Draft Survey
A Draft Survey is a method of determining the weight of bulk cargo loaded onto or discharged from a ship by measuring the vessel's displacement — read from the draft marks — before and after the operation. The difference in displacement, corrected for water density and for the weight of fuel, water and stores, gives the cargo weight.
Draft surveys are the standard way to weigh homogeneous bulk commodities such as coal, ore, grain and fertiliser, where weighbridges are impractical. Carried out by an independent surveyor, the survey is the basis for the bill of lading quantity and for settling accounts between shipper, receiver and carrier. Accuracy depends on careful draft readings on both sides, and precise measurement of the deductibles (bunkers, ballast, fresh water) on board.
For a bulk cargo worth millions, the draft survey is how the tonnage — and therefore the money — is agreed. There is no weighbridge for 50,000 tonnes of ore; the ship itself is the scale. A careful draft survey protects shipper and receiver alike, which is why it is done independently and forms the basis of the bill of lading quantity.
How does a draft survey measure cargo weight?
By reading the ship's displacement from its draft before and after, then correcting for water density and for bunkers, ballast and stores.
When is a draft survey used?
For homogeneous bulk cargoes — coal, ore, grain, fertiliser — where weighing each parcel is impractical.