Draft Survey: How Bulk Cargo Weight Is Measured at Sea
Quote from chief_editor on June 6, 2026, 6:52 pmHow draft surveys determine bulk cargo quantity at sea, the mechanics of displacement calculation, and common sources of error that lead to shortlanding disputes.
A draft survey is the standard method for determining the quantity of bulk cargo loaded onto or discharged from a vessel. It works by measuring the vessel's displacement before and after cargo operations—derived from the depth at which the hull sits in the water—and using the change in displacement to calculate cargo weight. Most bulk commodity contracts specify that the loading port draft survey is the final basis for invoiced quantity, making an understanding of how the survey works, and how errors arise, directly relevant to any trader buying by the shipload.
The Mechanics of Draft Survey
Every ocean-going vessel has draft marks—scales painted on the hull at defined positions, typically at six points: forward, midships, and aft on both the port and starboard sides. These marks show how deeply the hull is submerged. A survey crew reads all six marks before loading begins and again after loading is complete.
From the draft readings, the surveyor derives the vessel's displacement at each stage using the ship's hydrostatic tables—engineering data specific to that hull that convert draft to displacement in tonnes. The change in displacement between the two surveys represents the total weight added to the vessel.
The change in displacement includes more than cargo. The surveyor must also account for changes in ballast water, bunker fuel, fresh water, lubricating oil, and ship's stores between the two surveys. These deductions are derived from tank soundings taken at each stage, declared quantities from the vessel's stability computer, and sometimes direct measurement. The cargo weight is the change in total displacement minus these deductions.
Fresh water allowance is an additional correction. Seawater is denser than fresh water or brackish water, so the same vessel floats at different drafts in different water densities. If loading occurs in a river port or an estuary where water density is below the standard seawater density of 1.025 tonnes per cubic meter, the surveyor must apply a correction. Failure to apply this correction accurately produces a systematic error in the cargo weight figure.
Where Draft Survey Errors Arise
Draft surveys are accurate to approximately 0.3 to 0.5 percent in good conditions—calm water, stable vessel trim, no wind-driven list, and clear visibility of draft marks. Under real-world conditions, the tolerance is wider.
Ballast deduction errors are the most common source of significant variance. If the vessel's crew reports less ballast than was actually in the tanks before loading, the calculated displacement change will be inflated and the cargo weight will appear higher than it was. A careful surveyor measures ballast tanks independently—taking ullage readings and comparing against the tank's calibration tables—rather than relying entirely on crew declarations or the stability computer's logged values.
Reading errors occur when draft marks are obscured by fouling, paint, or waterline debris, or when the hull's trim makes it difficult to read midships marks accurately. In swell conditions, readings must be taken at the moment the hull is at its mean position, which requires experience and judgment. Two surveyors reading the same vessel simultaneously often reach slightly different figures.
Discrepancies between loading port and discharge port draft surveys are common. The loading figure is used for invoicing; the discharge figure, if lower, generates a shortlanding claim. The difference often reflects accumulated measurement errors at both ends rather than actual cargo loss, but disputes still arise. Buyers and sellers who want to avoid this should specify in the contract whether the loading or discharge figure governs, and appoint an independent surveyor—not the ship's agent—to perform each survey.
For certain cargo types, draft survey is less appropriate than alternative methods. For bagged goods loaded in containers, certified scale weights are more accurate. For liquid bulk at terminals with calibrated shore tanks, ullage measurement or flow meters provide a more precise figure. For coal or grain loaded through hoppers, belt scale totals can supplement or replace draft survey where the equipment is properly calibrated and certified.
The legal status of the draft survey certificate depends on the contract. Under most GAFTA voyage contracts, the loading port draft survey is binding on both parties unless fraud or manifest error can be demonstrated. A buyer who accepts loading port as final weight cannot subsequently dispute the quantity simply because the discharge port survey shows a lower figure—unless the difference exceeds the contractual tolerance or there is evidence of procedural error at loading.
For large transactions, buyers are well served by appointing their own surveyor to attend alongside the seller's appointed surveyor, rather than relying on a joint result from a single firm engaged by the vessel's agent. When both parties' surveyors are present and sign the same statement of facts, disputes about the survey procedure itself are much harder to sustain later.
