When by the river, children pass the time by throwing all sorts of things into the water. Branches, stones… From these life experiences, they all observe the same thing: the branch floats and the stone sinks. It is systematic. Since the stone is often heavier than the branch, a deduction is made: what is light floats, and what is heavy sinks.
But this biased view is quickly challenged when the children look at boats. They are heavy, yet they float. How is this possible?
Floating rhymes with density
Used in nautical terms, the buoyancy of a boat is a question of density. Density is the ratio of mass to volume. It depends mainly on two parameters.
First, the number of atoms per volume. The more atoms there are in a volume, the heavier the volume will be. Let’s take coal and diamonds as examples. Both materials are composed exclusively of carbon atoms. But the arrangement of these carbon atoms, their organisation, makes all the difference. For the same volume, coal has half the number of atoms as diamond. It will therefore be twice as light.
Apart from the organisation of the atoms, their mass must also be taken into account, which will depend directly on the chemical element. For example, for the same volume, a silver bar weighs half as much as a gold bar.
How is this density important for buoyancy?
The answer to this question was given to us about 250 years before our era. The tyrant of Syracuse, Hieron, had his goldsmith make a crown of solid gold. Once he received the crown, the tyrant began to doubt the craftsman. Had he replaced some of the gold with silver, a much cheaper metal? Hieron then asked Archimedes, a genius inventor, to check whether the crown was indeed made of pure gold. But the problem seemed insoluble. When gold and silver are amalgamated, it is impossible, simply by looking at it,to distinguish pure gold from gold amalgamated with silver. Therefore, another solution had to be found. One day while enjoying a bath, he observed that the water of said bath rose as he submerged himself. He then got out of his bath and shouted, naked, in the streets of Syracuse, “Eureka”, which means, “I have found it”! He immersed an ingot of pure gold of the same mass as the crown and noted the volume of water displaced. He did the same with a silver ingot of the same mass. Then, as he immersed the crown, he realised that the volume of water displaced was between those of the two ingots. The crown was therefore made of a mixture of the two metals. We don’t really know what happened to the goldsmith, though…
This story is said to be a legend. At least, to this day, no written record has been found of this story. This is probably a rumour that has been passed on for centuries and that has anchored Archimedes in history.
When an object is immersed in a fluid (a gas or a liquid), it will displace a certain volume of that fluid. The volume will depend on the density of the object and the fluid.
Definition of buoyancy
The object immersed in the fluid experiences a particular force called buoyancy. The more the fluid repels the object, the greater the buoyancy. In other words, the greater the density of the fluid and the smaller the density of the object, the greater the buoyancy. Or, in the case of boats: the lower the density of the boat, the greater the buoyancy. And for a boat to float, for it not to sink too much, it needs a lower density than water.
“Yes, but then how can a steel boat float? After all, steel is extremely dense!”
Indeed, steel has a density about 8 times that of water. But the boat is not just made of steel. It is, for the most part, composed of air! This is where the shape of the hull comes in! Take the world’s largest cargo ship , the Ever Ace, for example. At 400 m long, 62 m wide and almost 30 m high, it can carry up to 225,000 tonnes. It sounds insane. However, when we ratio this load to the total volume of the boat, the density is about 0.3. In other words, when this gigantic boat is loaded, its density is three times lower than that of water!
Buoyancy is therefore entirely dependent on the hull of the vessel, mainly its shape and material of construction. Today, the hull of most ships is made of infused polymer resin. This resin not only makes the hull watertight, but also allows air to be injected into the hull, which further reduces the watertightness of the boats.