Why are six draft marks read and not just two?

A ship's keel is not necessarily straight when loaded; hull hog or sag changes the actual displacement relative to the hydrostatic tables. Reading forward, midship, and aft marks on both sides allows calculation of the true mean draft after applying a correction for hog/sag. The midship draft (average of both sides) is compared to the mean of the forward and aft drafts; any difference indicates deflection which is corrected for in the displacement calculation.

What is the density correction and why is it needed?

Hydrostatic tables in the ship's stability booklet are computed for a standard density — typically 1.025 t/m³ (standard seawater). If the actual dock water density differs, the displacement read from the tables must be corrected. The formula is: corrected displacement = tabulated displacement × (actual density / 1.025). Dock water density is measured with a hydrometer at the ship's side; a difference of even 0.010 t/m³ can change the calculated displacement by 20-30 tonnes on a Panamax bulker.

What items are deducted to arrive at cargo mass?

Deductibles are the weight of liquids and constants that do not form part of the cargo: fuel oil (HFO, MDO, MGO), fresh water (FW tanks), ballast water, lube oil, sludge, slops, hydraulic oil, and the ship's constants (items not accounted for elsewhere — stores, spares, crew effects, paint). The surveyor must gauge every tank before and after loading and apply the relevant density to convert volume to mass.

What authority governs draft survey procedure?

The international standard is the UNECE Code of Practice for Draught Survey (ECE/TRANS/182). IMO Resolution A.715(17) requires that cargo masses on bulk carriers be determined to a recognised standard. The Intercargo Draft Survey Guidelines (2020) provide additional industry guidance. In most charter parties, the draft survey is agreed as the binding measure of the cargo loaded.

Draft survey procedure

A draft survey determines cargo mass by measuring the ship's displacement before and after loading (or discharging) and subtracting deductibles. It is the standard method of establishing bulk cargo weights at loading and discharging ports under charter party and commercial contracts, recognised by IMO Resolution A.715(17) and the UNECE Code of Practice for Draught Survey.

Reading the six draft marks

· Read forward port (FP), forward starboard (FS), midship port (MP), midship starboard (MS), aft port (AP), aft starboard (AS) marks.
· Drafts are read from the waterline to the nearest centimetre; repeat readings until three consecutive consistent values are obtained at each mark.
· Mean forward draft = (FP + FS) / 2; mean aft draft = (AP + AS) / 2; mean midship draft = (MP + MS) / 2.
· Longitudinal centre of flotation (LCF) correction converts the draft at the mark to the draft at the LCF position, which is the point at which displacement tables are entered.

Trim, hog/sag, and heel corrections

· Trim = mean aft − mean forward; enter the hydrostatic table with the corrected mean draft at LCF.
· First trim correction (FTC) = (TPC × trim × LCF distance from midship) / LBP.
· Second trim correction (STC) = (MCTC change per 100 mm trim × trim²) / (100 × LBP) — relevant when trim is large.
· Hog/sag correction: if the mean midship draft differs from the arithmetic mean of forward and aft drafts, a deflection correction of ±(2/3) × deflection is applied to the mean draft.
· Heel correction: if the ship lists, draft marks on one side are deeper than the other; correct to the upright mean before entering hydrostatics.

Density correction

· Measure dock water density with a calibrated hydrometer at the ship's side; take readings forward, midship, and aft and average.
· Corrected displacement (W) = tabulated displacement × (dock density / 1.025).
· In fresh water (density 1.000) a ship will sink deeper by the Fresh Water Allowance (FWA); in salt water it rides higher. TPC and FWA figures are taken from the stability booklet.

Deductibles

· All tanks are gauged and converted to mass using tank calibration tables and the known density of each liquid.
· Deductibles = ballast water + fuel oil (HFO + MDO + MGO) + fresh water + lube oil + sludge + constants.
· Ship's constants (typically 50–200 t depending on vessel age and size) must be agreed between charterer, owner, and surveyor before the survey; usually taken from the last dry-dock report.
· Cargo mass = (Displacement after loading − Deductibles after) − (Displacement before loading − Deductibles before) = W2 − W1 net of deductibles.

Common errors and accuracy

· Swell and passing vessels cause waterline oscillation; wait for calm conditions or average multiple readings.
· Incorrect hydrometer reading is one of the most frequent sources of error — ensure thermometer correction is applied.
· Fog or poor visibility may make reading distant marks impossible; a small boat should be used, not binoculars.
· A well-executed survey achieves accuracy of ±0.3% of cargo mass; errors above ±0.5% should be investigated.

Sources: UNECE Standard Code of Practice for Draught Survey (ECE/TRANS/182, 2nd ed.); IMO Resolution A.715(17) — Weighing of cargoes; Intercargo Draft Survey Guidelines (2020 edition). See also tonnes per centimetre (TPC), displacement, cargo securing manual.