How Do Ships Float? Explore Science Facts That Will Make You Wonder!

Abhijeet Modi
Oct 23, 2023 By Abhijeet Modi
Originally Published on Nov 05, 2021
Boat in sailing regatta.
Age: 3-18
Read time: 10.6 Min

Ships float in the water, based on the famous principle of Archimedes.

Did you know Seawise Giant, a ship twice the size of the Titanic was the largest ship to sink and then pulled up to sail again in 1989? It sank in the Iran-Iraq conflict that erupted in the '80s.

Fundamental to fluid mechanics, Archimedes principle is a law of physics. Archimedes' principle states that any upward buoyant force which is exerted on a body immersed in the fluid, partially or fully, is always equal to the amount of fluid that is displaced by the object.

Although it's relatively heavy, most modern ships which are made today are of steel. Strong lightweight metals like aluminium are used to make larger boats.

But how do ships float and what keeps them floating? The answer is air.

The air which is present inside the ship is denser than the water which helps the ship to float. So when the displacement of water is the same as its own weight it can float in the sea. One of the factors that determine the buoyancy of a boat is the content of salt in water.

Salty water is denser than freshwater which causes denser things to float in saltier water. After reading about the reasons behind floating boats, do check out how do plants grow and how do neon lights work?

Why does a ship float in water and how does that work?

Archimedes' principle is also known as the principle of buoyancy. It says that the upward (buoyant) force acting on a body submerged in a fluid is equal to the weight of the fluid displaced by the body.

A ship floats on the water because of something known as the Archimedes principle or the principle of buoyancy. When something is placed on the surface of the water, it can either float or sink to the bottom.

The principle of buoyancy states that the upward force acting on the body when displaced in a fluid is equal to the weight of the fluid displaced by the body.

Any object, either it sinks, or floats will experience an upward buoyant force. It is important to note that the weight of the water does not refer to its mass but rather the force acting on the mass of water due to gravity.

The weight of the body = (Mass of the body) X earth’s gravitational acceleration.

The earth’s gravitational acceleration is equal to 9.8 m/s2 or 10 m/s2.

If the weight of the ship (its mass multiplied by gravity) is more than the force due to buoyancy, the net force acting on it will be downwards and it will sink into the water.

If its weight is less than the weight of water displaced by the ship, then the net force acting on it will be upwards, and the ship will float in the water.

This principle works the same regardless of the size of the body.

The large passenger-designed ships which are mainly used for vacationing are categorized as Cruise Ships. Cruise ships embark on tours and journeys to various ports. They are not used for transport like ocean liners. Compared to them, cruise ships have less speed, hull strength, and agility.

The reasons that keep the cruise ship from tipping over are the low center of gravity and the ballast. Both these reasons give a combined effect on the buoyancy of the ship.

To keep the center low of gravity, all of the heaviest equipment is decked below. With the presence of ballast tanks in the ship, it is easier to maintain the balance of the ship. The role of these tanks is to counter waves and reduce rocking.

These tanks contain water that is pumped from side to side so that if the sea gets rough, the balance is maintained. Several ballast tanks are used for larger ships.

To help the cruise to roam through the ocean smoothly the shape of the cruise ship's hull is designed to be rounded and wide. These round edges increase the stability of the ship and help it move with minimal drag. It is surprising to know that strong winds cannot cause the ship to sink.

Why do ships float while a coin sinks?

Even if ships and coins are made of the same material and a ship is much larger and much heavier than a coin, a ship displaces more water as compared to its weight than the coin hence the net force acting on it is upwards and it floats on the surface of the water.

The volume of water an object will displace is equal to the volume of the object which is submerged underwater.

We know that the density of an object = the mass of the object/its volume.

Hence the net downward force (or the weight) = mass X g (acceleration due to gravity).

Or weight = density X volume of an object submerged in water X g.

Similarly, the upward force = the density of water X volume of water displaced X g.

This implies that the upward force on the body = density of water X volume of an object submerged X g.

Hence whether an object sinks, or floats underwater depends on the density of the object relative to the density of water.

If the density of the object is more than the density of water, it will sink to the bottom, if it is less than the density of water, the object will float and if it is equal to the density of water, the object will just float on the surface as the upward and downward forces are equal to each other and cancel each other out.

The buoyancy is caused due to the variation of liquid pressure with the variation of height in the fluid.

Essentially, as the depth of the water increases, the weight of the water above it exerts a downward force on the water below.

Now, Newton's third law states that every action has an equal and opposite reaction, which means that if a force is exerted on a body, it has to exert an equal and opposite back on another object.

For example, when you fire a gun, the force which the gun exerts on the bullet, exerts an equal force back on you as recoil.

If iron is denser than water how do ships float?

The density of iron is greater than the density of water, hence a raw block of iron will sink right to the bottom. So why does a ship that is made up of iron float on the surface of the water?

As we saw whether an object sinks or floats on the surface of the water is determined by its density relative to the water. Even though iron has a greater density than water, we need to keep in mind that the ship is not entirely made up of iron.

