Draft Survey and Determining Cargo Quantity
The method that determines, from the draught readings, how much cargo is actually on board: the basis of every commercial dispute over shortages or excess quantities.
Operational Explanation
The draft survey is the standard method for determining the quantity of cargo loaded or discharged by reading the draughts at the load line marks, corrected for trim, list, hull deflection (hog/sag) and water density, comparing the displacement before and after the operation.
Although it is a commercial method rather than a regulatory one, the draft survey is at the centre of a significant share of disputes between shipowner, charterer and cargo receiver: small reading or calculation errors, multiplied across the entire cargo, generate significant economic differences.
Regulatory Reference
The draft survey is not governed by any single specific convention, but follows established technical standards (typically ISO/NMSA guidelines or equivalent) and the contractual clauses of the individual charter party, which often specify the detailed procedure and the acceptable tolerances.
Scope of Application
Mainly dry cargo ships (bulk carrier, general cargo) where the cargo cannot be measured by other methods (e.g. flow meters as for liquids); applicable to every commercially significant loading/discharging operation.
Procedure / How to Complete It
- Read the draughts at the bow, stern and midships (on both sides if possible), before and after the loading/discharging operation.
- Correct the readings for trim, list and hull deflection (hog/sag).
- Determine the water density at the berth by direct sampling.
- Calculate the corrected displacement before and after the operation, subtracting the changes in ballast, fuel, fresh water and stores (constants).
- Compare the result with the quantity declared by the terminal/shore figures, documenting any significant discrepancies.
Practical Example
Example: a pre-loading draft survey shows a displacement of 12,450 t; post-loading, 24,680 t; after correcting for changes in ballast and fuel during the operation, the quantity of cargo loaded works out at 12,180 t, with a 0.3% discrepancy against the terminal's shore figures, within the contractual tolerance.
Real Cases
Common Mistakes Mistake Library
| Mistake | Consequence | How to avoid it |
|---|---|---|
| Imprecise draught readings, or readings taken by untrained personnel | A systematic error that propagates through the entire displacement calculation | Ensure the draft survey is conducted by experienced personnel, ideally with an independent surveyor present for commercially significant operations |
| Water density estimated rather than measured directly at the berth | Displacement calculation error, especially in areas with salinity variations (river mouths, port areas) | Always measure density by direct sampling at the specific location |
| Constants (ballast, fuel, fresh water, stores) not updated to reflect the actual situation | Inaccurate calculated cargo quantity | Verify and update all constants before every significant draft survey |
PSC Observations
Operational Tips
- For commercially significant operations, always request an independent surveyor for the draft survey, not just the ship's own personnel.
- Always measure the water density directly rather than estimating it, especially in areas with variable salinity.
- Precisely document all the constants used in the calculation, so the result can be defended in case of a dispute.
Checklist
- Draughts read at bow, stern and midships, before and after the operation
- Corrections applied for trim, list and hull deflection
- Water density measured directly at the berth
- Constants verified and updated
- Result compared with the shore figures and discrepancies documented