# How Does a Ship Float?: A Scientific Explanation of Buoyancy

Jagatheesh is a Mechanical Engineer and interested in writing on science and technology in our life.

Have you ever thought about how a ship floats, while a piece of iron sinks? Do you have any idea about the science behind the hot air balloon? Well, the answer is very simple. The mystery behind both of these things is the principle of buoyancy.

Before reading further, take a minute to thank the 3rd-century Greek mathematician Archimedes. He introduced the principle of buoyancy to the modern world. Ships, swimmers, hot air balloon, and submarines all function on the same principle, and this article will explain how it all works.

### The Science Behind How a Ship Floats

There are three basic concepts that explain how and why a ship can float:

1. Principle of Buoyancy: According to the principle of buoyancy, an object immersed in a liquid will face an upward force. When the upward force is more than the gravity (downward force), the object floats. The upward force exerted by the liquid is the buoyant force.
2. Archimedes' Principle: An object immersed in a fluid will experience an upward force from the fluid. The upward force is equal to the weight of the fluid displaced by the object.
3. Law of Flotation: Materials whose densities are lower than that of the liquid they are immersed in will float on that liquid. Wood and oil float on water, as the density of water is higher than the density of wood and oil.

## Floating and the Structure of a Ship

The structure of the ship is very important when it comes to floating. The design of the ship must ensure that it can displace water equal to the ship's own weight, as stated by Archimedes' principle.

The most important part of the design is the hull. The hull of a ship consists of hollowed-out shells of steel that contain an adequate quantity of air. It makes the ship less dense than water, which satisfies the law of flotation. The volume of air in the hull determines the floating and the load-carrying capacity of a ship.

The density of the ship—including the cargo, crew, and components—must be less than the density of water in order for the ship to float on top of the water.

A particularly important feature in the hull is its markings, referred to as the Plimsoll line or water line. Invented by English politician Samuel Plimsoll, these markings determine the load-carrying capacity and the space available for new cargo. They indicate the greatest depth a ship can immerse itself within.

## How Does a Ship Float?

If you see a ship launching video, then you might have noticed water rushing to the shore when the ship enters the water. This is because the ship displaces water equal to its weight and will immerse at a particular level in the water.

The air in the hull makes the density of the ship lower than the density of water. So the buoyant force (upward force) exerted on the ship by the water is higher than the downward force—thus allowing the ship to float in this condition.

When the ship is loaded, it will immerse to a certain level relative to the weight of cargo. The weight of the ship—including cargo, crew, and components—must have a lower density than the water, otherwise the ship will sink.

Plimsoll line indicates the safe level of immersion. Whe the ship is immersed beyond the Plimsoll line it will sink instead of reaching the destined port.

## Applications of Buoyancy in Daily Life

Here are just a few examples of how principles of buoyancy can be seen in everyday life.

• Hot Air Balloons: Hot air balloons are a perfect example of the principle of buoyancy. When the air inside the balloon is hot, it becomes less dense than the surrounding atmosphere, which makes the balloon float in the air.
• Swimmers: When you are swimming, the water displaced by your body is higher than your body weight. Our lungs act like a balloon when filled with air, which makes you float. Once the water enters your lungs, you will drown. After a few days, however, the bacteria inside the gut produce gases like methane, which make the dead body float in the water.
• Submarines: The buoyancy of a submarine is controlled by the ballast tank. When the tank is full, it increases the density of the submarine, which enables it to stay under the water. When the ballast tank is empty, air replaces the water. This makes drops the density to a level lower than the water, causing the submarine to float.
• Lactometers: A lactometer is the device used to test the purity of milk. It measures the relative density of milk for water. Archimedes' principle is working behind the lactometer as well.
• Life Jackets: Life jackets are the essential vests that save a person from drowning in the water by decreasing the total density of the person wearing it.

## What Caused the Titanic to Sink?

In the history of sea travel, there have been many ships that sank into the ocean. But the Titanic will always have a special place in history since it was the largest and fastest vessel at the time of its launch. Despite this, the Titanic sank into the Atlantic Ocean on its maiden voyage.

There are many theories all around the web on the failure of the Titanic. One article published in the New York Post states that the fire in the bunker weakened the hull, which allowed the iceberg to damage the hull with no trouble. The actual reason for the tragedy, however, is the fracture in the hull made by the iceberg. Water entered into the hull through the fracture and replaced the air. The total volume of the vessel increased with the water, which increased the ship's density to higher than that of the ocean water, which caused the ship to sink into the ocean.

## Cargo Ships Connect the World

More than two-thirds of the earth is filled with water, and there are hundreds of ships travelling in the sea every day thanks to science and the scientists who founded the principles for the betterment of travel.

Sea cargo contributes to over 90% of world trade, as it is the cheapest means of shipping essential and commercial goods. The invention of the ship helped connect the world with ease, and it was an important milestone in the history of humankind.

## References

• Bansal, R.K. A Textbook of Fluid Mechanics and Hydraulic Machines.

This content is accurate and true to the best of the author’s knowledge and is not meant to substitute for formal and individualized advice from a qualified professional.

Jagatheesh Aruchami (author) from Coimbatore, India on October 21, 2020:

Thank you Lakshmi for your feedback.

Jagatheesh Aruchami (author) from Coimbatore, India on October 21, 2020:

Thank you Umesh Chandra Bhatt for your kind words.

Lakshmi from Chennai on October 20, 2020:

Informative and well researched article .

Umesh Chandra Bhatt from Kharghar, Navi Mumbai, India on October 20, 2020:

Very informative article. Well presented.

Jagatheesh Aruchami (author) from Coimbatore, India on October 11, 2020:

Thank you, @Liz Westwood. I'm glad to hear that the article helped you to know about ships.

Liz Westwood from UK on October 09, 2020:

Ships and boats have always fascinated me. Your article gives interesting information about how they float.