A draft survey is a practical way to find the weight of cargo loaded onto a ship by measuring how deep the ship sits in the water. The key observation is simple: a ship sinks lower as more weight is added. By comparing the draft before loading and after loading, marine workers can estimate the weight of the added cargo.
This matters because ships often carry thousands of tons, and weighing each item directly is not realistic.
The method is based on Archimedes' principle, which says that a floating object displaces a weight of water equal to its own weight. When cargo is added, the ship must displace more water, so its waterline rises on the hull. Surveyors read draft marks at the bow, middle, and stern, then use hydrostatic tables for that ship to convert draft into displacement.
The cargo weight is found by subtracting the ship's displacement before loading from its displacement after loading, with corrections for water density, fuel, ballast, and stores.
Key Facts
- Archimedes' principle: buoyant force equals the weight of displaced water.
- For a floating ship: ship weight = weight of displaced water.
- Draft is the vertical distance from the waterline to the bottom of the hull.
- Cargo weight = final displacement - initial displacement.
- Displaced water mass = water density x displaced volume, or m = rho V.
- Seawater is denser than freshwater, so the same ship floats higher in seawater than in freshwater.
Vocabulary
- Draft
- The depth of a ship below the waterline, measured from the water surface to the bottom of the hull.
- Displacement
- The weight of water pushed aside by a floating ship, equal to the total weight of the ship and everything on it.
- Waterline
- The line on a hull where the surface of the water touches the ship.
- Buoyant force
- The upward force exerted by a fluid on an object placed in it.
- Ballast
- Water or other weight carried low in a ship to improve stability and control draft.
Common Mistakes to Avoid
- Using only one draft reading, which is wrong because ships may trim forward or aft. Read the bow, midship, and stern drafts to estimate the average draft more accurately.
- Ignoring water density, which is wrong because freshwater and seawater provide different buoyant force for the same volume displaced. Apply a density correction when the water is not standard seawater.
- Treating draft change as cargo weight directly, which is wrong because draft is a length, not a mass. Use the ship's hydrostatic tables to convert draft into displacement.
- Forgetting changes in ballast, fuel, or supplies, which is wrong because these also change the ship's weight. Correct for any added or removed non-cargo weight between the before and after readings.
Practice Questions
- 1 A ship's displacement before loading is 18,400 t and after loading is 24,950 t. What is the mass of cargo loaded?
- 2 A floating barge displaces 320 m3 of seawater with density 1025 kg/m3. What mass of water is displaced, and what is the barge's mass in kilograms?
- 3 A cargo ship has the same total mass in freshwater and seawater. Explain why its draft is deeper in freshwater than in seawater.