# Why do Ships Float?

Container ship in St. John’s harbor, Newfoundland, Canada (milehightraveler, iStockphoto)

Amy McDonald
5.72

Have you ever been on a ship and wondered how you’re staying afloat? The answer is buoyancy!

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 harbour?

## 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.

Did you know?

Archimedes is thought to have run naked through the streets shouting “Eureka!” (“I have it!” in Greek) after realizing that the amount of water displaced from his bath was equal to his body’s weight.

### 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!

Did you know?

The heaviest ship in the world is the Pioneering Spirit. It displaces about 900 000 metric tons of water. That’s equivalent to about 300 000 elephants!

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!

Did you know?

After striking the iceberg, the Titanic sank in 160 minutes. That’s only 2 hours and 40 minutes!

• Have you ever played with a toy boat in water and tried to make it sink? What did you do to make it sink?
• If you were trying to make an object float, what would you do to it?
• How are boats designed to both carry heavy loads and remain afloat?
• Why is the design of a personal floatation device important? What factors affect the design? Why do you have to know how much you weigh when you buy a PFD?
• Why does an object float? What is buoyancy?
• What is Archimedes Principle?
• How does the density of an object affect its buoyancy?
• Why is it possible for an extremely heavy ship to float? How does added cargo impact on the buoyancy of a ship? Could a ship have too much cargo? Explain.
• Why is making careful notes and observations an important aspect of science? How do we know that Archimedes developed the principle of we used to explain why things float? Archimedes carefully documented his findings on parchment. This parchment was later repurposed for another book. The existing manuscript is known as the Archimedes Palimpsest. Learn more about this important scientific record and how it is being conserved.
• The movie Titianic (1997) directed by James Cameron was hugely successful. Aside from the stellar cast, what aspects of that movie helped make it such a popular blockbuster? If you have watched this movie, did you enjoy it? If so, what did you like about it?
• This article can be used for Math & Physics teaching and learning related to buoyancy and fluids. Concepts introduced include Archimedes’ Principle, fluid, displaced, buoyant force, weight and mass.
• After reading this article, teachers could have students could have students complete a Concept Definition Map learning strategy to consolidate the concept of buoyant force. Ready-to-use Concept Definition Map reproducibles are available in [Google doc] and [PDF] formats.
• For a Nature of Science (NOS) extension, teachers could have students explore the website The Archimedes Palimpsest about the conservation of this manuscript that contains copies of Archimedes original writings, and this timeline of the Archimedes Palimpsest.

An article from the Pratt School of Engineering at Duke University that includes background information and engineering-focused activities to demonstrate buoyancy. The website also contains a video (2:47 min.) on buoyancy.

An animated short video (4:41 min.) from TED-Ed refuting the legend of Archimedes’ Eureka! while also explaining his law of buoyancy.

A website dedicated to information about the Archimedes Palimpsest, the medieval parchment manuscript that contains documentation of some of Archimedes theories, including On Floating Bodies

Informative animated video (7:52 min.) from Simple History on the sinking of the Titanic.

## References

Elert, G. (n.d.). Buoyancy. The Physics Hypertextbook

History.com. (2019, March 19). Titanic sinks.

Khan Academy. (n.d.). What is buoyant force?

Ross, R. (2017, April 25). Eureka! The Archimedes Principle. LiveScience.

Woodford, C. (2018, December 17). Ships and boats. Explain That Stuff.

## Amy McDonald

Amy McDonald is a postdoctoral fellow at the Alberta Centre for Toxicology located in the University of Calgary's Faculty of Medicine. Her Ph.D. is in analytical chemistry, and she really enjoys applying that knowledge to the field of toxicology. In her free time she enjoys doing science outreach, running (she is training for a 10K marathon right now), playing softball, and reading.