What Is a Draft Survey and How Is It Carried Out Step by Step?

A floating body displaces a weight of water equal to its own weight. As a ship takes on cargo she sinks deeper, and her displacement increases with her draught. The vessel's hydrostatic tables, produced by the shipyard, give the displacement corresponding to each draught.

The draft survey uses this relationship by establishing the total weight of the ship at the start and at the end of the operation. Once the non-cargo variable weights are deducted, the difference between the two figures is the weight of cargo loaded or discharged.

The method is standard for large, homogeneous dry bulk parcels such as iron ore, coal, grain, fertiliser and clinker. It becomes particularly important where there is no shore scale, where the accuracy of a belt weigher is disputed, or where the sales contract calls for an independent determination of quantity.

For bagged, unitised or break bulk cargo, quantities can usually be tracked by piece count and unit weight, so draft surveys are used less often. For small parcels the measurement uncertainty can be large relative to the cargo quantity, so the suitability of the method should be considered case by case.

The initial survey takes place before cargo operations begin. The surveyor and the ship's officers read the six draught marks together, take a dock water sample to measure density and sound the ballast, fresh water and bunker tanks.

The vessel's constant is also established at this stage. The constant represents unrecorded weights on board, the difference between the light ship plus known weights and the measured displacement, and is treated as unchanged for the rest of the calculation.

When loading or discharging is complete, the same procedure is repeated. Trim, deflection and density corrections are applied to the observed draughts to obtain the corrected displacement, and the deductibles such as ballast, fresh water and bunkers are subtracted.

On loading, the initial net weight is subtracted from the final net weight; on discharge the calculation is reversed. The result is the ship's figure for the cargo, which is normally compared with the B/L figure or the shore scale weight.

An interim draft survey may be carried out during long loading operations, where a cargo is split between several receivers or bills of lading, or where the loading plan requires a stop at a specific tonnage. It shows whether the operation is progressing towards the intended quantity.

Interim readings near the end of loading also help plan the final trimming pours, so that the vessel reaches her target draught and trim in a controlled way rather than by trial and error.

A handful of concepts is enough to follow the method. They appear throughout the report and in discussions with the ship's officers, and knowing them makes it easier to see what each stage of the calculation is doing.

The distinction between the constant and the deductibles matters most. Deductibles are measured afresh at every survey, whereas the constant is calculated once at the initial survey and assumed not to change during the operation.

The report records the observed draughts, dock water density, tank soundings, the corrections applied, the constant and the calculated cargo quantity. Weather, swell and current at the time of the readings, and any observations that may have affected accuracy, are noted as well.

Having the report countersigned by the ship's officers shows that the survey was carried out jointly and reduces the scope for later objections. Karan Gözetim provides draft survey services from its base in Gemlik; the scope of work and the party the report is issued to are agreed at quotation stage.