Section 7.4 Draft Surveys: Principles, Procedures, and Calculations (Initial, Intermediate, Final)

One of the most universally accepted and widely practiced methods for determining the quantity of cargo loaded onto or discharged from a bulk carrier is the draft survey. This meticulous procedure relies on fundamental naval architectural principles to calculate the weight of cargo by measuring the change in the vessel’s displacement. While often conducted by independent specialist surveyors, it is imperative that the Master and Chief Officer possess a thorough understanding of the principles, procedures, potential inaccuracies, and calculations involved. This knowledge enables them to effectively oversee the survey, verify its results, protect the shipowner’s interests, and address any discrepancies that may arise. Accurate draft surveys are crucial for Bills of Lading, customs declarations, freight calculations, and resolving commercial disputes.

1. Definition and Purpose of Draft Surveys:

Definition: A draft survey is a calculation of the weight of cargo loaded or discharged based on measurements of the changes in the vessel’s displacement between two specific conditions (e.g., before and after loading). It essentially “weighs” the ship by measuring how much water it displaces.

Primary Purpose: To determine, with a reasonable degree of accuracy, the quantity (weight) of bulk cargo loaded onto or discharged from the vessel. This figure is often the basis for:

Bills of Lading: The quantity stated on the B/L.

Freight Payment: Freight is often calculated based on the weight of cargo carried.

Customs Duties and Port Dues: Assessed based on cargo quantity.

Stock Reconciliation: For shippers and receivers.

Verification of Shore Scale Weights: Can be used to check or corroborate weights obtained from shore-based weighing equipment (e.g., belt scales, weighbridges), which can sometimes be inaccurate.

Prevention/Resolution of Shortage Claims: Accurate draft surveys provide crucial evidence in case of alleged cargo shortages.

2. Fundamental Principles of Draft Survey:

The draft survey is based on Archimedes’ Principle, which states that a body wholly or partially immersed in a fluid is buoyed up by a force equal to the weight of the fluid displaced by the body.

Displacement: The weight of the volume of water a ship displaces is equal to the total weight of the ship and everything on board (hull, machinery, cargo, fuel, water, stores, crew, etc.) at that particular draft.

Calculating Cargo Weight:

Weight of Cargo Loaded = Final Displacement – Initial Displacement – (Consumables taken onboard or consumed during loading) + (Ballast discharged during loading)

Weight of Cargo Discharged = Initial Displacement – Final Displacement – (Consumables taken onboard or consumed during discharging) + (Ballast taken onboard during discharging) The “consumables” part refers to changes in the weight of fuel, fresh water, ballast water, stores, etc., that occur between the initial and final draft readings.

3. Information and Equipment Required for a Draft Survey:

Accurate draft surveys require precise data and reliable equipment.

Ship’s Hydrostatic Particulars (Hydrostatic Tables/Curves):

Provided in the ship’s approved Stability Booklet.

These tables list key hydrostatic data for a range of drafts, including:

Displacement (Δ): Usually in metric tonnes, for salt water (density 1.025 t/m³) or sometimes fresh water.

Tonnes Per Centimetre Immersion (TPC): The weight required to change the mean draft by one centimetre.

Moment to Change Trim by One Centimetre (MTC or MCT1cm): The moment required to change the vessel’s trim by one centimetre.

Longitudinal Centre of Buoyancy (LCB): The longitudinal position of the center of the underwater volume.

Longitudinal Centre of Flotation (LCF): The longitudinal position of the centroid of the waterplane area. This is the point about which the ship trims.

Ship’s Tank Calibration Tables (Sounding Tables):

Provide volumes or weights of liquids in each tank corresponding to specific soundings or ullages, corrected for trim and list.

Draft Marks: Clearly marked and legible draft marks on the hull at six positions: forward (port & starboard), midships (port & starboard), and aft (port & starboard). The midship draft marks should ideally be located at or very near the LCF.

Measuring Equipment:

Sounding Tape and Bob/Plummet: For measuring soundings/ullages in tanks. Must be accurate and in good condition. Water-finding paste and oil-finding paste are also needed.

Hydrometer (or Salinometer): To measure the density of the dock water. Must be accurately calibrated and used correctly (with temperature correction if necessary). A special hydrometer sampling can is used to obtain an average water sample from near the vessel’s keel.

Draft Reading Equipment:

Visually: Reading the drafts directly from the hull marks. Requires good visibility, calm water, and proper viewing position.

Draft Tubes / Manometers: Some vessels have internal draft reading systems, but their accuracy must be regularly verified against visual readings.

