Science · 6 pages · 12 min
Why a boat made of steel can float
The hull is mostly air, so the average density is what floats even though the steel is dense.
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From the opening
A boat made of steel can float because the hull encloses a large volume of air, so the average density of steel, air, and cargo can be less than the density of water. The water pushes up with a force equal to the weight of the water the hull displaces.
Steel has not stopped being denser than water, and a solid steel brick of the same mass sinks. The shape is the trick, because a shell displaces far more water than the metal alone ever could.
The mechanism is more particular than the slogan. The buoyant force equals the weight of the displaced fluid, whether the object is wood, steel, or a bubble. As the hull settles lower it displaces more water, the upward force grows, and sinking stops when that force matches the weight.
Follow it one step further. Floating is not the same question as staying upright. A hull stays upright when a small tilt shifts the center of buoyancy enough to right it, and a steel barge can float and still capsize if the weight sits high or far to one side.
The case worth keeping in mind is specific. A steel mixing bowl in a sink floats until water laps the rim and fills it. Once it fills, the average density is that of steel plus water and the bowl goes to the bottom, even though the metal never changed.
That case has a boundary. The account assumes an unflooded hull displacing the fluid around it. A boat already full of water is no longer a light interior inside a shell, and a puncture ends the average-density trick.
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Inside
- Average density of the whole hull
- Buoyant force and displaced water
- Why a solid brick still sinks
- Floating is not the same as stable
- Where the idea stops applying
- A distinction worth keeping
Sources
- Archimedes principle of buoyancy
- Average density of a hollow hull versus solid steel