How draft surveys determine bulk cargo quantity at sea, the mechanics of displacement calculation, and common sources of error that lead to shortlanding disputes.
A draft survey is the standard method for determining the quantity of bulk cargo loaded onto or discharged from a vessel. It works by measuring the vessel's displacement before and after cargo operations—derived from the depth at which the hull sits in the water—and using the change in displacement to calculate cargo weight. Most bulk commodity contracts specify that the loading port draft survey is the final basis for invoiced quantity, making an understanding of how the survey works, and how errors arise, directly relevant to any trader buying by the shipload.
The Mechanics of Draft Survey
Every ocean-going vessel has draft marks—scales painted on the hull at defined positions, typically at six points: forward, midships, and aft on both the port and starboard sides. These marks show how deeply the hull is submerged. A survey crew reads all six marks before loading begins and again after loading is complete.
From the draft readings, the surveyor derives the vessel's displacement at each stage using the ship's hydrostatic tables—engineering data specific to that hull that convert draft to displacement in tonnes. The change in displacement between the two surveys represents the total weight added to the vessel.
The change in displacement includes more than cargo. The surveyor must also account for changes in ballast water, bunker fuel, fresh water, lubricating oil, and ship's stores between the two surveys. These deductions are derived from tank soundings taken at each stage, declared quantities from the vessel's stability computer, and sometimes direct measurement. The cargo weight is the change in total displacement minus these deductions.
Fresh water allowance is an additional correction. Seawater is denser than fresh water or brackish water, so the same vessel floats at different drafts in different water densities. If loading occurs in a river port or an estuary where water density is below the standard seawater density of 1.025 tonnes per cubic meter, the surveyor must apply a correction. Failure to apply this correction accurately produces a systematic error in the cargo weight figure.
Where Draft Survey Errors Arise
Draft surveys are accurate to approximately 0.3 to 0.5 percent in good conditions—calm water, stable vessel trim, no wind-driven list, and clear visibility of draft marks. Under real-world conditions, the tolerance is wider.
Ballast deduction errors are the most common source of significant variance. If the vessel's crew reports less ballast than was actually in the tanks before loading, the calculated displacement change will be inflated and the cargo weight will appear higher than it was. A careful surveyor measures ballast tanks independently—taking ullage readings and comparing against the tank's calibration tables—rather than relying entirely on crew declarations or the stability computer's logged values.
Reading errors occur when draft marks are obscured by fouling, paint, or waterline debris, or when the hull's trim makes it difficult to read midships marks accurately. In swell conditions, readings must be taken at the moment the hull is at its mean position, which requires experience and judgment. Two surveyors reading the same vessel simultaneously often reach slightly different figures.
Discrepancies between loading port and discharge port draft surveys are common. The loading figure is used for invoicing; the discharge figure, if lower, generates a shortlanding claim. The difference often reflects accumulated measurement errors at both ends rather than actual cargo loss, but disputes still arise. Buyers and sellers who want to avoid this should specify in the contract whether the loading or discharge figure governs, and appoint an independent surveyor—not the ship's agent—to perform each survey.
For certain cargo types, draft survey is less appropriate than alternative methods. For bagged goods loaded in containers, certified scale weights are more accurate. For liquid bulk at terminals with calibrated shore tanks, ullage measurement or flow meters provide a more precise figure. For coal or grain loaded through hoppers, belt scale totals can supplement or replace draft survey where the equipment is properly calibrated and certified.
The legal status of the draft survey certificate depends on the contract. Under most GAFTA voyage contracts, the loading port draft survey is binding on both parties unless fraud or manifest error can be demonstrated. A buyer who accepts loading port as final weight cannot subsequently dispute the quantity simply because the discharge port survey shows a lower figure—unless the difference exceeds the contractual tolerance or there is evidence of procedural error at loading.
For large transactions, buyers are well served by appointing their own surveyor to attend alongside the seller's appointed surveyor, rather than relying on a joint result from a single firm engaged by the vessel's agent. When both parties' surveyors are present and sign the same statement of facts, disputes about the survey procedure itself are much harder to sustain later.