Displacement and deadweight sound similar, but they serve very different roles in a draft survey. Understanding the distinction is essential because only one of these values can be used when applying density, trim and list corrections.
Displacement is the total weight of water the vessel pushes aside when it floats. By Archimedes’ Principle, this is the vessel’s true weight. It includes the light ship, cargo, ballast, bunkers, fresh water and every other weight on board.
Deadweight, on the other hand, represents the vessel’s carrying capacity. It covers cargo, fuel, water and stores, and is calculated by subtracting the light ship weight from the displacement.
In simple form:
Displacement = Light Ship Weight + Deadweight
For draft survey work, the key rule is that density corrections must always be applied to displacement. Using deadweight would leave out the light ship weight, which is a permanent and substantial part of the vessel’s mass. This would distort the corrected results and lead to inaccurate cargo calculations.
If a vessel’s documents show only deadweight, the surveyor must add the light ship weight to obtain the correct displacement before making any corrections. Mistaking a deadweight scale for a displacement scale is a common source of error, and one that directly affects the accuracy of the final cargo figure.
The draft survey ultimately relies on changes in displacement, corrected for all variables, to isolate the true weight of cargo moved.
