Why ships float.
Steel is eight times denser than water, yet a steel ship floats — because it's the hull's hollow shape, not its material, that keeps its average density below the sea's. Load this 20 m × 8 m hull past its limit below and watch exactly where that stops being true.
Cargo load mcargo
m ∝ V
0 theavier cargo · deeper draft900 t
Try
By Archimedes' principle, a floating object displaces exactly enough water that the buoyant force equals its own weight — it floats whenever its average density (total mass ÷ total hull volume, hollow interior included) stays below the water's. This models a simplified rectangular hull: a fixed 20 m × 8 m footprint, 5 m total depth, and a 120 t steel structure, with only the cargo mass adjustable — real hulls are curved, subdivided into compartments, and far more complex, but the core density/displacement relationship is the same. The line this page treats as the flood threshold is a simplified stand-in for the Plimsoll line (international load line), a real, legally mandated mark painted on the hull of every seagoing cargo ship showing the maximum draft it's permitted to load to — not just an analogy here, but the actual regulatory system this maps to. Seawater (ρ ≈ 1,025 kg/m³) is denser than freshwater (ρ ≈ 1,000 kg/m³), so the same ship needs to displace slightly less volume — and rides very slightly higher — at sea than in a river.