Electronic Draft Indicators: Increasingly common, but also need regular calibration and cross-checking.

Measuring Tape: For measuring distances (e.g., distance between draft marks, LCF to perpendiculars).

Calculator: For performing the calculations (though specialized draft survey software is often used by surveyors).

Vessel’s General Arrangement Plan: To identify locations of tanks, draft marks, etc.

Ballast Line Diagrams: To understand ballast tank connections.

4. Procedure for Conducting a Draft Survey:

A draft survey is a meticulous process involving several steps, performed for both the initial (before loading/after discharge) and final (after loading/before discharge) conditions.

Step 1: Reading the Drafts:

Read drafts at all six points: Forward Port (FP), Forward Starboard (FS), Midship Port (MP), Midship Starboard (MS), Aft Port (AP), Aft Starboard (AS).

Techniques for Accurate Reading:

Read as close to the water level as possible to minimize parallax error.

Use a draft reading tube or a “draft stick” in choppy water to dampen wave action.

Take multiple readings and average them if conditions are difficult.

Ensure draft marks are clean and legible.

Note if there is any list.

Calculate Mean Drafts:

Mean Forward Draft = (FP + FS) / 2

Mean Midship Draft = (MP + MS) / 2

Mean Aft Draft = (AP + AS) / 2

Calculate Mean of Means (Quarter Mean) Draft: This is the draft used to enter the hydrostatic tables.

Quarter Mean Draft (DM) = (Mean Forward + Mean Aft + (4 x Mean Midship)) / 6 (This is a common formula, but the exact formula specified in the ship’s stability booklet, often (Fwd + Aft + 6Mid)/8 or similar considering the midship position relative to LCF, should be used if different. The (F+A+4M)/6 is Simpson’s rule applied to draft readings, assuming midship is exactly halfway. More precise methods account for LCF position.)*

A simpler Mean of (Mean Forward, Mean Midship, Mean Aft) is sometimes used, but the Quarter Mean or a method accounting for LCF is generally more accurate. The key is consistency and using the method prescribed for the ship’s tables.

Step 2: Measuring the Density of Dock Water:

Take representative samples of the water in which the vessel is floating, typically from a depth of about half the vessel’s draft, near the midship section. A special sampling can is often used.

Use a calibrated hydrometer to measure the density.

Record the temperature of the water sample, as hydrometer readings may need temperature correction.

If density varies significantly along the ship’s length (e.g., in estuaries), multiple samples may be needed.

Step 3: Sounding/Ullaging All Tanks:

Meticulously sound or ullage all tanks containing liquids:

Ballast tanks

Fuel oil tanks (HFO, MDO/MGO – service, settling, storage)

Diesel oil tanks

Lubricating oil sumps and storage tanks (sometimes excluded if quantities are small and consistent, but best practice is to include significant quantities)

Fresh water tanks (potable and technical/feed water)

Sludge tanks, bilge holding tanks, sewage tanks (if quantities are significant and can change)

Any other tanks containing liquids (e.g., chemicals, slops).

Record soundings/ullages accurately.

Note any list of the vessel, as this will affect tank soundings (corrections for list are applied from tank calibration tables).

Calculate the weight of liquids in each tank using the tank calibration tables, applying corrections for trim and list.

Step 4: Determining Other Deadweight Items (Deductibles):

Estimate the weight of other non-cargo items onboard:

Crew and their effects (usually a standard figure).

Provisions and stores (estimate based on inventory or standard allowance).

Dunnage, lashings, spare parts (if significant and not part of lightship).

Sludge, bilge water (if not already accounted for from tank soundings).

These are often grouped under “Other Deductibles” or “Ship’s Constant” (see below).

Step 5: Applying Corrections to Displacement:

A. First Trim Correction (LCF Correction):

Purpose: To correct the displacement obtained from the hydrostatic tables (which is usually for an even keel condition or based on mean draft at LCF) for the effect of trim. The LCF is the point about which the vessel trims.

Calculation:

Trim = (Aft Draft – Forward Draft)

Distance of LCF from Midship (dLCF): From hydrostatic tables for the current mean draft.

TPC: From hydrostatic tables.

First Trim Correction (tonnes) ≈ (Trim (cm) x TPC x dLCF (m)) / L (Length Between Perpendiculars, or sometimes length between draft marks)

The sign of the correction depends on whether the LCF is forward or aft of midships and the direction of trim (by head or stern). This correction is added if trim causes the LCF to be more deeply immersed than the midship point from which the mean draft was partly derived, and subtracted if less immersed. Many loading instruments do this automatically.

