Draught Survey for Bulk Cargo: Full Method

A draught survey determines the weight of bulk cargo loaded or discharged from a ship by measuring the vessel’s displacement before and after the cargo operation and deducting all known changes in ballast, fuel, water, and stores. The method relies on Archimedes’ principle: displacement equals mass, so the difference between two accurate displacement readings, corrected for all non-cargo weight changes, equals the cargo mass. Under good port conditions on a Capesize or Newcastlemax, survey accuracy is within about 0.5 percent of the total cargo mass. The UN Economic Commission for Europe codified the procedure for coal cargoes in ECE/ENERGY/19, the Code of Uniform Standards and Procedures for the Performance of Draught Surveys of Coal Cargoes, and the same methodology is applied across virtually every dry bulk commodity: iron ore, grain, bauxite, fertilizer, sulphur, and scrap.

The physical principle: Archimedes and displacement

A ship floating freely displaces a volume of water whose weight exactly equals the ship’s total weight. Expressed formally, if $\rho$ is water density in t/m$^3$ and $V$ is the submerged volume in m$^3$, then displacement $\Delta$ in metric tonnes is:

The ship’s hydrostatic tables tabulate $V$ (and therefore $\Delta$ at standard density 1.025 t/m$^3$) as a function of draught. Read the draught accurately, enter the tables, correct for actual water density, and you have the total mass of ship plus everything in it. Do this twice, at the start and end of a cargo operation, and the difference is the mass of cargo moved, after subtracting the changes in everything that is not cargo.

The genius of the method is that it requires no contact with the cargo itself. No conveyor-belt weightometer, no shore scale, no weighbridge. For loose bulk commodities, which cannot practically be weighed in individual units, displacement is often the most reliable independent measure available.

The six draught marks and how they are read

A bulk carrier carries six permanently welded draught marks on the hull: forward port (FP), forward starboard (FS), midships port (MP), midships starboard (MS), aft port (AP), and aft starboard (AS). On most vessels the marks are Arabic numerals cut from flat bar, painted white on dark hull or dark on light hull, graduated in decimetres with alternating painted and unpainted bands representing individual centimetres or the leading edge of each decimetre.

Reading the marks requires a small boat or the ship’s gangway and a calm water surface. The surveyor reads the waterline intersection with the mark to the nearest centimetre, ideally from both the waterline (reading down) and slightly above (reading up) to bracket the true value and average them. On vessels with significant freeboard or in slight swell, optical instruments or laser rangefinders are sometimes used, but the standard method is still the direct visual read.

Draft marks are located at the forward and aft perpendiculars (or near them, with a correction for the distance from the mark to the perpendicular given in the stability booklet), and at the midships frame. Most modern bulk carriers and their hydrostatic tables reference a midships located at exactly half the length between perpendiculars (LBP/2), but older vessels or those that have been lengthened may carry mid-mark corrections in their tables.

Six individual readings are taken. If port and starboard differ by more than about 3 cm the vessel is heeled (listed) and a list correction may apply before averaging.

The quarter-mean: computing the true mean draught

The quarter-mean (or mean-of-means) calculation converts six individual draught readings into a single true mean draught that accounts for trim and for any difference between the midships draught and the average of the end draughts. It is the pivot of the entire survey calculation.

First, average port and starboard at each station:

Then form the apparent trim:

A positive value means the vessel is trimmed by the stern, which is the normal loaded condition for most bulk carriers. The draft marks for the quarter-mean are then combined:

This is the standard UNECE formula for the quarter-mean (also called the “sixth-of-the-sum” formula or the “1-6-1” formula). The midships reading is weighted six times because it is closest to the true mean waterplane and is least sensitive to trim effects. In practice, some surveyors use the simpler three-mean formula:

but ECE/ENERGY/19 specifies the quarter-mean as the preferred method for higher accuracy. The difference between the two is largest on heavily trimmed vessels or on ships where the centre of flotation lies far from midships.

The quantity $\frac{d_F + d_A}{2} - d_M$ is the deflection of the hull. If the midships draught is greater than the average of the end draughts, the vessel is sagging (hull bending concave upward amidships, typical when cargo is concentrated amidships). If midships is less than the average, the vessel is hogging. The deflection value enters the deflection correction described below.

Trim corrections: first and second

The trim correction adjusts the displacement derived from the quarter-mean draught to account for the location of the centre of flotation (LCF), which is almost never exactly at midships.

