The Surveyor Signed Off on Weight. The Draft Calculation Was Wrong.
Quote from chief_editor on June 8, 2026, 5:30 pmDraft surveys are conducted by surveyors using published displacement tables and field measurements. Both sources of error affect the final weight certificate.
A coal cargo loaded at Newcastle, Australia — 87,500 tonnes declared on the bill of lading — arrived at Jingtang port in China with a draft survey weight of 85,820 tonnes. The difference: 1,680 tonnes, or 1.92%. The cargo was worth approximately $120 per tonne. The weight difference represented $201,600 in dispute.
The buyer submitted a claim. The seller responded that the bill of lading weight was based on a draft survey at load port, conducted by a reputable Australian marine surveyor. The discharge draft survey was conducted by a Chinese surveyor using the same vessel's displacement tables. Both surveyors followed standard procedures.
At this point, both parties had technically correct weight certificates that differed by nearly 2%. The question was not whether someone committed fraud — it was whether the measurement methodology could account for the difference, and which certificate governed payment under the contract.
Two Surveyors, Same Vessel, Different Results: The Sources of Discrepancy
Draft survey weight is calculated as the difference in vessel displacement between the arrival and departure drafts. Displacement is read from the vessel's hydrostatic tables — documents produced during the vessel's initial classification that relate draft measurements to displacement weight. These tables are vessel-specific and are produced at the time of the vessel's construction; they may be updated after major structural changes or drydocking.
The sources of discrepancy between load port and discharge port draft surveys include: differences in the density of the water at each port (denser seawater at one port produces higher buoyancy and affects displacement calculations), trim corrections (vessels that are not exactly level fore and aft require corrections that depend on precise draft readings at bow and stern), freshwater allowance corrections (different correction factors apply depending on whether the port's water is salt, brackish, or fresh), and differences in how carefully the surveyors actually read the drafts — reading from a small boat alongside a large vessel in a swell introduces measurement uncertainty.
Industry estimates for the expected precision of draft surveys under good conditions are commonly cited as plus or minus 0.3 to 0.5% of cargo weight. Under less favorable conditions — poor weather, difficult access to the vessel's draft marks, outdated or imprecise displacement tables — the uncertainty is higher. A 1.92% discrepancy between two draft surveys on the same cargo is at the upper end of what might be explained by measurement methodology alone, suggesting that either the conditions at one port introduced additional measurement error or that the cargo weight genuinely changed — through moisture loss from the coal during a multi-week Pacific transit, which is physically possible for coal cargoes with high surface moisture.
The Governing Weight Certificate and Its Consequences
As with cargo quality, the question of which weight certificate governs for payment purposes is answered by the contract, not by the relative merits of the two measurement methodologies. A contract that specifies load port draft survey as final pays on the load port figure. A contract specifying discharge port draft survey pays on the discharge figure. A contract that says nothing about governing weight methodology leaves the parties to dispute the question under the applicable law.
For a dispute of this size — $201,600 — formal arbitration is economically marginal but not irrational. Industry practice in the coal and iron ore trades is to negotiate these discrepancies commercially: a percentage-based allowance is agreed in the contract for weight variance, and discrepancies within the allowance are absorbed without claim. Discrepancies above the allowance trigger claims that are typically settled through negotiation rather than formal arbitration, with each party's actual measurement uncertainty bearing on the credibility of their certificate.
The buyer who does not understand that their discharge survey has its own measurement uncertainty, and who presents it as a precise fact against a potentially equally imprecise load port survey, is engaging in a negotiation where both sides have the same fundamental problem: neither measurement is perfectly accurate, and neither party can demonstrate that their result is closer to the true weight than the other.
Draft surveys are conducted by surveyors using published displacement tables and field measurements. Both sources of error affect the final weight certificate.
A coal cargo loaded at Newcastle, Australia — 87,500 tonnes declared on the bill of lading — arrived at Jingtang port in China with a draft survey weight of 85,820 tonnes. The difference: 1,680 tonnes, or 1.92%. The cargo was worth approximately $120 per tonne. The weight difference represented $201,600 in dispute.
The buyer submitted a claim. The seller responded that the bill of lading weight was based on a draft survey at load port, conducted by a reputable Australian marine surveyor. The discharge draft survey was conducted by a Chinese surveyor using the same vessel's displacement tables. Both surveyors followed standard procedures.
At this point, both parties had technically correct weight certificates that differed by nearly 2%. The question was not whether someone committed fraud — it was whether the measurement methodology could account for the difference, and which certificate governed payment under the contract.
Two Surveyors, Same Vessel, Different Results: The Sources of Discrepancy
Draft survey weight is calculated as the difference in vessel displacement between the arrival and departure drafts. Displacement is read from the vessel's hydrostatic tables — documents produced during the vessel's initial classification that relate draft measurements to displacement weight. These tables are vessel-specific and are produced at the time of the vessel's construction; they may be updated after major structural changes or drydocking.
The sources of discrepancy between load port and discharge port draft surveys include: differences in the density of the water at each port (denser seawater at one port produces higher buoyancy and affects displacement calculations), trim corrections (vessels that are not exactly level fore and aft require corrections that depend on precise draft readings at bow and stern), freshwater allowance corrections (different correction factors apply depending on whether the port's water is salt, brackish, or fresh), and differences in how carefully the surveyors actually read the drafts — reading from a small boat alongside a large vessel in a swell introduces measurement uncertainty.
Industry estimates for the expected precision of draft surveys under good conditions are commonly cited as plus or minus 0.3 to 0.5% of cargo weight. Under less favorable conditions — poor weather, difficult access to the vessel's draft marks, outdated or imprecise displacement tables — the uncertainty is higher. A 1.92% discrepancy between two draft surveys on the same cargo is at the upper end of what might be explained by measurement methodology alone, suggesting that either the conditions at one port introduced additional measurement error or that the cargo weight genuinely changed — through moisture loss from the coal during a multi-week Pacific transit, which is physically possible for coal cargoes with high surface moisture.
The Governing Weight Certificate and Its Consequences
As with cargo quality, the question of which weight certificate governs for payment purposes is answered by the contract, not by the relative merits of the two measurement methodologies. A contract that specifies load port draft survey as final pays on the load port figure. A contract specifying discharge port draft survey pays on the discharge figure. A contract that says nothing about governing weight methodology leaves the parties to dispute the question under the applicable law.
For a dispute of this size — $201,600 — formal arbitration is economically marginal but not irrational. Industry practice in the coal and iron ore trades is to negotiate these discrepancies commercially: a percentage-based allowance is agreed in the contract for weight variance, and discrepancies within the allowance are absorbed without claim. Discrepancies above the allowance trigger claims that are typically settled through negotiation rather than formal arbitration, with each party's actual measurement uncertainty bearing on the credibility of their certificate.
The buyer who does not understand that their discharge survey has its own measurement uncertainty, and who presents it as a precise fact against a potentially equally imprecise load port survey, is engaging in a negotiation where both sides have the same fundamental problem: neither measurement is perfectly accurate, and neither party can demonstrate that their result is closer to the true weight than the other.