✓ Sources checked · 10 September 2026 · View verification
A ship floats when it displaces water whose weight balances the ship’s total weight. A hollow watertight hull provides displacement volume; steel alone does not have to be less dense than water.
By Matan · Updated
Include hull, cargo and everything aboard
For a floating ship at rest without other vertical support, ρwaterVdisplaced = mtotal. The displaced volume is the submerged hull volume, not its complete internal volume. Adding cargo generally increases draft because more water must be displaced. [1]
Worked cargo-induced draft change
Assume a box-shaped barge has waterplane area 40 m² and receives 20,000 kg of cargo. In fresh water of density 1000 kg/m³, additional displacement is 20 m³, so draft increases by 20/40 = 0.50 m. With assumed seawater density 1025 kg/m³, the same cargo increases draft by 0.488 m.
Enough buoyancy does not guarantee upright floating
The location of the centre of mass and how the centre of buoyancy shifts during heel determine restoring behavior. High cargo or partly filled tanks can affect stability even when the vessel still has enough displacement volume.
Flooding changes the problem
Water entering previously watertight spaces reduces effective reserve buoyancy and can alter weight and stability, depending on the accounting model. The draft example assumes vertical sides, unchanged waterplane area and adequate freeboard. It is a teaching calculation, not a cargo-loading limit.
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How this article was checked
Sources checked; calculations checked where shown. Main claims and model assumptions were checked against the references below. Example arithmetic and units were checked separately. Numerical examples are illustrative calculations, not experimental measurements.
Source checking and editing assisted by AI. No independent human scientific review is recorded for this revision. This record applies to the version dated 10 September 2026.
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Cite this article
Matan. “Why Ships Float.” Your Physicist. Originally published 2026-06-11; updated 2026-09-10. https://your-physicist.com/why-ships-float/
Revision history
10 September 2026: Replaced repeated analogies with cargo/draft calculations and separated flotation from stability. Added references and a transparent verification record.
