In maritime logistics and cargo operations, measurement accuracy is critical. Imagine a large ship loaded with thousands of tonnes of cargo, and the need to determine that cargo's weight with minimal error. This is where the draft survey comes in: a method that gives the net quantity of cargo without unnecessary complications. Based on the principles of hydrostatics, it is the standard way of determining weight in port. In this article, we explain what a draft survey is, how it is carried out and why it is indispensable in shipping operations.
Submit an applicationA draft survey is the process of determining a vessel's displacement by measuring its draft. Draft is the depth to which the hull is immersed in the water. In essence, the survey relies on Archimedes' principle: the weight of the vessel equals the weight of the water it displaces. The difference in displacement before and after loading or discharge gives the precise quantity of cargo.
The method is used in the maritime industry to calculate the weight of cargo lots, ballast or fuel. It is particularly important where direct weighing of the cargo is impossible or impractical.
The surveyor who performs the procedure must have in-depth knowledge of hydrostatics and ship construction. Unlike other methods, a draft survey can take account of any changes in the vessel's condition, including the contents of the ballast tanks and the vessel's constant. The procedure is governed by international rules, such as the standards of the IMO (International Maritime Organization), and must be carried out in accordance with them to obtain reliable results.
A draft survey is a systematic process that requires careful preparation. It usually has an initial and a final stage, before and after cargo operations. Below we go through the main steps so that you can see how it works in practice.
First, the surveyor inspects the vessel, checking that all the necessary documents and technical equipment are in place. It is important to make sure that the crew is ready to cooperate and that the vessel is lying in calm conditions, without heavy swell or strong current.
Next, the drafts are read. Readings are taken at six key points: forward, aft and midships, on both the port and starboard sides. The draft is read from the draft marks on the hull, which are given in metres or feet. The mean draft is calculated as the average of these readings.
After the readings, the trim (the difference between the forward and aft drafts) and the list, which shows the vessel's heel, are determined. If the trim is greater than zero, the vessel is trimmed by the stern; if it is less than zero, by the head.
Water samples are then taken to determine the water density. The density of the dock water (the water in which the vessel is floating) depends on temperature, salinity and location (river or sea). The standard value for seawater is 1.025 t/m³, but the actual value may differ. The sample is taken with special equipment, and its density is measured with a hydrometer.
Next, ballast, fuel and other liquids on board are accounted for. The surveyor checks the levels in the tanks and uses the vessel's tables to calculate their weight. This includes corrections for temperature-related changes in volume.
Finally, the displacement is calculated from the vessel's hydrostatic tables. These tables form part of the vessel's documentation and give the displacement, TPC (tonnes per centimetre immersion) and other parameters for different drafts and trims.
The procedure may be carried out in several stages if operations extend over a long period. If there is swell or other interference, the readings are repeated for greater accuracy.
The accuracy of a draft survey is usually 0.5-1% of the cargo weight, but it depends on many factors. Weather plays a part: swell, wind or currents can distort the draft readings, so measurements are best taken in calm water.
Water density. If it is not correctly accounted for, the results will be wrong. Temperature affects the expansion of liquids in the tanks and requires corrections.
Vessel condition: list, trim, hull deflection caused by the cargo. The hydrostatic tables must be up to date; if the vessel has been modified, they may no longer be accurate.
Human error: inaccurate reading of the marks, incorrect sampling or calculation errors. The crew must cooperate by providing complete information on ballast and fuel.
Freeboard and wave height are also taken into account. In the case of a significant list, the readings are corrected by calculation to obtain the mean draft.
To minimise errors, the surveyor always takes several readings and cross-checks the data. This makes it possible to achieve results that are close to ideal.
The surveyor is the hands-on specialist who carries out the entire process. Surveyors must be certified and follow the applicable rules. The survey report sets out all readings, calculations, corrections and the final weight. It is an official document and is used to issue the certificate.
The report includes the date, place, a description of the vessel, the water samples, calculation tables and any remarks. It sets out the calculations in full so that any party can check the data.
Draft surveys have a wide range of applications, from loading in one port to discharge in another. They are often combined with other inspections, such as a bunker survey.
A draft survey is one of the key stages of cargo surveying, and it determines how accurately the quantity of cargo carried is calculated. TopFrame specialists take the vessel's draft readings using certified instruments and proven methods, providing reliable data on the actual quantity of cargo on board.
We work in accordance with international standards and the principle of independence, which ensures objectivity and the trust of all parties to the transaction. Every TopFrame report is a document backed by experience, attention to detail and responsibility to the client.
Draft surveys are part of our survey services in ports in Russia and abroad.