The centre of flotation (LCF) is the centroid of the waterplane area. When a ship trims, it pivots about the LCF, not about the midships mark. If the LCF is aft of midships (common in loaded bulk carriers), a vessel trimmed by the stern will have a greater displacement than an untrimmed vessel at the same quarter-mean draught. The first trim correction accounts for this.

The first trim correction in metric tonnes is:

where $T$ is the apparent trim in metres, TPC is the tonnes per centimetre immersion at the mean draught (from hydrostatic tables, converted appropriately), $l_{LCF}$ is the distance in metres of the LCF from the midships mark (positive aft), and $L$ is the length between perpendiculars. This correction is additive when the vessel is trimmed by the stern and the LCF is aft of midships.

The second trim correction (the Mull correction or the wedge correction) is needed when trim is large relative to ship length, typically exceeding 0.5 percent of LBP. It corrects for the fact that the waterplane area changes as the ship trims: the relationship between trim and displacement change is not perfectly linear. The second trim correction is:

where $\frac{d(\text{MCT1cm})}{dD}$ is the rate of change of the moment to change trim one centimetre with draught, read from the hydrostatic tables at the mean draught. ECE/ENERGY/19 gives worked examples showing that on a typical Panamax (LBP 200 m) with 0.5 m trim, the second trim correction is on the order of a few tonnes and can be neglected, but on a vessel with 2 m trim it may reach 20 to 30 tonnes.

Both trim corrections are applied to the displacement at the quarter-mean draught before any density correction.

Hull deflection correction

Bulk carriers flex longitudinally under cargo load. When iron ore or coal is concentrated amidships, the hull sags at the centre. When the same commodity is trimmed to fill the end holds before the midships holds (a common loading sequence on self-unloading vessels), the vessel hogs. The midships draught reading captures this deflection, and the correction ensures the mean draught reflects the true average waterplane rather than a deflected one.

The deflection (positive for sag, negative for hog) is:

A sagging vessel (positive $\delta$) reads a higher midships draught than the true mean, so the corrected quarter-mean will be slightly less than the raw quarter-mean. The deflection correction in centimetres applied to the quarter-mean draught is:

ECE/ENERGY/19 gives the correction as $-(3/4)\delta$ applied to the midships mean before computing the quarter-mean, which produces the same result in terms of the final adjusted quarter-mean. The deflection can be substantial: a 300-metre Capesize fully loaded with iron ore may sag 300 to 400 mm amidships, adding several thousand cubic metres of displacement if not corrected.

Dock water density and the hydrometer correction

The hydrostatic tables are always constructed at standard saltwater density 1.025 t/m$^3$. Port dock water is almost never exactly 1.025. River ports (Newcastle, NSW; Richards Bay; Gladstone) have densities from about 1.000 to 1.010. Estuarine anchorages vary with tide and season. Even open-anchorage ports in tropical regions may read 1.020 to 1.024 because of fresh-water river plumes.

The surveyor takes water density samples at the ship’s side using a certified ASTM-pattern hydrometer (typically calibrated at 15°C), with a thermometer to apply the temperature correction. ECE/ENERGY/19 requires at least two samples at different depths, typically 0.5 m below the surface and 1.0 m below, averaged together. If surface fresh-water runoff from rain is present, a deeper sample may be taken. The hydrometer reads to 0.0005 t/m$^3$ (0.5 kg/m$^3$) and is re-calibrated against a reference solution at each survey.

The density-corrected displacement is:

where $\Delta_{table}$ is the displacement at the true mean draught read from the hydrostatic tables (which assume 1.025), and $\rho_{measured}$ is the hydrometer reading in t/m$^3$.

On a Capesize at 180,000 t displacement, a density error of 0.001 t/m$^3$ (one digit on the hydrometer) produces:

This is the largest single source of error in most draught surveys, which is why ECE/ENERGY/19 devotes considerable attention to hydrometer calibration, sampling technique, and temperature correction. In ports with strong density stratification (river plumes over saline layers), sampling only at the surface has caused systematic errors of 300 to 500 tonnes on large vessels.

The deductibles

Deductibles are all masses on board that change between the initial and final survey but are not cargo. The cargo weight is isolated by subtracting the net change in deductibles from the gross displacement change.

Ballast water is typically the largest single deductible, and the most significant source of error. A fully ballasted Panamax may carry 20,000 to 30,000 tonnes of ballast, and a 1 percent sounding error translates directly to 200 to 300 tonnes cargo error. The difficulty is compounded by tank geometry: ballast tanks on bulk carriers include wing tanks, double-bottom tanks, and fore/aft peak tanks, many with complex shapes that require printed ullage tables or computer calculation for accurate volume at any given sounding.