B. Second Trim Correction (Layer Correction / Nemoto’s Correction):

Purpose: A further, usually smaller, correction to account for the change in the shape of the underwater volume due to trim, as the TPC and MTC values change with draft.

Calculation: More complex, often involving the derivative of MTC.

Second Trim Correction (tonnes) ≈ (Trim (cm))² x (dMTC/dz or 50 x (MTC₂ – MTC₁)) / L

Where dMTC/dz is the rate of change of MTC per unit change in draft, or (MTC₂ – MTC₁) is the difference in MTC values for drafts 50cm above and below the current mean draft.

This correction is usually positive.

C. Density Correction:

Purpose: Hydrostatic tables are usually compiled for a standard density (e.g., salt water at 1.025 t/m³). If the actual dock water density is different, the displacement must be corrected.

Calculation:

Corrected Displacement = Displacement from Tables (for 1.025) x (Actual Dock Water Density / 1.025)

Alternatively, a correction in tonnes can be calculated: Density Correction (tonnes) = (Displacement from Tables – Constant) x ((Actual Density – Standard Density) / Actual Density). (The “Constant” here refers to the vessel’s lightship or a known fixed weight). A simpler approach is: Displacement Correction = (Actual Density – Standard Density) * Volume of Displacement.

D. Heel (List) Correction (Usually Minor):

If the vessel has a significant list, a small correction to displacement might be applicable, but it’s generally best to conduct draft surveys with the vessel as upright as possible. Tank soundings must be corrected for list.

E. Hog/Sag Correction (Deflection Correction):

Purpose: If the vessel is hogging (bent upwards amidships) or sagging (bent downwards amidships), the midship draft reading will not accurately reflect the true mean immersion. This affects the displacement calculated using the midship draft.

Assessment: Hog or sag can be indicated by comparing the observed midship draft with the mean of the forward and aft drafts.

If Mean Midship Draft < (Mean Fwd Draft + Mean Aft Draft) / 2 => Vessel is Hogging.

If Mean Midship Draft > (Mean Fwd Draft + Mean Aft Draft) / 2 => Vessel is Sagging.

Calculation: The correction is often complex and may require specific formulas or tables provided in the ship’s stability booklet, or it’s handled by the loading instrument if it has this capability. It typically involves the TPC and the amount of deflection.

Correction (tonnes) ≈ Deflection (cm) x TPC x Correction Factor (often around 0.6 to 0.8, depending on hull form).

The sign of the correction depends on whether it’s hog or sag and how the initial displacement was derived. If using quarter mean draft, the effect is somewhat averaged out, but for precise surveys, a specific deflection correction is better.

Step 6: Calculating Net Displacement and Cargo Quantity:

Net Corrected Displacement: Start with displacement from tables for the Quarter Mean Draft, then apply all relevant corrections (trim corrections, density correction, deflection correction).

Deductible Weights: Subtract the total weight of all non-cargo items (ballast, fuel, FW, lubes, stores, crew, constant – see below) from the Net Corrected Displacement.

Initial Survey: The result is the vessel’s “Constant” (if it’s a lightship survey) or the “Weight before Loading.”

Constant: The apparent weight of the ship when all known deductibles are removed from the lightship displacement. It accounts for accumulated stores, spare parts, mud/sludge in tanks, minor structural additions, paint, etc., not included in the original lightship weight. A consistent, small positive constant is normal. Large variations in the constant between surveys should be investigated.

Final Survey: The result is the “Weight after Loading.”

Cargo Loaded = (Weight after Loading) – (Weight before Loading) – (Any consumables loaded/consumed during the cargo operation itself, if significant and not accounted for in the initial/final deductible calculations). (More simply: (Final Net Disp. – Final Deductibles) – (Initial Net Disp. – Initial Deductibles)).

6. Types of Draft Surveys (When They Are Done):

Initial Draft Survey (Before Loading / After Discharge):

Conducted when the vessel is in its light condition (or after discharging a previous cargo) before commencing loading.

Purpose: To establish the vessel’s displacement and the total weight of all deductibles (ballast, fuel, FW, stores, crew) onboard before any new cargo is loaded. This allows for the calculation of the vessel’s “Constant” if it’s a true lightship survey, or to establish a baseline “empty” weight.

Intermediate Draft Survey(s) (During Loading or Discharging):

Conducted one or more times during the cargo operation, often at the request of shippers, charterers, or for the ship’s own monitoring.

Purpose: To determine the quantity of cargo loaded or discharged up to that point. Useful for:

Monitoring progress against nominated quantities.

Verifying shore figures at intervals.

Making decisions about trimming or completion of certain holds.

