A draft survey is an independent method of determining cargo weight by reading a vessel's draft marks before and after loading or discharge and calculating the change in displacement. It is widely used for dry bulk cargoes, either as an alternative to shore scale figures or to cross-check them.
What is Draft Survey?
A draft survey starts with reading the forward, aft and midship draft marks on both sides of the vessel. These readings are corrected for trim, hull deflection (hog/sag) and the density of the dock water to calculate the vessel's displacement at that moment.
The same procedure is carried out before and after the operation. After deducting variable weights on board such as ballast, fresh water and bunkers (the deductibles), the difference between the two displacements gives the quantity of cargo loaded or discharged.
The calculation relies on the vessel's approved hydrostatic tables and tank calibration tables. A widely used reference is the UN/ECE Code of Uniform Standards and Procedures for the Performance of Draught Surveys of Coal Cargoes; although written for coal, its methodology is commonly applied to other dry bulk cargoes as well.
A draft survey is a calculation method, and the reliability of the result depends on the conditions at the time of survey: swell, current, the accuracy of ship's documents and tank soundings. The survey conditions and any limitations are therefore stated clearly in the report.
Why does it matter?
In dry bulk trade, bill of lading quantity, invoice value and freight are usually tied to tonnage. Where shore scale figures are unavailable or disputed, a draft survey provides an independent quantity determination from the vessel itself.
Comparing load port and discharge port draft survey results is an important input when assessing alleged in-transit shortages. Carrying out readings jointly with the parties and recording the conditions reduces the scope for later objections.
Interim draft readings also help monitor trim and cargo distribution against the loading plan, making it easier to adjust the final quantity towards the intended figure.
Who is it for?
- Exporters and importers of dry bulk cargo such as ores, grain, coal, fertiliser and clinker
- Parties wishing to independently verify the bill of lading quantity
- Operations without shore scale figures, or where a comparison with scale figures is required
- Shipowners, charterers and cargo insurers
- Traders whose sales contract requires independent quantity determination
Scope of service
- Initial and final draft readings
- Reading forward, aft and midship draft marks on both sides
- Measuring dock water density with a hydrometer
- Sounding ballast, fresh water and bunker tanks, or recording ship's soundings
- Displacement calculation from hydrostatic tables with trim, deflection and density corrections
- Determining the vessel's constant and comparing it with previous records
- Interim draft surveys where required
- Report including the calculation sheet and survey conditions
The exact scope is agreed at the quotation stage according to the nature of the operation.
Key checkpoints
- 01Reading conditions
- Swell, current, list and any difference between the quay side and offshore side are assessed; readings are taken in the calmest conditions available and repeated if necessary.
- 02Water density
- Density samples are taken at a depth close to the vessel's draft and at suitable points along her length; in ports affected by river outflow or fresh water, density variation is given particular attention.
- 03Tank soundings
- Ballast and fresh water tanks are sounded and trim corrections applied from the calibration tables; tanks that cannot be sounded or confirmed full or empty are noted in the report.
- 04Ship's documents
- The surveyor confirms that hydrostatic and calibration tables belong to the vessel and are approved and current; the distances of the draft marks from the perpendiculars are used in the corrections.
- 05Consistency of the constant
- The constant found at the initial survey is compared with the vessel's previous records; an unusual value may indicate a reading or tank-related problem.
How does the process work?
- 01
Preparation
Vessel particulars, cargo type and operation plan are obtained, together with the vessel's hydrostatic tables and tank calibration documents.
- 02
Initial survey
Before operations start, draft readings, water density and tank soundings are taken jointly with the ship's officers and the constant is calculated.
- 03
Interim survey (if required)
Where requested or required by the loading plan, interim readings are taken during the operation to monitor progress.
- 04
Final survey
On completion, the same measurements are repeated and weather and sea conditions at the time are recorded.
- 05
Calculation and report
Cargo quantity is calculated from the difference in displacement and reported together with the calculation sheet and observations.
Information we need from you
- Vessel name, IMO number and port
- Cargo type and declared quantity
- Estimated commencement and completion times
- Party in whose name the report is to be issued
- Any survey method or special instructions specified in the contract
Reporting
- Initial and final draft survey report
- Displacement calculation sheet
- Survey conditions and observation notes
- Interim draft survey records (on request)
Report content and format are agreed according to your request.
Key terms
- Displacement
- The weight of water displaced by the floating vessel, equal to the total weight of the ship, cargo, bunkers, ballast and everything else on board.
- Trim
- The difference between forward and aft drafts. A trim correction is applied so that displacement is calculated as if the vessel were on an even keel.
- Hog / Sag
- Longitudinal deflection of the hull. When the midship section rises it is hogging; when it sinks it is sagging. It is detected from the midship draft and corrected for.
- Deductibles
- Measurable non-cargo weights such as ballast water, fresh water, fuel and lubricating oil, which are deducted from displacement when calculating cargo quantity.
- Constant
- The sum of weights on board other than lightship and measurable deductibles, such as unpumpable residues, stores and spare gear.
Frequently asked questions
What does a draft survey actually measure?
It measures the vessel's draft, not the cargo directly. Cargo quantity is calculated from the change in displacement before and after the operation, after deducting variable weights on board.
Why can draft survey results differ from shore scale figures?
Each method has its own measurement tolerance. Swell, current, errors in ship's documents and tank soundings can all affect a draft survey result, which is why survey conditions are stated in the report.
Which cargoes is a draft survey suitable for?
It is generally used for dry bulk cargoes where determining weight from the vessel is practical. Suitability for a particular cargo is assessed at the quotation stage.
Do the ship's officers need to take part?
Yes. Tank soundings and access to ship's documents normally require working together with the ship's officers, and readings are agreed between the parties.
Can a draft survey be carried out in bad weather?
Swell and current make draft reading more difficult. Readings may be repeated and averaged, or the survey may be postponed to a better time by agreement with the parties; the conditions are stated in the report.
Why does the constant matter?
The constant directly affects the cargo calculation. Checking whether the initial value is consistent with the vessel's previous records helps reveal reading or tank-related errors.