Draft Survey in Bulk Cargo: How Vessel Weight Is Determined
Quote from chief_editor on April 28, 2026, 7:59 amHow draft surveys determine bulk cargo weight, what factors affect accuracy, and when draft survey is and is not suitable for quantity determination.
A draft survey is a method of determining the weight of bulk cargo loaded onto or discharged from a vessel by measuring the vessel's displacement before and after the cargo operation and calculating the weight difference. It is widely used in dry bulk commodity trade — for grain, coal, ore, fertilizer, and similar commodities — when conveyor belt scales or weighbridge measurement are unavailable or impractical. The accuracy of a draft survey depends on the precision of measurements taken, the accuracy of the vessel's hydrostatic tables, and the correctness of deductions for bunkers, ballast, and stores consumed or added during the cargo operation.
How a Draft Survey Is Conducted
The draft survey process begins before cargo operations start. The independent surveyor reads the vessel's draft — the depth to which the vessel is immersed in water — at six points: forward, midship, and aft on both the port and starboard sides. These six readings are used to calculate the vessel's mean draft, corrected for the vessel's trim (the difference between forward and aft drafts) and list (any sideways tilt). The surveyor records the water density using a hydrometer, since seawater, brackish water, and fresh water have different densities, and a one-millimeter difference in draft at high displacement can represent a significant tonnage difference.
The surveyor then reads the quantity of bunkers (fuel oil and diesel) in the vessel's tanks, the ballast water in ballast tanks, and stores and consumables. These are recorded as the initial constants. The total displacement calculated from the mean draft is reduced by these deductible quantities to arrive at the light displacement — the weight of the vessel without cargo.
After cargo operations are complete, the same measurements are repeated. The displacement after loading minus the displacement before loading, with corrections for any bunkers, ballast, or stores added or consumed during the operation, gives the weight of cargo loaded. The result is expressed as the surveyor's figure — typically in metric tons.
Accuracy Limitations and When Draft Survey Is Not Suitable
Draft survey is subject to several sources of inaccuracy that are important for commodity traders to understand before relying on it for quantity determination under a contract.
The hydrostatic tables — the vessel's calibration data relating draft to displacement — may be outdated or inaccurate. A vessel whose hull has been modified, repaired, or that has accumulated marine growth may have actual hydrostatic properties that differ from those in the published tables. An inaccuracy of a few centimeters in draft at a displacement of 50,000 metric tons can produce a calculation error of several hundred tons.
Trim and list corrections require mathematical assumptions that introduce calculation uncertainty, particularly for vessels with complex hull forms or significant trim during cargo operations.
Wave action and vessel motion during draft reading in an exposed anchorage can make accurate draft measurement difficult. Standard practice requires the surveyor to take multiple readings and average them, but conditions at some ports make reliable readings impossible.
For these reasons, draft survey accuracy is typically quoted as plus or minus 0.5% of the surveyed cargo quantity for a careful survey in good conditions. In adverse conditions, the uncertainty can be significantly larger. GAFTA contracts recognize this by specifying a survey differential tolerance within which differences between load port and discharge port surveyed quantities are absorbed without claim.
Draft survey is appropriate as the primary measurement method when belt scale or weighbridge measurement is not available at the relevant port, and when the parties' contract assigns quantity determination to draft survey results. It is not appropriate when high measurement precision is required — for example, in transactions where small quantity differences have large financial consequences — or when vessel or sea conditions would make accurate readings impossible.
The practical implication for commodity traders is to understand their contract's measurement provisions before the vessel loads, not after a shortage is discovered at discharge.
Keywords: draft survey bulk cargo weight determination accuracy | draft survey method bulk cargo weight, vessel displacement cargo calculation, draft survey accuracy limitations, GAFTA survey differential draft survey, bill of lading quantity draft survey
Words: 726 | Source: Industry knowledge — WorldTradePro editorial research; GAFTA Contract No. 100 survey provisions; BIMCO guidance on draft surveys; ISO 9001 measurement standards | Created: 2026-04-11
How draft surveys determine bulk cargo weight, what factors affect accuracy, and when draft survey is and is not suitable for quantity determination.