Free-surface effect (movement of liquid surface in a partially full tank) is not a direct measurement error but it makes tank readings more sensitive to vessel motion. ECE/ENERGY/19 recommends that, where possible, ballast tanks be pressed full or emptied at the time of survey to eliminate free-surface uncertainty. In practice, this is not always possible because ballast management during loading must maintain stability margins.

Fuel is measured by ullage (the distance from the tank top to the liquid surface) or by sounding (depth from the tank bottom), with temperature correction for density. The density of HFO varies between approximately 0.930 and 0.995 t/m$^3$ at 15°C depending on grade and origin, so the grade-specific density must be known. A 0.005 t/m$^3$ density error on 500 tonnes of HFO produces a 2.5-tonne error, small relative to ballast but material when the total deductible package is being reconciled carefully.

The survey constant

The survey constant (vessel constant or unmeasured weight) is the difference between the lightship displacement calculated from the hydrostatic tables and the true lightship as established by the most recent inclining experiment or lightweight survey. Every ship accumulates mass after it is built. Paint builds up: successive dry-docking coats can add 20 to 50 tonnes to a large bulk carrier over a decade. Structural reinforcements, additional equipment, modifications to the bilge system, or the accumulation of scale in tanks all increase lightship weight without being reflected in the original hydrostatic tables.

The constant is determined by conducting a displacement survey in a condition where all deductibles can be measured very precisely (typically immediately after dry docking when tanks are clean and calibrated, fuel and water are known quantities), computing the expected displacement from the hydrostatic tables after deducting all measured masses, and taking the residual. A well-maintained large bulk carrier typically has a constant in the range of 100 to 400 tonnes. Constants above 600 tonnes should trigger a review of the hydrostatic tables and a fresh lightweight survey.

The constant is subtracted as a deductible at both surveys. Because it is the same value at both surveys, it cancels out of the cargo calculation mathematically. This means that an incorrect constant does not directly introduce error into the cargo weight, as long as the same constant is applied consistently at both ends. The constant matters in disputes about the lightship displacement for other purposes (vessel valuation, deadweight calculations, charter-party compliance on displacement-based freight).

The step-by-step procedure

A full commercial draught survey for a bulk cargo proceeds in the following sequence:

Pre-loading (initial) survey. The surveyor boards the vessel before loading starts, or after discharge is complete. All six draught marks are read, usually twice (ascending from water level and descending from ship’s rail), and the values are averaged. The surveyor then takes two dock water density samples and records their temperatures. All deductible tanks are sounded: HFO, diesel, lube oil, fresh water, ballast water, and any slops or waste-oil tanks. Trim, list, and the absence of free communication between tanks are verified. The hydrostatic tables are consulted to confirm that the current condition is within their valid trim range. A tally of all stores loaded or discharged since the last certified lightweight survey is noted.

Displacement calculation (initial condition). The six draught readings are processed through the quarter-mean calculation to give the apparent mean draught. The trim corrections (first and, if needed, second) are applied. The deflection correction is applied. The corrected mean draught is entered into the hydrostatic tables to read displacement in saltwater. The density correction converts this to the actual water density. The sum of all measured deductibles (with tank soundings converted to mass using calibrated ullage tables) is subtracted, giving the initial lightship-equivalent displacement.

During cargo operations. The surveyor does not need to remain continuously aboard during loading, but must verify that no undocumented transfers of ballast, fuel, or water occur. The ballast plan is agreed with the master before loading, and any deviations must be recorded. Transfers that happen outside the agreed plan, or that cannot be measured, introduce errors that may be irreconcilable.

Post-loading (final) survey. The same procedure as the initial survey: six draught readings, density, all deductibles. Trim and list must be within the valid range of the hydrostatic tables. Heavy weather, passing traffic creating swell, or discharge pump pressure from barges alongside can make the draught marks difficult to read accurately and may require waiting.

Cargo calculation. Final displacement (density-corrected, trim-corrected, deductibles deducted) minus initial displacement (same corrections) equals cargo weight. The UNECE ECE/ENERGY/19 calculation form presents each step in a standard tabular format, which both the surveyor and the master’s representative sign.

Survey report. The completed report documents all raw readings, all corrections, and the final figure. It is signed by the independent surveyor, a representative of the ship (usually the chief officer), and, if present, a receiver’s surveyor. The report forms the basis for the bill of lading quantity, the freight calculation, and any subsequent cargo-shortage claim.

