Draft Survey and Determining Cargo Quantity
A draft survey determines cargo quantity by comparing displacement before and after the operation, accounting for changes in non-cargo weights. Readings, corrections and deductions must be traceable; comparison with shore figures, tolerance and surveyor appointment follow the protocol and applicable conditions.
Operational Explanation
A draft survey derives cargo quantity from the change in the ship’s displacement. Draughts are read at the forward, aft and midship draught marks on both sides, using safe access and the adopted protocol. The readings and the ship’s hydrostatic tables give initial and final displacement after the relevant corrections for trim, list, hull deflection (hog/sag) and water density.
The displacement difference alone is not yet the cargo: changes in ballast, fuel, fresh water and other non-cargo weights recorded at both surveys must be accounted for. Identify lightship and the ship’s constant separately, avoiding deduction of the same weight twice. The report must allow readings, conditions, tables and deductions to be traced; the example below expressly assumes unchanged lightship and constant.
Regulatory Reference
UNECE — Uniform Code of Draught Survey (1992), coal cargoes; Britannia P&I — Draft Survey Calculation. The UNECE code concerns coal; use for other bulk cargoes should be stated and agreed. Check the contract and applicable provisions for method, appointment and tolerances. A dispute may also involve legal duties and the evidentiary value of documents; it is not necessarily purely contractual.
The UNECE code, ECE/ENERGY/19 (3 February 1992), Part 1, page 2, describes accuracy within 0.5% conditional on suitable approved ship information and work by qualified, experienced surveyors. This technical statement does not establish a universal contractual tolerance for differences against shore figures. Keep method accuracy, actual survey uncertainty and the agreed acceptance criterion separate. UN Digital Library — ECE/ENERGY/19, Part 1, p. 2.
Scope of Application
Operations where the charter party or other contract, Company procedure or agreement uses a draft survey to determine quantity. The UNECE Code is specific to coal cargoes; use by analogy must be agreed and stated.
Procedure / How to Complete It
- Identify the agreed contract, method, protocol and tolerance.
- Document drafts, density, constants, ballast, fuel and consumables with the associated uncertainty.
- Compare results on the agreed basis, not against a universal threshold.
- Investigate discrepancies against contract, conditions and documented uncertainty.
- Assess an independent surveyor by contract, cargo value, dispute risk and Company procedure.
Practical Example
Illustrative loading example: corrected displacements are initially 12,450 t and finally 24,680 t, a difference of 12,230 t. If total non-cargo weights increase by 50 t, with unchanged lightship and constant, cargo = 12,230 − 50 = 12,180 t. Against a hypothetical shore figure of 12,217 t, the difference relative to that figure is about −0.30%. Acceptability requires the agreed tolerance; the number alone does not establish contractual compliance.
What Typically Goes Wrong
Common Mistakes Mistake Library
| Mistake | Consequence | How to avoid it |
|---|---|---|
| Confusing the ship’s constant with variable ballast and consumables | Incorrect non-cargo deductions distort the cargo quantity | Measure ballast and consumables at initial and final surveys and check lightship and constant separately |
| Changing weights during readings | Draught readings and deductions may describe different ship conditions | Coordinate operations and suspend ballast and bunker transfers during the survey |
| Using assumed densities or wrong correction signs | Corrected displacement and resulting quantity are unreliable | Document samples and instruments and verify ship-table conventions |
What the PSCO Checks
PSC checks safety and applicable statutory requirements, including stability, loading and ship limits. It does not normally certify commercial quantity or resolve a discrepancy between draught survey and shore figures.
Operational Tips
- Do not use 0.5% as an international default.
- Agree method, joint readings and discrepancy handling before the operation.
- Independent appointment is mandatory only where required by contract or another applicable procedure; otherwise it is a risk-management choice.
Preparation checklist
- Charter party/contract checked
- Protocol and tolerance agreed
- Uncertainty and conditions documented
- UNECE applicability to coal or analogous use stated
- Surveyor appointment checked against contract and risk
FAQ
Related Topics
Certified substantive revision: 15 October 2026 · page fingerprint 811e1b3355a4