A draft survey is a method of determining the weight of bulk cargo loaded onto or discharged from a vessel by measuring the vessel's displacement before and after the cargo operation and calculating the weight difference. It is widely used in dry bulk commodity trade — for grain, coal, ore, fertilizer, and similar commodities — when conveyor belt scales or weighbridge measurement are unavailable or impractical. The accuracy of a draft survey depends on the precision of measurements taken, the accuracy of the vessel's hydrostatic tables, and the correctness of deductions for bunkers, ballast, and stores consumed or added during the cargo operation.
How a Draft Survey Is Conducted
The draft survey process begins before cargo operations start. The independent surveyor reads the vessel's draft — the depth to which the vessel is immersed in water — at six points: forward, midship, and aft on both the port and starboard sides. These six readings are used to calculate the vessel's mean draft, corrected for the vessel's trim (the difference between forward and aft drafts) and list (any sideways tilt). The surveyor records the water density using a hydrometer, since seawater, brackish water, and fresh water have different densities, and a one-millimeter difference in draft at high displacement can represent a significant tonnage difference.
The surveyor then reads the quantity of bunkers (fuel oil and diesel) in the vessel's tanks, the ballast water in ballast tanks, and stores and consumables. These are recorded as the initial constants. The total displacement calculated from the mean draft is reduced by these deductible quantities to arrive at the light displacement — the weight of the vessel without cargo.
After cargo operations are complete, the same measurements are repeated. The displacement after loading minus the displacement before loading, with corrections for any bunkers, ballast, or stores added or consumed during the operation, gives the weight of cargo loaded. The result is expressed as the surveyor's figure — typically in metric tons.
Accuracy Limitations and When Draft Survey Is Not Suitable
Draft survey is subject to several sources of inaccuracy that are important for commodity traders to understand before relying on it for quantity determination under a contract.
The hydrostatic tables — the vessel's calibration data relating draft to displacement — may be outdated or inaccurate. A vessel whose hull has been modified, repaired, or that has accumulated marine growth may have actual hydrostatic properties that differ from those in the published tables. An inaccuracy of a few centimeters in draft at a displacement of 50,000 metric tons can produce a calculation error of several hundred tons.
Trim and list corrections require mathematical assumptions that introduce calculation uncertainty, particularly for vessels with complex hull forms or significant trim during cargo operations.
Wave action and vessel motion during draft reading in an exposed anchorage can make accurate draft measurement difficult. Standard practice requires the surveyor to take multiple readings and average them, but conditions at some ports make reliable readings impossible.
For these reasons, draft survey accuracy is typically quoted as plus or minus 0.5% of the surveyed cargo quantity for a careful survey in good conditions. In adverse conditions, the uncertainty can be significantly larger. GAFTA contracts recognize this by specifying a survey differential tolerance within which differences between load port and discharge port surveyed quantities are absorbed without claim.
Draft survey is appropriate as the primary measurement method when belt scale or weighbridge measurement is not available at the relevant port, and when the parties' contract assigns quantity determination to draft survey results. It is not appropriate when high measurement precision is required — for example, in transactions where small quantity differences have large financial consequences — or when vessel or sea conditions would make accurate readings impossible.
The practical implication for commodity traders is to understand their contract's measurement provisions before the vessel loads, not after a shortage is discovered at discharge.
Keywords: draft survey bulk cargo weight determination accuracy | draft survey method bulk cargo weight, vessel displacement cargo calculation, draft survey accuracy limitations, GAFTA survey differential draft survey, bill of lading quantity draft survey
Words: 726 | Source: Industry knowledge — WorldTradePro editorial research; GAFTA Contract No. 100 survey provisions; BIMCO guidance on draft surveys; ISO 9001 measurement standards | Created: 2026-04-11