Draft Survey Explained: How Cargo Weight Is Calculated

What Is a Draft Survey?

A draft survey, also written as a draught survey in British maritime English, is a method used to determine the approximate mass of cargo loaded onto or discharged from a ship. It is based on the principle that a vessel's displacement changes as its total weight changes.

Displacement is the weight of the water displaced by a floating vessel and, under the relevant floating conditions, corresponds to the vessel's total weight. When cargo is loaded, the ship generally sits deeper in the water. When cargo is discharged, it generally sits higher.

By measuring the vessel's draught before and after a cargo operation and applying the necessary corrections, the surveyor calculates the change in displacement. Changes in ballast water, fuel, fresh water and other weights must also be considered to estimate the cargo quantity accurately.

A draft survey provides a calculated estimate based on measurements and vessel data. It is not a direct weighing of the cargo, and its accuracy depends on the quality of the readings, the vessel's hydrostatic information, the operating conditions and the corrections applied.

Why Are Draft Surveys Important?

Cargo quantity is an important part of a commercial shipping operation. Differences between the quantity declared by a shipper, the quantity recorded on shipping documents and the quantity calculated at the vessel can lead to financial disagreements and cargo claims.

For bulk cargoes, weighing every tonne individually at the vessel may not be practical. A draft survey offers an alternative method of estimating cargo mass using the vessel's displacement and the relevant supporting measurements.

An independent surveyor provides a documented record of the initial and final conditions, readings, calculations and relevant observations. This helps the parties review how the reported cargo quantity was determined and investigate differences when they arise.

Draft surveys are commonly associated with bulk cargo loading and discharging operations. The method and its suitability depend on the vessel, cargo, available data and any requirements agreed between the parties.

When Is a Draft Survey Carried Out?

For loading operations, the initial survey is generally conducted before loading and the final survey after loading is completed. For discharging operations, the initial survey is generally conducted before discharge and the final survey after discharge.

The survey arrangements should be agreed in advance, including the timing of readings, access to the vessel, the availability of relevant documents and how any operational changes will be recorded.

• Before loading: An initial survey establishes the vessel's displacement and relevant onboard weights before cargo is loaded.
• After loading: A final survey determines the vessel's displacement after loading, allowing the estimated loaded cargo quantity to be calculated.
• Before discharging: An initial survey records the vessel's condition before cargo discharge begins.
• After discharging: A final survey records the vessel's condition after discharge, helping estimate the quantity discharged.
• During quantity investigations: Additional measurements or a review of earlier survey records may help investigate discrepancies between cargo quantity figures.

How a Draft Survey Works

A draft survey involves more than simply reading the numbers painted on the side of a vessel. The surveyor must collect the relevant measurements, review the vessel's hydrostatic data, calculate corrected displacement and account for other weights that may change during the operation.

A typical survey includes the following steps, although the exact procedure may vary according to the vessel, operating conditions and agreed scope.

1. Review the vessel's particulars, approved hydrostatic tables and relevant tank information needed for the calculations.
2. Record the initial draught readings at the forward, midship and aft positions on both the port and starboard sides, where accessible.
3. Measure the density of the surrounding water using suitable equipment and an appropriate sampling procedure.
4. Determine the quantities of ballast water, fuel, fresh water, lubricating oil and other relevant onboard weights.
5. Apply the appropriate corrections for draught, trim, hull deflection and water density, as required by the vessel's data and the adopted survey method.
6. Calculate the initial displacement and account for the relevant non-cargo weights to establish the initial net displacement.
7. Repeat the required measurements after loading or discharging has been completed.
8. Calculate the final net displacement and compare it with the initial net displacement to estimate the cargo quantity loaded or discharged.
9. Review the figures, document discrepancies and prepare the survey report with the supporting readings and calculations.

Understanding Vessel Draught Readings

Draught is the vertical distance between the vessel's keel and the waterline. Draught marks painted on the hull allow the surveyor to observe how deeply the vessel is sitting in the water.

Readings are normally taken at three longitudinal positions: forward, amidships and aft. Where access permits, readings are taken on both the port and starboard sides, giving six observed readings.

The port and starboard readings at each position are assessed to determine the relevant mean draughts. These readings help identify differences caused by the vessel's list and form the basis for subsequent calculations.

The forward and aft readings show the vessel's trim, while the midship readings provide additional information about the hull's condition and possible deflection. The appropriate method is then used to determine the draught value required for the hydrostatic calculations.

Readings must be taken carefully because the waterline may move due to swell, waves, passing vessels or other local conditions. Poorly maintained or obscured draught marks can also affect the result. Where readings are difficult, the surveyor should record the limitations and follow an appropriate measurement procedure.