The base of the ship is designed such that it is hollow, the air inside the ship is much less dense than water. This keeps the total density of the ship lesser than that of water and keeps the ship afloat on the surface.

The base of the ship is made broad so that it can displace as much water as possible while it is submerged, which increases the upward buoyant force.

If the weight or displacement tonnage of the ship increases, for example, due to the loading of cargo, the downward force on the ship is increased. To counter this force, the ship submerges a little more into the water of the ocean to displace more volume of water and increases the buoyant force to counter the downward force.

A ship can sink in the ocean if somehow its density increases, for example, if it has a hole in the bottom and water is leaking into it. The water inside the ship will replace the air and increase the ship’s mass.

A fleet of sailing boats during offshore race

Which are the biggest ship in the world?

The biggest ship in the world is the TI class of supertankers. The ships include TI Africa, TI Asia, TI Oceania, and TI Europe. Originally, these ships were named Hellespont Alhambra, Hellespont Fairfax, Hellespont Metropolis, and Hellespont Tara.

They are active for four years and are still in service. These ships are Ultra Large Crude Carrier which carries 503,409,900 L. These crude oil tankers were made by a South Korean shipping company called Daewoo Shipbuilding and Marine Engineering.

All four ships were constructed for the shipping company called Hellespont Group. Later in 2004, all four ships were purchased by the Belgian Shipowner Euronav NV. Unless on a ballast voyage, these ships cannot travel through the Suez Canal.

Unlike metal or ironwood isn’t as dense as water hence wood naturally displaces more water than its weight and the buoyant force is greater than the downward gravitational pull of the earth due to the wood's weight. Hence dry wood floats on the surface of the water.

It is important to keep in mind that wood is made up of cellulose, cellulose is a polymer of beta D glucose monomers, meaning many glucose bonds together to form a long chain of cellulose. Cellulose is the same material from which cotton is made of.

Raw cellulose is denser than water and has a density of 1.5 g/mL, which is 1.5 times denser than that of water.

Then how does wood float on water? The answer to this is in the structure of the wood. The cellulose makes up only a fraction of the entire volume of wood.

There are many hollow spaces inside the wood’s structure. Wood is a porous tissue, similar to a very hard sponge.

In a living tree, this wood is filled up with resins. This is why wood that is freshly cut from trees sinks into the water. After cutting the wood it is allowed to dry up and the sacs are filled with air which reduces the density of wood making it easier to float on water.

This is why wooden boats float on water more easily and only a small part of the boat is submerged as compared to a boat made up of steel or iron.

However, there are also some drawbacks to using wood to make boats. The obvious one is the availability of wood.

Deforestation is one of the biggest issues our planet is facing and is a major contributor to climate change hence, cutting wood to make giant ships is not a good idea at all. Other factors are the weakness of wood to biological agents like fungus.

When wood gets soaked in water, it creates moisture, which is a perfect environment for fungus to grow in.

When wood stays wet for a long time, it soaks water into its pores which can make it denser and cause it to sink. Also, wood is not as sturdy as other metals which makes it undesirable to build ships, however, small boats of wood are widely used around the world.

Yes, the materials used to make ships such as steel or any other metal (not iron, iron is not resistant to attack from saltwater in the sea and corrodes easily. Iron is not a suitable material to make a ship) is denser than water.

This should make ships sink right to the bottom of the seas like a block of metal in a tub of water.

However, ships aren’t just blocks of metal, ships are made hollow from the inside. These hollow structures are filled with air which is less dense than water and makes the total density of the ship less than that of water.

Ships are designed in such a way that they experience a very large buoyant force due to the amount of water they displace.

A simple experiment to understand how Archimedes principle works is to take a bowl made of steel or any other metal and place it on a bucket filled with water.

The bowl is hollow, that is it's filled with air which makes its density less than that of water and makes it float. You can add more weight to the bowl and see how much weight it can hold until it sinks.

Instead of weights water can also be used to observe a similar effect.

The weights resemble the cargo on a ship. The greater the cargo loaded on the ship, the greater its weight and it’ll displace more water and submerge more into the water.

Due to this principle, the water at depth exerts another upward force on the water at the surface which is experienced as the buoyant force by the object on the surface of the water.

Here at Kidadl, we have carefully created lots of interesting family-friendly facts for everyone to enjoy! If you liked our suggestions for how do ships float?

Explore science facts that will make you wonder! Then why not take a look at bagworm life cycle: interesting facts on moths for kids revealed! Or blue whale diet: why do this marine mammal like krill as food!

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Written by Abhijeet Modi

Master of Computer Science

Abhijeet Modi picture

Abhijeet ModiMaster of Computer Science

An experienced and innovative entrepreneur and creative writer, Abhijeet holds a Bachelor's and Master's degree in Computer Application from Birla Institute of Technology, Jaipur. He co-founded an e-commerce website while developing his skills in content writing, making him an expert in creating blog posts, website content, product descriptions, landing pages, and editing articles. Passionate about pushing his limits, Abhijeet brings both technical expertise and creative flair to his work.

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