Background
Draught survey accuracy depends on careful execution at both ends of the cargo operation. The light displacement (vessel before loading) and loaded displacement (vessel after loading) are each calculated from observed draught readings at six positions (forward port and starboard, midships port and starboard, aft port and starboard), corrected for trim, list, hull deflection (hogging or sagging), and water density, then converted to displacement using the vessel’s hydrostatic tables. Subtraction of light from loaded displacement, after accounting for changes in deductibles (fuel, fresh water, ballast, lube oil, stores) between the two surveys, yields the cargo weight. Surveyor accuracy is typically ±0.5 percent on Capesize vessels and ±1.0 percent on Handysize vessels under good conditions, deteriorating in heavy weather, congested anchorages, or strong currents.
Survey Procedure
A standard bulk cargo draught survey proceeds in defined stages.
Initial survey (before loading or after discharge) establishes the empty (or part-loaded) reference. The surveyor reads the draughts at all six marks, measures the seawater density at the ship’s side using a hydrometer or refractometer, and verifies all deductibles by sounding fuel, fresh water, ballast, lubricating oil, and bilge tanks. The constants of the vessel (lightweight, hull deflection coefficients, trim correction tables) are read from the trim and stability booklet.
Loading or discharge operation proceeds with periodic monitoring of trim, draft, and ballast water transfers. The surveyor remains aboard or attends key milestones to verify that no significant changes in deductibles occur outside documented quantities.
Final survey (after loading or before discharge) repeats the initial survey at the loaded condition. Six draught readings, density measurement, and deductible soundings are repeated.
Cargo calculation subtracts initial displacement (corrected for initial deductibles) from final displacement (corrected for final deductibles) to give net cargo weight. The full calculation involves:
- Mean of forward, mean of midships, mean of aft draughts (averaging port and starboard)
- Apparent trim from forward and aft means
- Mean of means draught (with hog/sag correction from midships reading)
- True mean draught after trim correction
- Displacement at true mean draught from hydrostatic tables
- Density correction (hydrostatic tables typically reference 1.025 t/m³ saltwater; correction applied for actual measured density)
- Trim correction from longitudinal centre of flotation (LCF)
- Deductible adjustments
Survey report documents all readings, corrections, and the final cargo figure, signed by the surveyor and the master.
Hydrostatic Tables
The vessel’s hydrostatic tables in the trim and stability booklet are the foundation of the calculation. The tables provide, for each draught from light to scantling:
- Displacement in saltwater density
- Tons per centimetre immersion (TPC)
- Moment to change trim 1 cm (MCT1cm)
- Longitudinal centre of buoyancy (LCB)
- Longitudinal centre of flotation (LCF)
- Vertical centre of buoyancy (VCB or KB)
The surveyor interpolates between table entries to find the displacement at the true mean draught. Modern surveys use the ship’s loading computer to perform the interpolation, but the manual calculation remains the standard reference.
Density Correction
Saltwater density varies between approximately 1.000 t/m³ (fresh water) and 1.030 t/m³ (high-salinity tropical seas). The standard hydrostatic tables reference 1.025 t/m³, so the actual displacement is corrected for the measured density at the survey location. The correction is approximately:
$$\Delta_{actual} = \Delta_{table} \times \frac{\rho_{measured}}{1.025}$$A density measurement error of 0.001 t/m³ on a Capesize at 180,000 tonnes displacement yields a 175 tonne error, almost a tenth of one percent, larger than most other survey errors combined. Surveyors take multiple density samples at different depths and compare to verify accuracy.
Trim and Hull Deflection Correction
A vessel rarely floats at perfectly even keel. The mean of the six readings is the “apparent” mean and requires corrections:
Trim correction accounts for the vessel pitching (forward draught different from aft). The longitudinal centre of flotation (LCF) is rarely exactly at midships, so a trimmed vessel displaces a different volume than an even-keel vessel at the same mean draught. The correction is:
$$\text{Trim correction} = \text{Trim} \times \frac{\text{LCF distance from midships}}{L}$$Hull deflection correction (hogging or sagging) accounts for the longitudinal bending of the hull from cargo distribution. Hogging (cargo concentrated at ends) makes the midships rise; sagging (cargo concentrated amidships) makes the midships dip. The correction is derived from comparing the midships mean to the average of forward and aft means.
Deductibles
Deductibles are all consumable and ballast quantities that change the vessel’s displacement but are not cargo. Standard deductibles include:
- Heavy fuel oil
- Diesel oil / marine gas oil
- Lubricating oil (sump and storage)
- Fresh water
- Ballast water (often the largest deductible)
- Slops and bilges
- Constants (lashing equipment, dunnage, ship’s stores)
Each deductible is sounded or measured at both surveys. Differences between the two surveys are subtracted from the displacement difference to isolate the cargo weight. Imprecise deductibles (e.g., partially-empty fuel tanks with sloping bottoms) are a common source of survey error.
Common Discrepancies
Draught survey figures sometimes differ from shore-scale figures by 0.5 percent or more. Common causes include:
- Density measurement error (single biggest source)
- Heavy weather during surveys (waves prevent reliable draught reading)
- Hull deflection (especially on long ships in heavy seas)
- Inaccurate hydrostatic tables (older vessels with welded modifications)
- Ballast water sounding errors (non-standard tank shapes, residue)
- Free-surface effects in partially full tanks
- Squat at speed (vessel must be at rest for survey)
- Survey timing relative to tank topping-off
When discrepancies arise, the survey can be re-performed or supplemented with shore-scale, ullage, or tape measurements as supporting evidence.
See also
References
- BIMCO/UN ECE Code of Uniform Standards and Procedures for the Performance of Draught Surveys of Coal Cargoes
- ASTM F1321, Standard Guide for Conducting a Stability Test (Lightweight Survey and Inclining Experiment)
- IMO Resolution A.749(18), Code on Intact Stability
- Industry guidance from CIMM (Chartered Institute of Marine Mining) and various P&I clubs