What Are Hydrostatic Tables?

Hydrostatic tables are vessel-specific reference data used to determine properties such as displacement at different draughts. They may also provide information needed to calculate corrections for trim and other relevant conditions.

After determining the appropriate corrected draught, the surveyor uses the vessel's hydrostatic information to obtain the corresponding displacement and other values required by the calculation method.

These tables are important because vessels differ in hull design, dimensions and displacement characteristics. Data from another vessel should not be substituted for the vessel being surveyed.

The surveyor should use the appropriate approved vessel data and check that the readings fall within the range covered by the relevant tables. Missing, unsuitable or inaccurate hydrostatic information can undermine the reliability of the calculated cargo quantity.

Why Water Density Matters

Water density affects how deeply a vessel sits in the water for a given weight. A vessel floating in freshwater will generally sit deeper than the same vessel at the same weight in seawater because freshwater is less dense.

Hydrostatic tables commonly use a specified reference water density. If the actual water density at the survey location differs from that reference, the displacement obtained from the tables must be corrected using the appropriate method.

The surveyor measures the water density using suitable equipment, such as a properly maintained hydrometer, and follows an appropriate sampling procedure. Depending on local conditions, density may vary with location and depth, so a single reading may not always represent the surrounding water adequately.

Incorrect density measurements or the use of an unsuitable correction can affect the calculated displacement and therefore the estimated cargo quantity. Water density should be measured and documented as part of the survey procedure.

Trim, List and Hull Deflection

A vessel does not always float on an even keel. Its forward and aft draughts may differ, and it may also lean to one side. These conditions affect how the observed draught readings are interpreted.

Trim describes the difference between the vessel's forward and aft draughts. A vessel trimmed by the stern has a greater draught aft, while a vessel trimmed by the bow has a greater draught forward. The applicable trim corrections depend on the vessel's hydrostatic data and the survey calculation method.

List refers to the vessel's inclination to port or starboard. Comparing readings on both sides helps identify this condition and determine the appropriate treatment of the readings.

Hull deflection, commonly described as hogging or sagging, can also influence the relationship between draught readings at different positions. The surveyor should consider the relevant readings and apply the appropriate correction where required by the adopted method.

These corrections are important because the displacement calculated from an uncorrected draught may not accurately represent the vessel's actual condition. The appropriate procedure depends on the vessel and its hydrostatic information.

Ballast Water and Other Weight Corrections

A vessel carries more than cargo. Ballast water, fuel, diesel, lubricating oil, fresh water, stores and other onboard items also contribute to its total weight.

These weights must be considered because they can change between the initial and final surveys. For example, a vessel may take in or discharge ballast water during cargo operations, or consume fuel while waiting at anchorage or alongside.

The surveyor reviews the relevant tank soundings, tank calibration information and other available records to determine the quantities of non-cargo items that need to be included in the calculation. Tank readings may require appropriate corrections for trim, list and other applicable factors.

The vessel's lightship weight and any applicable ship's constant must also be treated correctly. Lightship weight represents the vessel's weight without cargo, fuel, ballast, stores and other variable loads, according to the applicable vessel definition and documentation. The ship's constant represents weight differences not otherwise accounted for and must be handled consistently with the survey method.

Care is needed to avoid omitting a relevant weight or deducting the same item twice. The treatment of each item should be clear in the calculation and supported by the available information.

How Is Cargo Quantity Calculated?

The above expressions explain the basic principle, not a complete calculation procedure. Actual surveys require the appropriate treatment of ballast, fuel, fresh water, stores, lightship weight and ship's constant, as applicable. The calculation must also follow the vessel's hydrostatic data and the agreed survey method.

The final figure is an estimate based on the measurements and information available. It should be reviewed alongside the relevant cargo documents and any shore-based weighing records where applicable.

• Corrected displacement: Displacement determined from the relevant draught readings and hydrostatic data after the applicable corrections.
• Net displacement: Corrected displacement adjusted for the relevant non-cargo weights according to the adopted survey method.
• Estimated cargo loaded: Final net displacement minus initial net displacement.
• Estimated cargo discharged: Initial net displacement minus final net displacement.

Common Causes of Draft Survey Discrepancies

When a discrepancy arises, the parties should review the initial and final survey records, the vessel data, tank measurements, operational logs and the relevant cargo documents. The investigation should focus on the available evidence rather than assume a cause before the records have been examined.

