What Is a Draft Survey?
A draft survey figures out how much bulk cargo got loaded or unloaded by measuring how deep the ship sits in the water before and after. You read the draft marks on the hull, calculate the displacement change, and that gives you the cargo weight. It is the standard method for commodities like iron ore, coal, grain, and fertilizer, anything that pours into a ship's hold. The big advantage: you do not need to move the cargo across a weighbridge. The ship itself becomes the scale.
How a Draft Survey Works in Practice
The procedure has been standardized since 1992 under the UNECE Code of Uniform Standards for Draught Surveys of Coal Cargoes (document E/ECE/ENERGY/19). It breaks into six steps.
Step one: read the draft marks. You need six readings, forward, midship, and aft, port and starboard. Surveyors usually do this from a small boat or the quayside using calibrated optics. Readings go to the nearest centimetre. Two people often read independently and compare, if they disagree by more than 2 cm, you re-read.
Step two: measure water density. Seawater varies from roughly 1.020 to 1.030 g/cm³ depending on temperature and salinity. Hydrostatic tables assume 1.025 tonnes/m³. If a vessel sits in river water at 1.000, it sinks about 2.5% deeper for the same weight. Skip the density correction on a 100,000-tonne cargo and you have built in a 2,500-tonne error, serious money. A digital densitometer with temperature compensation is far more reliable than a glass hydrometer, especially in cold water.
Step three: sound all tanks. Ballast tanks, fuel tanks, fresh water, bilges, everything containing liquid gets measured. You apply the ship's tank calibration tables to convert depth readings to volume to weight. This is where surveys most often go wrong. Tanks are rarely perfectly rectangular, and calibration tables can be out of date after repairs or modifications.
Step four: calculate initial displacement from hydrostatic tables using corrected mean draft. This corrects for hull bending (hog or sag) and trim. A vessel hogged by 10 cm in heavy seas can throw your mean draft off by enough to matter.
Step five: repeat everything after cargo operations. Step six: subtract, adjust for consumables changed between readings, and you have your cargo weight.
Practical pitfall: Wind, waves, and current are the surveyor's enemies. Reading draft marks in anything over 0.5 m swell is guesswork dressed up as measurement. Experienced surveyors wait for calm or average multiple readings. But in a busy port, the terminal wants you done fast, and that pressure leads to sloppy readings.
Why Draft Surveys Matter Financially
The 0.5% figure you hear everywhere is not an IMO regulation. It is a commercial trade allowance adopted by P&I Clubs, Gard, Japan P&I Club, West of England, as a practical threshold. If the difference between load-port and discharge-port draft survey figures exceeds 0.5%, that is probably a real physical shortage, not measurement noise, and it usually triggers a formal cargo claim. For a Panamax carrying 75,000 tonnes of iron ore at USD 120/tonne, 0.5% is USD 45,000, enough to fight over.
Under the UNECE Code, accuracy as tight as 0.1% has been demonstrated in controlled conditions with independent surveyors on the same vessel. That is the theoretical ceiling. In day-to-day port conditions, most experienced surveyors aim for 0.3-0.5% and are happy with it.
Technology Changing the Game
Computer vision is the biggest leap. High-res cameras capture draft mark images, and algorithms extract readings with sub-centimetre consistency, no parallax, no squinting from a bouncing boat. Digital densitometers feed density data straight into the calculation. The real value is not speed, though saving an hour helps. It is the audit trail: timestamped photos attached to every reading, a tamper-proof record that survives a courtroom cross-examination better than a surveyor's handwritten notebook.
Frequently Asked Questions
How accurate is a draft survey?
In calm conditions, a properly conducted survey hits within 0.5% of actual cargo weight. The UNECE Code documents accuracy as tight as 0.1% under ideal conditions. Rough water, strong current, or ballasting operations still in progress all degrade accuracy substantially.
How long does a draft survey take?
A manual survey runs 2-4 hours end to end, reading marks, sounding tanks, doing the calculations. Digital platforms with automated hydrostatic lookup and density integration can cut this below an hour. But the bottleneck is rarely the math; it is getting safe boat access to all six draft marks on a large vessel at a busy berth.
Free tool: Try our online Draft Survey Calculator — enter 6 draft readings and get Quarter Mean, trim corrections, and cargo weight instantly.
Related Terms
- Ballast Water, water carried for stability; measuring it accurately is one of the hardest steps in a draft survey.
- Displacement Tonnage, the physical principle the whole method rests on.
- Marine Surveyor, the qualified professional who conducts the survey.
- Load Line, the Plimsoll mark; defines the maximum legal draft.