Vessel Draft Survey Procedure

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A vessel draft survey determines cargo quantity using the vessel's displacement and the weights to be deducted. The procedure comprises five main activities: checking documents, reading drafts, measuring water density, determining deductions and calculating results using the vessel's hydrostatic information.

1. Check the vessel's condition and documents

• Assess the vessel's longitudinal trim and transverse list at the time of the survey.
• Check the hydrostatic tables, tank capacity tables and sounding/ullage correction tables.
• Verify the draft mark positions, their distances from the perpendiculars, the length between perpendiculars and the sign conventions.
• Prepare information on ballast tanks, fresh water, fuel, bilge water and other weights affecting the result.

2. Read all six draft marks

Observe and record all six readings directly in the survey notebook. Where additional equipment is needed to read the marks, arrange safe access and viewing conditions. The quality of these readings underpins the entire calculation.

Draft marks are not necessarily located at the perpendiculars. Correct the drafts using the actual distances and conventions in the vessel's documentation. Do not automatically add or subtract a correction without checking the measurement positions.

3. Sample and measure the water density around the vessel

After reading the drafts, take water samples from the area in which the vessel is floating and measure density using a suitable hydrometer. Density varies with salinity and temperature, so record the actual measurement at the time of the survey. Values of 1.025 t/m³ for seawater and 1.000 t/m³ for fresh water are common references and do not replace measurements at the site.

4. Determine the deductions

Measure or check the levels in tanks containing ballast water, fresh water, FO/DO, bilge water and other liquids within the calculation scope. Use the vessel's tank tables and record the temperature and density needed to convert volumes to mass.

5. Calculate using the vessel's hydrostatic tables

After correcting the forward, midships and aft drafts, denoted Fcd, Mcd and Acd respectively, determine the true trim and weighted mean draft:

Use MMM to look up displacement and the required parameters in the hydrostatic tables, including TPC, LCF and MCTC as defined in the vessel's documentation. Apply the first and second trim corrections and any other specified corrections. Use the correct LCF sign convention and MCTC variation data; do not treat every case as a subtraction using one universal formula.

Where the hydrostatic tables use a reference density of 1.025 t/m³, convert the trim-corrected displacement to the actual water conditions using the density ratio:

When determining cargo on board at a particular time, also account for the vessel's lightship weight and constant. When determining cargo loaded or discharged, compare the initial and final survey results and account for changes in weight during cargo operations.

Survey records and ITVC services

The report should show the draft readings, water density, tank data, hydrostatic tables used, corrections and calculation results. This article introduces the work sequence; the specific calculations must follow the documentation for each vessel and the applicable survey procedure.

ITVC provides marine survey services in Hai Phong, Ho Chi Minh City, Da Nang, Nghi Son, Vung Tau, Phu My and other locations by agreement.