• Draught reading errors: Poor visibility, damaged marks, waves, swell or inconsistent readings can affect the observed draught.
• Water density errors: An unrepresentative sample, unsuitable equipment or incorrect density data can affect the displacement correction.
• Incomplete tank measurements: Missing or inaccurate ballast, fuel, fresh water or other tank readings can distort the weight corrections.
• Incorrect vessel data: Using unsuitable hydrostatic tables or incorrect vessel particulars can affect the calculation.
• Trim and hull deflection: Inadequate treatment of trim, list, hogging or sagging can introduce errors into the calculated displacement.
• Changes during operations: Ballasting, deballasting, bunkering, freshwater transfers or other weight changes between readings can affect the result if not properly recorded.
• Unrecorded weight changes: Changes in stores, equipment, liquids or other onboard items may affect the calculation if they fall within the relevant scope and are not accounted for.
• Procedural differences: Different measurement practices, assumptions or calculation methods may produce different results.

Draft Survey Accuracy and Limitations

A draft survey is an indirect measurement method, and the accuracy of the result depends on the quality of the readings, the vessel's characteristics, the surrounding conditions and the calculation procedure.

Industry guidance recognises that draft survey results can vary, even when care is taken. The level of uncertainty depends on the particular survey conditions, and a single accuracy percentage should not be treated as a guarantee for every vessel or cargo operation.

Conditions such as swell, restricted access to draught marks, variable water density, incomplete tank information and significant changes in onboard weights can affect the result. These limitations should be considered when interpreting the final quantity.

A well-documented report makes the method and supporting figures traceable. Where a discrepancy is material, the parties may need additional checks or a review of the available records before drawing conclusions.

Draft Survey vs Shore Weighing

Draft surveys and shore-based weighing are different methods of determining cargo quantity. A draft survey estimates cargo mass from changes in vessel displacement, while shore weighing uses suitable weighing equipment to measure cargo passing through or over a weighing system.

The methods may produce different figures because of differences in measurement principles, equipment, operating conditions, timing and the procedures used. Neither result should be assumed to be conclusive without considering the circumstances and the applicable contractual arrangements.

For bulk cargo operations, the relevant contract, charter party, port procedures and agreed measurement method may determine which figures are used for commercial purposes. An independent survey report can help document the vessel-side calculation and provide a basis for comparison.

When Should You Appoint an Independent Draft Surveyor?

The surveyor should be appointed early enough to arrange access and coordinate the initial and final surveys with the vessel and relevant parties. The scope, reporting requirements and any applicable commercial instructions should be clarified before attendance.

An independent surveyor records and assesses the available evidence within the agreed scope. The survey does not automatically determine liability for a shortage or replace the contractual process for resolving a dispute.

• When loading or discharging bulk cargo and an independent estimate of cargo quantity is required.
• When the shipper's, receiver's and vessel's quantity figures need to be compared.
• When the cargo value or contractual arrangements justify an independent survey record.
• When a previous cargo operation has resulted in quantity discrepancies or claims.
• When a charter party, sale contract or other agreed arrangement calls for a survey.
• When a documented record of measurements, calculations and relevant observations is needed for further review.

Choosing a Draft Surveyor

A useful survey report should allow the relevant parties to understand how the reported cargo quantity was calculated. Clear supporting records are especially important where the survey results will be compared with other quantity figures or used during a commercial investigation.

• Relevant experience with draft surveys and the type of vessel or cargo involved.
• Familiarity with hydrostatic tables, draught readings, water density measurements and the treatment of onboard weights.
• Suitable measurement equipment and an appropriate procedure for recording the readings.
• A clear report showing the relevant observations, calculations, corrections and assumptions.
• Independence from the parties whose cargo quantity figures are being assessed.
• Availability that matches the planned loading or discharging schedule, subject to confirmation.

Draft Surveys in Sri Lanka

Draft survey attendance in Sri Lanka depends on the port, vessel, cargo operation, access arrangements and surveyor availability. Arrangements should be confirmed in advance so that the initial and final surveys can be coordinated with the vessel and the relevant parties.

Earndeck Shipping can assist with draft survey arrangements and prepare reports documenting the measurements, calculations and observations included in the agreed scope. To discuss a requirement, see our draft survey service or contact us with the vessel name, port, cargo type, operation and expected survey date.

References and Further Reading

For practical guidance on draught surveys and good practice, see the North of England P&I Association's guide to draught surveys.

For common sources of error and their potential effect on cargo quantity calculations, consult West of England P&I Club's guidance on inaccuracies in draught surveys.

For additional calculation guidance, see Britannia P&I's draft survey calculation guidance.

Frequently Asked Questions