Draft Survey Basics: How Cargo Weight Is Really Measured

For bulk cargoes — grain, coal, ore, fertiliser — the bill of lading quantity is usually decided not by a weighbridge but by a draft survey: measuring the ship's weight before and after loading by reading its drafts. Millions of dollars rest on those draft readings, so every deck officer should understand how the number is built.

The principle: displacement difference

A floating ship displaces a weight of water equal to its own total weight (Archimedes' principle). If you know the ship's displacement before loading and after loading, the difference — minus everything that is not cargo — is the cargo loaded:

• Cargo = (Displacement after − deductibles after) − (Displacement before − deductibles before)

The whole survey is therefore two careful sets of draft readings, each turned into a displacement via the ship's hydrostatic tables, each corrected for the real conditions at the time.

Step 1: read the drafts properly

Read all six draft marks — forward, midships, and aft, on both port and starboard sides. Take the mean of each pair, then combine: the "mean of means" is (mean forward + mean aft + 4 × mean midships) ÷ 6, which gives the midship draft proper weight. Read to the nearest centimetre, in calm water, with the ship steady — a rolling ship gives rolling numbers.

Step 2: apply the corrections

The draft you read is rarely the draft the tables want. Standard corrections, in order:

• Trim correction — if the ship is trimmed by the stern or head, the mean draft must be corrected using the trim and the hydrostatic data (first and second trim corrections). Large trims make large corrections: keep trim small during surveys.
• List correction — a listed ship reads deeper on one side; correct using the centreline draft formula.
• Density correction — hydrostatic tables assume seawater at 1.025 t/m³. Measure the actual dock water density with a hydrometer and correct the displacement proportionally. In brackish river berths this correction alone can be hundreds of tonnes.

Step 3: subtract the deductibles

Displacement includes everything on board, so deduct it all: fuel oil, diesel oil, lubricating oil, fresh water, ballast water, stores, and the ship's constant (the slow accumulation of paint, mud, and stores residue — found by comparing with a known lightship survey). What is left is cargo.

The mistakes that ruin surveys

• Reading drafts in a swell or while the ship is moving — wait for calm, or take the mean of several readings
• Guessing the water density instead of measuring it with a hydrometer
• Forgetting ballast exchanges between the two surveys
• Using an outdated or wrong "constant" — recheck it whenever you can
• Rushing the surveyor — a good independent surveyor is your best protection, and theirs

Work through draft surveys step by step with SMART SEAMEN's free draft survey calculator — get started.

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