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A draft survey is the method used to determine the quantity of bulk cargo (grain, ore, coal, fertiliser, sugar, etc.) loaded or discharged by measuring the change in a vessel's displacement. Because weighbridges or conveyor belt scales are not always reliable or available, and because a ship's displacement can be calculated accurately from its draft marks and hydrostatic tables, the draft survey is accepted internationally as a legally recognised method of cargo measurement for the purpose of determining bill of lading quantities.
The fundamental principle is simple: displacement before loading minus displacement after loading equals the weight loaded (net of all other changes). In practice, applying the necessary corrections accurately — for trim, hull flexure, water density, and deductibles — requires care, experience, and access to the vessel's calibrated hydrostatic tables. A 0.1% error in a 150,000-tonne capesize cargo is 150 tonnes, worth tens of thousands of dollars at prevailing bulk freight rates.
Surveys are conducted twice: once before loading (initial survey) and once after loading is complete (final survey). The difference in displacement gives the cargo weight. For discharge, the order is reversed — initial on arrival at discharge port (loaded), final after discharge (empty).
Drafts are read at three stations — forward perpendicular (FP), midships (amidships), and after perpendicular (AP) — and on both port and starboard sides, giving six readings. This accounts for any list and allows calculation of the true mean draft at each station.
Average port and starboard at each station to get the mean forward draft (F), mean midships draft (M), and mean aft draft (A). These three figures are used for all subsequent calculations.
If the vessel is not on an even keel, the displacement scale in the hydrostatic tables applies to the draft at the LBP (length between perpendiculars) midpoint. The trim correction adjusts the apparent mean draft to the hydrostatic draft using the TPC (tonnes per centimetre) and LCF (longitudinal centre of flotation) from the vessel's hydrostatic tables.
A vessel's hull flexes under its own weight and cargo loading — hogging (hull curves upward in the middle, giving a falsely high midships draft) or sagging (hull dips in the middle, giving a falsely low midships draft). The hog/sag correction adjusts the mean draft to account for this hull deformation. The formula uses the six draft readings and a constant from the hydrostatic booklet.
The corrected mean draft is used to enter the vessel's hydrostatic tables (which vary by vessel and are provided in the stability booklet) to determine the displacement in salt water at the tabulated draft.
Hydrostatic tables are computed for salt water (density 1.025 t/m³). If the actual dock or port water density differs (measured with a calibrated hydrometer sampled well clear of fresh-water inflow), a correction is applied: Displacement (actual) = Displacement (tables) × (actual density / 1.025).
From the displacement figure, subtract all 'deductibles' — items on board that are not cargo: ballast water (measured by sounding), fresh water (FW tanks sounded), fuel oil (measured by sounding or by tank calibration), diesel oil, lubricating oil, constants (crew and stores, taken from the constant sheet), and any dunnage or grain fittings. The result is the net cargo weight.
Load ports at river mouths (Rotterdam, New Orleans, Chittagong) may have dock water significantly less dense than salt water. A density of 1.010 instead of 1.025 in a capesize results in many hundreds of tonnes of error if not corrected. The hydrometer sample must be taken at the waterline and close in time to the draft readings.
Ballast tanks declared as empty may still contain several hundred tonnes of water in the garboard strakes (bottom of the tank). A proper sounding, ullage measurement, and if necessary a visual inspection via the tank access is required. Surveyors routinely check all tanks marked zero.
The constant sheet — crew, provisions, spares, tools, mooring ropes, anchors and cables — is often underestimated by operators. A ship's actual constant may be significantly higher than the figure on the form, particularly on older vessels where layers of paint and accumulated stores are never re-measured.
On large bulk carriers, hog/sag can amount to 10–20 cm, which translates into hundreds of tonnes. Surveyors who do not record and apply the hog/sag correction introduce systematic error. The correction requires all six drafts to be read — not just forward and aft.
Night readings, vessel movement in a swell, or heavy rain can make draft marks difficult to read. Discrepancies between port and starboard at the same station greater than 5–10 cm should prompt re-reading. Some ports require an independent observer from the terminal or port authority.
The charterparty typically specifies who appoints the draft surveyor. Common arrangements:
Where the two surveyors disagree, the charterparty may provide for a third surveyor (jointly appointed) to be the tiebreaker. In practice, the cargo officer's own sounding figures and draft observations, recorded in the cargo log, provide an independent reference for any subsequent dispute.
The cargo officer (typically the Chief Officer) participates directly in the draft survey — reading draft marks, measuring tank ullages, providing the constants sheet, and countersigning the surveyor's report. The figures in the survey determine the bill of lading quantity, which in turn determines the freight invoice. If the surveyor's figure differs from a shore-based weighing system, a shortage claim or excess claim follows. Accurate, contemporaneous records in the cargo log are the ship's protection in any subsequent dispute.