Plate 34 · Matter
Why a Steel Ship Floats
A steel screw dropped in water sinks straight to the bottom. Yet a cargo ship built from thousands of tonnes of steel can take on more than ten thousand tonnes of cargo and still float. The steel is the same. What changed is its shape.
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The Same Steel
This cargo ship is 140 metres long, and empty it weighs about 6,000 tonnes, mostly steel. Melt that much steel into a solid cube and it would be only about 9 metres on each side.
Put the cube in the sea and it sinks to the bottom. The same amount of steel built into the shape of a ship floats.
The same 6,000 tonnes of steel · one cube, one empty ship
The Water Pushes Back
Anything put into water shoves some of the water aside. Water presses harder the deeper you go, so it pushes up on the bottom of an object harder than it presses down on the top. The difference is an upward push. How strong? Exactly as strong as the weight of the water pushed aside. Archimedes worked this out more than 2,000 years ago.
The steel cube pushes aside only about 760 cubic metres of water, which weighs about 780 tonnes: nowhere near enough to hold up 6,000 tonnes of steel. A ship is mostly empty space inside. Its bottom only has to sink about 2.7 metres before the seawater it pushes aside weighs 6,000 tonnes, enough to hold the whole ship up.
Load It, and It Sinks Deeper
Put cargo aboard and the ship settles lower, pushing aside more water, so the push grows until it holds up the whole weight again.
With 10,000 tonnes of cargo, this ship sits about 7.2 metres deep. It would take more than 18,000 tonnes before the sea reached the deck. Real ships are never loaded that far: a line painted on the hull marks how high the water may come.
Deeper in a River Than at Sea
Seawater has salt dissolved in it, so each cubic metre weighs about 25 kilograms more than fresh water. The same ship has to push aside the same weight of water, so in fresh water it sinks a little deeper: about 2.5% deeper.
That is why a ship carries more than one load line: one for fresh water, one for the sea, and others for tropical and winter seas. The first mark is named after Samuel Plimsoll, a British MP who campaigned for it. British law required ships to carry one from 1876; later the government, not the owner, decided where it went, and since 1930 an international agreement has applied it worldwide.
Load lines: TF tropical fresh · F fresh · T tropical · S summer · W winter
Your Turn
Load the ship and see how deep it sinks. Then switch to river water, seawater or Dead Sea water and see how high the same ship floats. Load too much and it goes under.
Try thisWith 17,000 t of cargo, switch from river to Dead Sea: how much higher does the ship ride?
What the model leaves out
The hull is treated as straight-sided, with its underwater part filling 70% of the box around it; a real ship has a pointed bow and rounded bottom, so draft and load don’t match exactly. A ship also has to stay upright, which depends on how high its weight sits; that isn’t calculated here. Dead Sea water is taken as about 1,240 kilograms per cubic metre; the real figure varies with season and depth.
A common misreading
“Heavy things sink and light things float” isn’t right. A huge ship weighs far more than a pebble, yet it floats. What matters is whether the water an object pushes aside can hold up its weight — in other words, whether the whole object, including the air inside it, is lighter than the same volume of water.
Sources
Follow the question
Plate 17Why Ice FloatsFreeze most liquids and the solid sinks in its own liquid. Ice floats. The secret is in the little hexagons water molecules build.
Plate 35The V Behind a ShipA big ship, a small boat, even a swimming duck: the V-shaped wake behind them opens at almost exactly the same angle, about 19.5° on each side.