Accuracy and sources of error

Under ideal conditions (calm port, stable vessel, clean marks, calibrated instruments, freshly surveyed tanks), a large bulk carrier survey achieves 0.5 percent accuracy. That is the figure cited in the UK P&I Club’s guidance on draught surveys and is consistent with the UNECE ECE/ENERGY/19 discussion of achievable precision. On a Capesize loading 170,000 tonnes, 0.5 percent is 850 tonnes: not trivial for a commodity worth $50 to$100 per tonne.

The dominant error sources, roughly in decreasing order of their practical contribution, are:

Dock water density. A 0.002 t/m$^3$ error on a large vessel produces a 300 to 350 tonne error, as computed above. Stratified water columns, surface rain dilution, and uncertified hydrometers are the main failure modes.

Ballast tank sounding. Partially full tanks with complex geometry, sloping bottoms, and structural frames are difficult to sound accurately. Residue from previous ballast operations (mud, scale, biological matter) can add 20 to 100 tonnes of unrecorded mass and corrupt the sounding-to-volume relationship.

Draught mark reading. In swell, each mark oscillates by 10 to 30 cm. The surveyor must interpolate the mean waterline position while the mark moves. Heavy fouling of the hull at the waterline, incomplete paint on the marks, or poor lighting at night all degrade reading accuracy.

Hydrostatic table accuracy. Hydrostatic tables are constructed from the original building drawings. Structural modifications, additional steel welded internally, or conversion work not captured in a fresh lightweight survey make the tables inaccurate. Tables more than 10 years old without a reconciling lightweight survey are a recognised risk factor.

Trim correction. The LCF position in the hydrostatic tables is an interpolated value, and errors in the LCF distance propagate directly into the first trim correction. On a Panamax with 1 m trim and an LCF 1 m off mid, a 0.5 m error in LCF position produces approximately 10 to 15 tonnes of error.

Free-surface correction on ballast tanks. The free surface of liquid in partially full tanks reduces the effective stability and, separately, makes the sounding unstable during vessel motion. ECE/ENERGY/19 recommends pressing tanks hard or emptying them to eliminate this.

Currency of the vessel constant. A constant that has not been verified since the last major repair period may be wrong by 100 to 300 tonnes on older vessels with extensive modification histories.

On Handysize vessels (25,000 to 40,000 DWT), each error source represents a larger percentage of the total cargo and survey accuracy degrades to 0.75 to 1.0 percent under similar conditions. The absolute error from density or sounding may be the same in tonnes, but it is a larger fraction of a 20,000-tonne cargo than of a 150,000-tonne cargo.

The UNECE ECE/ENERGY/19 coal cargo code

The UN Economic Commission for Europe published ECE/ENERGY/19, Code of Uniform Standards and Procedures for the Performance of Draught Surveys of Coal Cargoes, in 1995. Although titled for coal, the code is applied industry-wide to dry bulk cargoes because it provides the most complete and internationally agreed description of the procedure.

ECE/ENERGY/19 specifies:

• The quarter-mean formula (1-6-1 weighting) as the preferred method for deriving the true mean draught
• The first and second trim corrections and their calculation formulae
• Minimum hydrometer calibration requirements and the temperature correction for water density
• Tank sounding procedures, including minimum reading frequency and the requirement for certified ullage tables
• The form and content of the survey report, including all fields that must be completed and signed
• Definitions of the survey constant and its verification
• Requirements for the attending surveyor’s qualifications and independence from the cargo owners

The code was developed jointly by coal exporters (principally Australia, South Africa, and Colombia) and importers (principally Japan, South Korea, and European power utilities) who needed a consistent framework to settle bill-of-lading quantity disputes. Before ECE/ENERGY/19, differences between ship’s draught figures, shore conveyor-belt weightometers, and port scale figures were reconciled by negotiation rather than by a defined technical standard, producing persistent commercial disputes.

ECE/ENERGY/19 does not itself have the force of international maritime law the way SOLAS or MARPOL do. It is a voluntary technical code, but it has been incorporated by reference into many charter parties (particularly COAL-OREVOYs and Gencon with rider clauses), into port state stevedoring agreements in major coal-loading terminals, and into P&I club survey protocols. When a charter party specifies “draught survey as per UNECE ECE/ENERGY/19,” the code’s procedure is contractually binding and any deviation from it can void the evidential weight of the survey.

