Why a loaded ship sits lower

A ship floats because it pushes aside water, and the water pushes back with a force equal to the weight of what was pushed aside (Archimedes' principle). Add cargo and the ship must sink deeper to displace more water. For a ship with fairly straight sides, the extra depth is the added mass divided by the water's density times the area of the hull at the waterline.

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Floating hullThe empty hull already sits a few meters deep, displacing water equal to its own weight. That depth is called the draft.
Add cargoEvery tonne loaded must be matched by another tonne of displaced water, so the hull settles lower.
Draft marksNumbers painted on the bow and stern let the crew read exactly how deep the ship sits, and so how much cargo is aboard.
Fresh waterFresh water is less dense than seawater, so the same ship floats deeper in a river port. Load lines include a separate fresh water mark for this.

Archimedes in a formula

A floating ship displaces a volume of water whose mass equals the ship's mass. If the hull sides are nearly vertical near the waterline, adding a mass m lowers the ship by m / (rho x A), where rho is water density and A is the waterplane area, the hull's footprint at the waterline.

This model uses a 150 m by 25 m hull with a waterplane that fills 85 percent of that rectangle, and an empty ship of 12,000 tonnes. Real hulls flare and narrow, so naval architects use hydrostatic tables, but this wall-sided version is the standard first estimate.

Try it: load 10,000 t in seawater and note the total draft, then switch to fresh water. The ship sinks about 17 cm deeper without a single box added.

Tonnes per centimeter

Crews use a handy number called tonnes per centimeter immersion: how much cargo sinks the ship one more centimeter. It is rho times waterplane area divided by 100. For this hull it is about 33 tonnes per centimeter in seawater.

Sources

Principles of Ship Performance, Chapter 3: Hydrostatics (EN400)U.S. Naval Academy, Naval Architecture and Ocean Engineering
Predictive Model for Hydrostatic Curves of Chine-Type Small Ships Based on Deep LearningJournal of Marine Science and Engineering (MDPI, open access)
Buoyancy: Archimedes PrincipleNASA Glenn Research Center

How to cite this model

Avid Solutions International. (2026, November 11). Why a loaded ship sits lower [Interactive 3D model]. Avid Field Lab. https://avidsolutionsintl.com/explore/cargo-ship-buoyancy

Last updated November 11, 2026

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