft due to pitching or rolling.

Draft Marks

To monitor and control draft, ships have draft marks painted on the hull at the bow, midship, and stern. These marks are vertical numerical indicators (usually in decimetres or feet) that allow crew and port inspectors to quickly assess how deep the vessel is sitting in the water.
For example:

• If the draft marks read 9.2 metres at the stern and 8.8 metres at the bow, the mean draft would be 9.0 metres.

These readings are vital for calculating displacement and ensuring the ship does not exceed safe load limits.

Types of Draft

Depending on how and where it is measured, several specific types of draft are recognised in naval architecture:

1. Design Draft: The draft for which a ship is originally designed, balancing cargo capacity and performance.
2. Load Line Draft (Plimsoll Line): The maximum legal draft to which a ship can be loaded, marked on the hull by the Plimsoll line. It ensures safety under different sea conditions and water densities.
3. Deep Load Draft: The draft when the ship is fully loaded to its maximum permissible limit.
4. Light Draft: The draft when the vessel is empty or carrying minimal cargo and ballast.
5. Tropical, Summer, and Winter Drafts: Variants defined by the International Load Line Convention, reflecting differences in water density and climatic conditions. For example, a ship may have a smaller permissible draft in fresh water or winter conditions due to reduced buoyancy.
6. Mean Draft: The average of the forward and aft drafts, commonly used in ship stability and displacement calculations.
7. Trimmed Draft: When the draft differs between the bow and stern, the ship is said to have a trim, which affects handling and fuel efficiency.

Draft and Displacement

Draft and displacement are closely related concepts.

• Displacement is the weight of the volume of water displaced by the hull, equivalent to the ship’s total weight.
• The greater the draft, the larger the displaced volume, meaning a heavier load.

By reading draft marks, naval officers can calculate displacement and ensure compliance with safety regulations.

Relationship with the Plimsoll Line

The Plimsoll Line—a marking on the ship’s hull—indicates the maximum safe draft under varying conditions (freshwater, tropical seawater, winter seawater, etc.). Introduced in the 19th century by British politician Samuel Plimsoll, this system prevents overloading and ensures maritime safety by defining the legal limits for cargo weight.
Each line on the Plimsoll mark corresponds to a specific loading condition. Exceeding the marked draft makes a vessel unsafe and illegal to sail.

Draft in Port and Channel Operations

Port authorities and harbour pilots carefully consider a vessel’s draft before permitting entry into a port or channel.

• Maximum Allowable Draft (Channel Depth): Determines whether a vessel can safely navigate without running aground.
• Under-Keel Clearance (UKC): The minimum vertical distance between the ship’s keel and the seabed; maintaining adequate UKC is vital for safe navigation.

In shallow harbours, ships may need to lighten cargo, wait for high tide, or use dredged channels to avoid grounding.

Significance in Ship Design

For naval architects, draft is a central design parameter. It affects:

• Hull form and hydrodynamics
• Stability and buoyancy calculations
• Centre of gravity and trim adjustments
• Fuel economy and speed efficiency