1.3 Introduction to Draught Marks and Load Lines on Merchant Vessels
A merchant ship’s safe passage at sea relies heavily on a delicate balance between a vessel's weight and its buoyancy.
Draught marks and load lines are the essential markings that ensure this balance is maintained.
This chapter looks their purpose, design, and how to interpret them for safe and legal operation of merchant vessels.
Draft: When a ship floats in water, the draft is the depth of the hull which is submerged, measured from the waterline to the lowest point of the keel.
Freeboard: This is the vertical distance between the waterline and the uppermost continuous deck (the main deck or weather deck) - essentially, the part of the hull that remains above water.
Maintaining sufficient freeboard is crucial for the safe operation of any vessel. An overloaded ship sits lower in the water, reducing its stability and increasing the risk of flooding, compromising buoyancy and potentially leading to disaster.
However, a ship's draft is not static and can fluctuate depending factors including:
Cargo Loading: The primary influence on draft is cargo weight. As a ship loads cargo, it displaces water and sinks lower. Conversely, unloading cargo allows it to rise in the water.
Fuel and Supplies: Changes in fuel and onboard supplies also affect draft. Burning fuel consumes weight, causing the ship to rise slightly. Similarly, taking on new supplies adds weight and increases draft.
Water Density: Saltwater is denser than freshwater. Ships entering freshwater from saltwater will experience a slight rise in draft due to the increased buoyancy.
Temperature: Water density also varies with temperature. Colder water is denser, causing a slight decrease in draft compared to warmer waters.
Draft marks provide a real-time snapshot of the ship's draft, while load lines indicate the maximum permissible draft for specific conditions.
Together, they form a comprehensive system for safe and legal cargo loading and maritime transport.