Quantity

Draft survey: weighing cargo by displacement

Published 28 August 2026 · updated 22 September 2026

A draft survey does not weigh cargo. It weighs the ship, loads her, weighs her again, and calls the difference cargo. Everything that makes the method useful and everything that makes it argued over follows from that one sentence: the quantity is never measured directly, it is inferred from how much deeper the hull sits.

The method

The surveyor reads the draught at six marks, forward, midships and aft on both sides, and takes the mean. Those readings are corrected before they are used, because a ship is not a rigid box sitting level:

• Trim, the difference between forward and aft draught, requires a correction because the draught marks are not at the perpendiculars and the centre of flotation is not amidships.
• Hog and sag, the bending of the hull, is why the midships readings are taken at all. The mean of means gives a better answer than a simple average of forward and aft.
• Density. Hydrostatic tables are computed for a standard density, so a sample of the dock water is taken and the displacement corrected to the density actually floating her.

The corrected mean draught is taken into the ship’s hydrostatic tables, which return displacement. From that displacement the surveyor deducts everything aboard that is not cargo: ballast, bunkers, fresh water, lubricating oil, stores, and the constant, which is the catch-all for the weight the ship carries that nobody itemises. What remains is the light ship plus cargo, and the difference between the initial and final surveys is the cargo figure.

Where the error comes from

The West of England P&I Club’s loss prevention guidance is blunt about the precision on offer: draught surveys are not an exact science, and the accuracy of the calculated cargo figure may typically vary by between 0.5 and 1.0 per cent. On a Panamax cargo, one per cent is a number big enough to fund a dispute.

The Club’s worked example makes the leverage plain. An error of two centimetres in the mean draught moves the displacement by roughly 68 tonnes on a Handysize and by about 300 tonnes on a Capesize. Two centimetres is not a careless reading. It is a small swell, a rusted mark, or a survey taken from the quay instead of from a boat.

Two honest figures that disagree

The shore has its own answer: a weightometer on the conveyor, a weighbridge on the trucks, or a tally. Ship and shore figures will not match, and the interesting part is that neither is automatically wrong. They measure different things by different methods with different error bands, and a difference inside a fraction of a per cent is the normal condition of the trade rather than evidence of loss.

What matters commercially is which figure the contract makes binding. Freight is usually earned on the bill of lading quantity, and the bill of lading is normally signed on the shore figure. If the master believes that figure overstates what came aboard he does not simply refuse: he issues a letter of protest, records the ship’s own figure, and preserves the position. A protest costs nothing at the time and is often the only contemporaneous document that exists when a shortage claim arrives eighteen months later.

Why the discharge survey is the one that hurts

A shortage claim is the discharge port figure set against the bill of lading figure. If the loading port bill was signed on a generous shore figure and the discharge port runs a careful draft survey, the ship appears to have lost cargo she never carried. Nothing was stolen and nothing was spilled: two measurement systems were compared as if they were the same instrument.

The defences are all documentary and all cheap if done at the time. A draft survey at load, a protest against any figure the master disputes, the density record, the ballast soundings, and the survey working sheets themselves. The parties who lose these arguments are usually the ones who have the answer and cannot prove they had it then.

The clauses that decide it

Because the method has a known error band, the contract is where the tolerance lives. A sale contract commonly sets a franchise, a percentage inside which no claim lies. A charterparty may name whose survey governs, whether a joint survey is required, and who pays the surveyor. These are negotiated terms and they are worth reading before the cargo moves rather than after.

The standardised procedure most often referred to is the United Nations Economic Commission for Europe code for draught surveys of coal cargoes, written for coal and used far beyond it because nothing better is public. It exists precisely because uniform forms and a uniform sequence remove the discrepancies that otherwise become commercial disputes.

The one thing to carry away

A draft survey figure is an inference from a displacement, and an inference is only as good as the deductions made from it. When a quantity is in dispute, the question is almost never whether somebody read a mark wrongly. It is whether the ballast, the constant and the density were the same at both ends of the subtraction.

References

• West of England P&I Club, Loss Prevention Bulletin: Inaccuracies in Draught Surveys. westpandi.com
• Britannia P&I, Loss prevention guidance: preventing bulk shortage claims, 2025. britanniapandi.com
• United Nations Economic Commission for Europe, Code of Uniform Standards and Procedures for the Performance of Draught Surveys of Coal Cargoes, 1992.
• IMO, International Convention on Load Lines, 1966. imo.org
• IMO, International Maritime Solid Bulk Cargoes (IMSBC) Code. imo.org

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