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Buoyancy

Buoyancy and how ships float explained showing 1m3 of water displaced supporting a tonne of weight and the same principle floating a 50,000 tonne ship

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It’s always been mind-boggling to me how some of the largest and heaviest things we build on Earth are ships that float on water. Cruise ships are like floating towns, some able to accommodate over 5,000 people. How can it be that these gigantic, impossibly heavy objects made of steel can float on water? The key is displacement and realising the considerable weight that the water already supports.

In a lake or ocean, each drop of water is supported by the water around it. 1m³ of fresh water weighs 1 metric tonne, which is 1,000 kg. If a ship displaces 1m³ of water, the buoyant force of the surrounding water supports 1 tonne of the ship’s weight. Therefore, a ship weighing 50,000 tonnes will just float if it displaces 50,000m³ of water.

Sea water contains salt, which makes it denser than freshwater. As the denser water supports a greater weight, a ship will float slightly higher in salt water than in fresh. The International Load Line, previously called the Plimsoll line, shows how deep a boat sits in the water.

To realise the immense power of water, you can try submerging a biscuit tin in a bath. It’ll put up quite a fight.

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Published 25 Aug 2024

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Keep exploring

Iceberg floating orientation explained: Icebergs are usually drawn floating vertically, while a stable iceberg orientation is usually on its side

Iceberg orientation

Eponym examples, explanation and etymology explained in a sketch with Erno Rubik of the Rubik's Cube, the Jacuzzi brothers, the 4th Earl of Sandwich, Lázló Biró, Etienne de Silhouette and Rudolf Diesel and his diesel engine

An explanation of the plimsoll line, the international load line, for how high a ship sits in the water

The Plimsoll line

Goodhart's law illustration showing a manager frustrated by 1000's of tiny nails when measuring on number of nails made, and pulling their hair out when presented with giant nails when measuring on weight

Goodhart’s Law: when a measure becomes a target, it ceases to be a good measure.

Vorfreude explanation and example: what is vorfreude? the pleasure in anticipation shown by someone looking enjoying looking forward to their upcoming holiday

Strand line illustration: a beach showing the line where debris is stranded by high tide

Nearly every iceberg you see in a picture or diagram is probably floating the wrong way. This was what I learned (from Megan Thompson-Munson) after sketching Biz Stone's brilliant saying about the myth of overnight success. With some approximation, the density of ice is around 900 kg/m3, and seawater is around 1,000 kg/m3. Therefore, the fraction of an iceberg that's submerged is around ~900/1000 or 0.9. So, about 90% of an iceberg is below the surface and 10% above, which is partly why they can be so dangerous. While most iceberg pictures get this part more or less correct, most of these icebergs will be floating vertically. In reality, a tall, thin iceberg will likely topple, so most icebergs end up floating on their side, not their tips, even though we rarely draw them this way. I remember learning about a fascinating experiment with children of different ages estimating which glass holds more water: a tall, slender one filled high or a wider glass filled to a lower level. Younger children almost always chose the glass with the higher water level as the most water, even when it was significantly less than the shorter and wider glass. I wonder if it's part of why we draw icebergs vertically, at least when we're using them as a metaphor. It's easier to grasp quantities vertically, and we commonly underestimate volume spread over a wider area. We may still want to draw our icebergs tall and deep to make our point, but now, at least, we can do so with the knowledge that they're not like the real ones. If you want to see it yourself, Joshua Tauberer made a brilliant draw-an-iceberg-and-see-how-it-will-float game. I recommend you give it a go so it sinks in forever (sorry). Also see: Overnight success Why ice doesn't sink Ice-cream, gelato, sorbet Know your poles: penguins or polar bears, frozen ice or land…Nearly every iceberg you see in a picture or diagram is probably floating the wrong way. This was what I learned (from Megan Thompson-Munson) after sketching Biz Stone's brilliant saying about the myth of overnight success. With some approximation, the density of ice is around 900 kg/m3, and seawater is around 1,000 kg/m3. Therefore, the fraction of an iceberg that's submerged is around ~900/1000 or 0.9. So, about 90% of an iceberg is below the surface and 10% above, which is partly why they can be so dangerous. While most iceberg pictures get this part more or less correct, most of these icebergs will be floating vertically. In reality, a tall, thin iceberg will likely topple, so most icebergs end up floating on their side, not their tips, even though we rarely draw them this way. I remember learning about a fascinating experiment with children of different ages estimating which glass holds more water: a tall, slender one filled high or a wider glass filled to a lower level. Younger children almost always chose the glass with the higher water level as the most water, even when it was significantly less than the shorter and wider glass. I wonder if it's part of why we draw icebergs vertically, at least when we're using them as a metaphor. It's easier to grasp quantities vertically, and we commonly underestimate volume spread over a wider area. We may still want to draw our icebergs tall and deep to make our point, but now, at least, we can do so with the knowledge that they're not like the real ones. If you want to see it yourself, Joshua Tauberer made a brilliant draw-an-iceberg-and-see-how-it-will-float game. I recommend you give it a go so it sinks in forever (sorry). Also see: Overnight success Why ice doesn't sink Ice-cream, gelato, sorbet Know your poles: penguins or polar bears, frozen ice or land

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