Have you ever seen one of those giant cruise ships? Or a cargo ship piled high with containers? Or perhaps even an aircraft carrier covered with jet planes? How are these huge ships able to float on water? Water is much less dense than the steel these ships are made of. So why aren’t these ships sinking to the bottom of the harbours?

In this post, Pritish Kumar explained buoyancy, how Archimedes’ Principles work and how this applies to ships.

What is buoyancy?

We can thank Archimedes for being the first to explain the principle behind this puzzle. Archimedes was a Greek scientist who was born in 287 BCE. This principle is known as buoyancy or Archimedes’ Principle.

Archimedes’ Principle states that the force exerted on an object in a fluid is equal to the weight of fluid displaced (moved out of the way) by the object. This force is called buoyant force. The buoyant force pushes upwards against the object. Gravity exerts a downward force on the object (its weight), which is determined by the object’s mass. So if the force exerted downward on the object by gravity is less than the buoyant force, the object will float.

How does buoyancy relate to density?

If a block of wood measuring one cubic centimeter (1 cm x 1 cm x 1 cm) is placed in a container of water, the amount of water displaced will equal the weight of the block of wood. But what about if a block of the same size is made of lead? Lead has a much higher density than wood. If a one cubic centimeter block of lead is placed in a container of water, the amount of water displaced will equal the weight of the block of lead.

In the case of the wood, the weight of the water displaced is small. The buoyant force is greater than the gravitational force, so the wood floats. The lead is denser than the wood. That means it contains more mass in the same volume. So, more water is displaced by the lead than the wood. The gravitational force on the lead exceeds the buoyant force, so the lead sinks.

How do buoyancy and density apply to ships?

How can this principle be applied to ships? Ships are enormous steel vessels. A ship can have a mass of hundreds of thousands of tonnes. Steel is much denser than water, so you would think that massive steel ships would sink, right?

Well, think again! What helps keep ships afloat is their shape and what is inside them. Ships are not solid pieces of steel. Instead, they are mostly hollowed-out shells of steel. There are all sorts of components inside the ship. For example, the ship’s engine, fuel, and cargo may be inside. But most importantly, there is air inside a ship.

The air that is inside a ship is much less dense than water. That’s what keeps it floating! The average density of the total volume of the ship and everything inside of it (including the air) must be less than the same volume of water. As a ship is set in water, it pushes down and displaces an amount of water equal to its weight.

The closer the total density of the ship is to the density of the same volume of water, the greater the amount of the ship that will be in the water. If the average density of the ship is ever greater than the density of water, then the ship will sink beneath the surface of the water.

When a ship sinks?

It is because water enters the ship. This forces out the air, making the average density of the ship greater than that of the water. One of the most famous disasters is the sinking of the RMS Titanic. The ship struck an iceberg off the south coast of Newfoundland in April of 1912. The iceberg tore open several small holes in the hull of the ship, letting water into the bow. As more water entered the ship, the air was forced out. This caused the ship to sink to the bottom of the ocean.

Just like every other ship that ever sank, the Titanic ultimately went to the bottom of the ocean because of (a lack of) buoyancy.