What is a draught survey and how is cargo weight calculated?

A draught survey turns the ship itself into the weighing instrument, and its accuracy depends entirely on how carefully the surveyor reads the draughts and accounts for everything aboard that is not cargo, which is why a properly conducted survey is a demanding, methodical job rather than a quick glance at the marks.

The surveyor reads draught marks at the forward, midships and aft positions, on both port and starboard sides where accessible, correcting each reading for the distance of the mark from the perpendiculars and for any list, to arrive at draughts corrected to the forward, midships and aft perpendiculars. From these, applying the ship's hydrostatic particulars and any correction for hull deflection, hogging or sagging, the mean draught and the corresponding displacement are taken from the ship's hydrostatic tables or stability software.

The displacement found this way includes everything aboard, not just cargo, so the surveyor must establish the deadweight constant items separately: bunkers, fresh water, ballast water, lubricating oil, stores and any other consumables, each measured by sounding or by transfer records, and converted to weight using their known densities. These are deducted from the total displacement to leave the lightship weight plus cargo, and comparing the before-loading and after-loading, or before and after discharge, figures, both similarly corrected, gives the weight of cargo moved.

Two corrections matter enough to single out. Water density at the time of the survey must be measured, using a hydrometer sample taken from alongside at draught mark depth, because a ship floats deeper in less dense water and shallower in denser water, and applying the wrong density skews the whole calculation. Trim also affects the accuracy of draught readings and the deflection correction, so the ship is kept as stable and as close to even keel as reasonably possible during the readings, and readings are taken close to still water, avoiding excessive swell that makes the marks hard to read consistently.

Because cargo quantity disputes between shipper, receiver and shipowner can run to real money, draught surveys are often conducted or witnessed by an independent surveyor, and the ship's own officers keep their own figures for comparison. A well-trained chief officer treats draught survey accuracy as a professional discipline: sloppy readings or an overlooked density correction can shift the calculated cargo figure by a meaningful margin on a large bulk parcel.