When bulk cargo such as grain, fertiliser, clinker or mineral concentrate is loaded onto a vessel, the parties need an agreed figure for how much was shipped. A draft survey is one of the most widely used ways to establish that figure.
What is a draft survey?
A draft survey determines the weight of cargo by measuring how deep the vessel sits in the water before and after loading (or discharge). Using the vessel's hydrostatic tables, the surveyor converts the readings into the vessel's displacement. The difference between the two displacements, after adjustments, gives the weight of cargo loaded or discharged.
What the surveyor checks
- Draft readings at forward, midship and aft marks on both sides of the vessel
- Density of the water the vessel is floating in
- Ballast water, fresh water and fuel quantities on board
- Corrections for trim, list and hull deflection
- The vessel's constant (weights not otherwise accounted for)
Why independence matters
Small errors in a reading or a correction can translate into many tonnes of cargo. When the survey is carried out by an independent surveyor, shippers, receivers, vessel owners, insurers and financiers can all rely on the same figure. That shared, impartial record is often what prevents a quantity dispute from turning into a costly claim.