Draught surveys in cargo quantity disputes

Cargo shortage claims in dry bulk trade are frequently resolved by comparing the ship’s draught survey figure against the shore weightometer or port scale figure. The two methods have different error characteristics, and the standard approach is to take the draught survey as prima facie evidence of quantity shipped or received and to accept shore scale as corroborating evidence, not the reverse.

Charter parties on voyages carrying iron ore, coal, and grain typically specify that the bill of lading quantity be based on the ship’s draught survey if there is a discrepancy, because the draught survey is an independent, methodology-defined measurement taken at the time of loading or discharge. Shore weightometers are subject to calibration drift, spillage on conveyor belts, moisture content variation between weighing and loading, and the lag between conveyor measurement and hold filling.

When a discrepancy exceeds 0.5 percent, the receiver or charterer typically appoints an independent surveyor to attend discharge and conduct a final draught survey, which is then compared to the bill of lading figure. Differences within the charter-party tolerance (often 0.5 to 1.0 percent, expressed as a “certificate of quantity” clause) are settled at the contract price for the certified tonnage. Differences above the tolerance may trigger a formal claim.

P&I clubs (UK P&I, Standard Club, Steamship Mutual) provide guidance noting that the most common sources of disputes are density stratification in port (where the surveyor sampled only the surface layer), ballast transfers not recorded between surveys, and the use of outdated hydrostatic tables. Surveyors engaged for both parties conducting a joint survey simultaneously, with agreed readings signed by both, reduce dispute risk substantially, though joint surveys are not always contractually required or practically possible.

List correction

When the vessel is not upright, port and starboard draught marks read different values. If a vessel has a 1-degree list to port, the port marks are lower and the starboard marks are higher by approximately $\frac{B}{2} \times \tan\theta$, where $B$ is the beam and $\theta$ is the angle of list. For a Capesize with 50 m beam and 1 degree of list, this difference is approximately 0.44 m, which would produce a significant draught error if only one side were read.

The standard correction is to average port and starboard at each station, which cancels the list effect provided the ship’s waterplane is symmetric about the centerline. Most loaded bulk carriers meet this condition. The averaging approach fails if the vessel has a structural asymmetry (a large blister, a one-sided repair, or significant bilge water on one side only), which is rare but occasionally encountered after collision damage.

The list also means that the apparent midships draught differs from the true draught at the keel centerline. For lists below 1.5 degrees, this secondary correction is negligible. ECE/ENERGY/19 recommends that if list exceeds 0.5 degrees at the time of survey, the cause should be investigated and corrected before proceeding, because free liquid surfaces in ballast tanks (a common cause of list in bulk carriers during loading) introduce free-surface uncertainty into the sounding measurements.

Relation to the ship’s hydrostatic tables and stability booklet

The hydrostatic tables are the computational heart of the draught survey. They are produced by the shipbuilder from the hull lines plan, checked by the classification society, and compiled into the ship’s trim and stability booklet, which is an approved document under SOLAS Chapter II-1. The tables give, for each draught at even keel in standard saltwater:

• Displacement ($\Delta$) in metric tonnes
• Volumes of displacement in m$^3$
• Tonnes per centimetre immersion (TPC)
• Moment to change trim one centimetre (MCT1cm, also called MTC)
• Longitudinal centre of buoyancy (LCB) from the aft perpendicular
• Longitudinal centre of flotation (LCF) from the aft perpendicular
• Vertical centre of buoyancy (KB)

For trim other than even keel, the tables provide corrections or separate “trimmed displacement” tables at various trim values. Some vessels carry only the even-keel displacement table and rely on the trim corrections described above to arrive at the trimmed displacement; others carry multi-trim hydrostatic tables that allow direct entry at the actual trim.

Modern loading computers (see marine stability booklet and loading computer) automate the interpolation and apply all corrections numerically. However, ECE/ENERGY/19 explicitly requires that the surveyor be capable of performing the complete calculation manually from the approved tables, because loading computer software varies between vessels, may not match the approved tables exactly, and is not itself a class-approved calculation in the sense that the underlying tables are.

Interaction with lightweight, deadweight, and displacement concepts

Understanding the draught survey requires clarity on three related but distinct weight concepts, covered in depth in the lightweight versus deadweight article. Lightweight (LWT or LDT) is the mass of the ship as built, including hull, machinery, and all permanently installed equipment, but excluding cargo, fuel, water, and stores. Deadweight (DWT) is the total mass the vessel can carry above its lightweight, equal to the difference between loaded and lightship displacement. The cargo deadweight is the DWT less the consumables (fuel, water, stores, crew, constants).

