How Is a Draft Survey Calculated? Trim, Deflection and Density Corrections

The calculation relies on the vessel's approved hydrostatic tables, which list displacement and a number of coefficients for each draught. Understanding these values is the key to understanding why each correction is applied.

Because of hull shape, draught marks are rarely located exactly at the forward and aft perpendiculars or at midships. When the vessel is trimmed, the distance between a mark and its perpendicular means the observed reading differs from the true draught at the perpendicular.

Each reading is therefore corrected using the distance of the mark from the perpendicular and the observed trim. The distances are normally given in the draught mark plan or the hydrostatic booklet. Skipping this step can produce a noticeable error, particularly on initial surveys in ballast where trim tends to be large.

Depending on how weight is distributed, the hull bends longitudinally. If midships rises relative to the ends the vessel is hogging; if it sinks she is sagging. In either case the simple average of forward and aft draughts does not represent the true immersion.

In practice, deflection is allowed for by a weighted mean that gives extra weight to the midship draught. The common formula adds the forward draught, the aft draught and six times the midship draught and divides by eight, often called the mean of mean of means. This quarter-mean draught is used to enter the tables for displacement, TPC, MTC and LCF.

Hydrostatic tables are calculated for an even-keel condition. On a trimmed vessel the centre of flotation is not at midships, so displacement taken straight from the tables needs correcting. The first trim correction, sometimes called the layer correction, uses trim, TPC and the distance of LCF from midships; its sign depends on whether LCF lies forward or aft of midships and on the direction of trim.

The second trim correction, often associated with Nemoto, allows for the shift of LCF as trim changes. It uses the difference between MTC values read a little above and below the mean draught and is always additive. With small trim its effect is minor, but with a large trim it should not be ignored.

The tables assume a reference seawater density. If the dock water is lighter or heavier, the vessel floats deeper or shallower for the same weight. The trim-corrected displacement is therefore adjusted in proportion to the ratio between measured dock water density and table density.

The accuracy of this step depends on taking samples at a representative depth and using a hydrometer whose calibration matches the basis of the tables. On a large vessel, even a small density error translates into a significant number of tonnes.

Stripped of detail, a draft survey calculation follows the same sequence every time. Keeping to that sequence matters, because each correction takes the previous result as its input and swapping steps around can produce a different answer. A clear, step-by-step calculation sheet also makes it much easier for the ship's officers and other parties to check the figures.

The deductibles, meaning ballast, fresh water, bunkers, lubricating oil and other variable weights, are subtracted from the corrected displacement to give the net weight. At the initial survey, light ship is subtracted from this net figure to establish the constant.

The same process is repeated at the final survey, and the difference between the two net weights is the cargo quantity. A frequently cited framework for the method is the UN/ECE Code of Uniform Standards and Procedures for the Performance of Draught Surveys of Coal Cargoes. Karan Gözetim carries out draft surveys including these calculations from its base in Gemlik.