If loading multiple parcels or at multiple berths.

Final Draft Survey (After Loading / Before Discharging):

Conducted once all cargo loading is completed and before the vessel sails (or upon arrival before commencing discharge).

Purpose: To determine the vessel’s final displacement and, by comparison with the initial survey (and accounting for any changes in deductibles during loading), to calculate the total quantity of cargo loaded. This is often the definitive figure for the Bill of Lading.

7. Accuracy and Potential Sources of Error in Draft Surveys:

While draft surveys are widely accepted, they are subject to potential inaccuracies. Achieving high accuracy (e.g., within +/- 0.5% of the actual cargo weight) requires meticulous attention to detail.

Common Sources of Error:

Inaccurate Draft Readings: Due to rough water, poor visibility, parallax error, dirty/illegible draft marks, or incorrect reading technique.

Incorrect Density Measurement: Using an uncalibrated or dirty hydrometer, not taking a representative water sample, or errors in temperature correction.

Inaccurate Tank Soundings/Ullages: Due to sounding tape errors, incorrect application of trim/list corrections from tables, “phantom” soundings in viscous oils, or unrecorded internal tank transfers.

Errors in Hydrostatic Tables or Tank Calibration Tables: Though rare if approved, wear and tear or structural changes to the vessel over time could theoretically affect them if not updated.

Incorrect Calculation/Application of Corrections: Mathematical errors or misunderstanding of how to apply trim, list, or deflection corrections.

Unknown Weights Onboard (Poorly Estimated Constant): If the vessel’s “constant” (unaccounted for weights) is not accurately known or varies significantly, it will affect the cargo calculation. Regular monitoring of the constant is important.

Entrapped Water/Mud: Significant quantities of mud or water trapped in ballast tanks (if not properly drained or accounted for) or in void spaces.

Vessel Movement During Survey: If the vessel is ranging at the berth or if there are significant changes in mooring line tension.

Rapid Changes in Water Density: In some estuarial ports.

Human Error: Simple mistakes in reading, recording, or calculating.

8. Role of Independent Surveyors:

Draft surveys are very often conducted by independent, qualified marine surveyors appointed by shippers, charterers, receivers, or owners.

Impartiality: They are expected to provide an unbiased calculation of the cargo quantity.

Expertise: They are specialists in draft survey procedures and calculations.

Joint Surveys: It is common for surveyors representing different parties (e.g., shipper and receiver, or owner and charterer) to conduct the survey jointly to agree on readings and calculations, minimizing disputes.

Ship’s Staff Involvement: Even when independent surveyors are onboard, the Chief Officer (and sometimes the Master) should:

Witness all draft readings and density measurements.

Accompany surveyors during tank soundings.

Provide accurate information about consumables and any changes during the operation.

Conduct their own parallel draft survey calculations as a cross-check.

Review the surveyor’s report and calculations before signing, querying any discrepancies.

9. Master’s and Chief Officer’s Responsibilities:

Provide Suitable Conditions: Ensure the vessel is as upright as possible, with minimal trim (if feasible for readings), and that mooring lines are reasonably slack (but safe) to allow the vessel to float freely during draft readings.

Ensure Accurate Data Provision: Provide precise and up-to-date information on all tank soundings and other deductibles to the attending surveyor(s).

Facilitate the Survey: Allow surveyors safe and unrestricted access for reading drafts and sounding tanks.

Witness Key Measurements: Personally witness or delegate a responsible officer to witness all critical measurements (drafts, density, key tank soundings).

Perform Own Calculations: The Chief Officer should always perform the ship’s own draft survey calculations independently, even if an external surveyor is appointed. This is crucial for verification.

Compare and Reconcile: Compare the ship’s figures with the surveyor’s figures. Investigate and try to reconcile any significant differences before the surveyor finalizes their report.

Sign Survey Report with Remarks (if necessary): If there are unresolved discrepancies or if the Master/CGO disagrees with the surveyor’s findings or methods, the report should be signed “for receipt only” or with a formal remark detailing the point of disagreement. A Letter of Protest may be issued if the discrepancy is significant and could prejudice the owner’s interests.

Maintain Records: Keep copies of all draft survey reports, supporting calculations, and any related correspondence.

In conclusion, the draft survey is a cornerstone of bulk cargo quantity determination. Its accuracy relies on meticulous procedure, precise measurements, correct application of hydrostatic principles, and careful calculation. For the Master and Chief Officer, a comprehensive understanding of this process is essential not only for compliance and operational efficiency but also for safeguarding the significant commercial interests tied to every tonne of cargo that crosses the ship’s rail.