In a draught survey, the “initial displacement” is the total displacement of the vessel in its starting condition: lightweight plus all deductibles present at the time of the initial survey. The “final displacement” is the total at the end. The cargo weight is the difference in total displacement minus the difference in deductibles. This is why the survey constant is so important: it represents the difference between the tabulated lightweight and the actual lightweight, and if it is wrong it will affect the absolute displacement at both surveys equally (so it cancels), but it may affect other compliance calculations.

The trim and list interaction

A bulk carrier’s trim changes substantially during loading as cargo fills the holds in sequence. A typical loading sequence for a large Capesize loading iron ore might start by filling the aft holds to clear the berth’s shallow section, then progressively filling forward holds, then completing with the aft holds to achieve the desired departure trim. During this process the trim may swing from 2 m by the stern to 1 m by the head and back again, with associated changes in LCF position and the applicability of the trim corrections.

ECE/ENERGY/19 requires that the draught survey be conducted with the vessel as close to even keel as practicable, and specifically recommends that trim not exceed about 0.5 to 1.0 percent of LBP at the time of either survey. When trim is large, the second trim correction becomes material, and the accuracy of the LCF data from the hydrostatic tables becomes critical. Trim optimization for fuel efficiency on loaded passages, discussed at trim optimisation, is a separate consideration from survey trim.

Relationship to the IMSBC Code

Draught surveys are the primary quantity verification method for cargoes listed in the IMSBC Code. Several Group A cargoes (those that may liquefy) carry specific provisions about moisture content testing and the flow moisture point (FMP), and the loading quantity per hold must stay within the IMSBC-specified angle of repose and TML limits. The draught survey verifies the total quantity loaded across all holds but does not verify per-hold distribution. That per-hold distribution is monitored separately through the ship’s stability and strength calculations in the loading computer.

For specific commodity contexts:

• Coal: IMSBC schedule: Coal draught surveys are the primary application of ECE/ENERGY/19 and are specified in major coal export terminal contracts.
• Iron ore: IMSBC schedule: Iron ore is Group C (does not liquefy) but is one of the densest dry bulk cargoes, making per-hold weight distribution a strength concern and accurate total quantity verification commercially important.

Limitations

Draught surveys are not applicable in all circumstances. Key limitations include:

Heavy weather. Swell above about 0.25 m directly degrades the accuracy of draught mark readings. ECE/ENERGY/19 warns that surveys conducted with swell present should note this as a caveat on the survey report. On exposed anchorages (common at Australian coal terminals during cyclone season), survey accuracy may degrade to 2 to 3 percent.

Vessel condition. Heavily fouled hull or waterline obscuring the draught marks, or missing mark numerals, prevents accurate reading. Similarly, hydrostatic tables that have not been updated after structural modification or repair introduce systematic errors that cannot be corrected without a fresh lightweight survey.

Warm or turbid water. High-turbidity dock water prevents visual confirmation of the waterline against the draught mark. In some ports, diver-assisted underwater reading is used, but this is not standard and not covered by ECE/ENERGY/19.

Calibration chain gaps. If the hydrometer was last calibrated more than 12 months before the survey, or if the tank ullage tables were last verified more than a defined interval, ECE/ENERGY/19 treats the survey data as provisional. Surveyors are required to present calibration certificates.

Multi-port loading. When cargo is loaded across multiple ports on a single voyage, separate draught surveys at each port are required. The overall cargo figure is the sum, but errors accumulate. A series of five port calls, each with 0.3 percent error, does not guarantee the total is within 0.3 percent; each error can be the same sign, summing to 1.5 percent on the total.

Ship modifications. Vessels that have been converted, lengthened, or substantially modified without a follow-up inclining experiment may have hydrostatic tables that no longer match the actual hull. The hydrostatics and Bonjean article covers the relationship between the hull form, the Bonjean curves, and the derivation of the hydrostatic data that underpins every draught survey.

See also

• Bulk carrier
• Lightweight versus Deadweight
• Trim and list
• Hydrostatics and Bonjean
• Marine stability booklet and loading computer
• IMSBC Code
• Coal: IMSBC schedule
• Iron ore: IMSBC schedule
• Trim optimisation
• Bill of lading
• Statement of facts

Related calculators:

• Displacement at draught with density correction
• TPC (Tonnes Per Centimetre Immersion) by Simpson’s rule
• Mean draught from three-station readings
• Ullage table lookup by trim
• Bunker density and temperature correction