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Lesson: Ballast Systems in Submarines

Submarine With Water and Air Gushing from Ballast Tanks

(Courtesy MIT Sea Grant, Sea Perch Program and Robert Valtos, author). Level: Grades 7-12. Group Size: 4-6 students. Time Required: One 45-minute period.

Ballast Systems in Submarines


This lesson helps  students understand how a submarine controls its buoyancy through the use of  ballasts. Students construct a model ballast system, and then learn to control its vent valves to make it submerge and surface.

Created by the MIT Sea Grant Sea Perch Program as part of a set of introductory lessons leading to the creation of a remotely operated underwater vehicle (ROV), this lesson can stand alone, or be taught in conjunction with the complete downloadable Sea Perch Curriculum.


The students will

  • Construct a model of a submarine’s ballast system
  • Develop an understanding of an active ballast system

National Science Content Standards:

  • Unifying Concepts and Processes
  • Science as an Inquiry
  • Physical Science
  • Science and Technology for grades 9-12


(from the Sea Perch lesson on buoyancy)

Who are scientists? They are people whose minds never stop running–not even in the bath tub. One famous scientist was Archimedes, a Greek physicist, mathematician, and inventor known as the “father of experimental science.” Archimedes lived most of his life in Syracuse, Sicily, now an island of Italy.

According to one story, the king of Syracuse asked Archimedes to tell him whether his new crown was made of pure gold. Archimedes was bathing when the solution to this challenge came to him. When he stepped into his bathtub, it overflowed, and  he realized that his body had displaced a certain amount of water. That ‘s when — so the story goes — Archimedes ran out into the streets naked, calling “Eureka!”–I have found it!

He realized that if the crown were pure gold, it would displace the same amount of water as the chunk of pure gold that was supposed to have  been used to make the crown. He then conducted a test and learned that the crown was not pure gold, and that the King had been cheated by the crown-maker.

So, what was the test, and how did it lead to Archimedes’ theory of buoyancy?  A treatise he wrote called On Floating Bodies seems to provide the explanation. In it, what is today known as the Archimedes’ principle states that a body immersed in a fluid experiences a buoyant force equal to the weight of the fluid it displaces. If the golden crown had weighed as much as the solid block of gold, when immersed in water, both should have displaced an equal amount of water.

Archimedes may have immersed both separately or may have used a more elaborate test, placing both on a scale that was then immersed in water, as illustrated below. If the crown were less dense than the gold, it would displace more water, given its greater volume; it would also experience greater buoyancy than the chunk of gold, causing the scale to tip.


Buoyancy is the tendency of an object that is immersed in a fluid to rise. Archimedes discovered that when an object is placed in water (or any other fluid) it displaces a certain amount of water. The upward force from that displaced water is equal to the weight of the displaced water. Whether this force is strong enough to make an object float/ rise depends on the density (mass per volume) of the object. Some objects, such as submarines, can adjust their density. A submarine becomes more or less dense – allowing submerging or rising – by taking water into its tanks or forcing it out, using compressed air.

Ideas for illustrating the concepts of volume, mass, and buoyancy:

  • Raise the following question: why is it that an iron ball placed in water will sink and a boat made out of iron can float? The difference is the density. The boat has more volume per mass than the iron ball.
  • An experiment: give each learner plasticine, let them roll it into a ball and place it into water, it will sink. Let them try and change the shape of it until it floats (into a similar shape of a boat) more on sink and float

Submarine Diving Diagram and CaptionTechnology: how does a submarine work?

A submarine is a ship that operates underwater. Submarines are used mostly as a weapon of war. Others are built for peaceful uses such as sunken treasure hunt and ocean research.

Most submarines are shaped like a cigar with tapered ends and an almost round body. At the front and the back end are placed the diving planes, which control the ships up and down movements.

A tall structure called a coning tower rises above the top of the submarine and it contains the control and navigation centre. (It includes the periscope, a kind of telescope that is used for observing the surface when the ship is submerged.) A submarine operates on the surface just like any other ship. When it goes under water, sea water is allowed to enter and fill tanks called ballast tanks. They are placed around the living space. To make the submarine surface , the lower valves open and compressed air from inside the submarine forces the water out of the tanks. To make the submarine dive the valves open to allow air to escape from the top of the tanks.



A submarine controls its ballast by allowing water to fill ballast tanks located around the ship. To make a submarine submerge, vent valves at the top of the ballast tanks open, allowing air to escape and water to fill the tanks through holes in the bottom. To make a submarine surface, the vent valves are shut and high pressure air is released into the tanks, forcing the water out through the holes in the bottom.

Safety Note: Caution must be exercised when drilling the hole into the bottle cap. For greatest safety, teachers should perform this step ahead of time, supplying students with a pre-drilled cap.


  • 3/8 inch drill bit and drill
  • 5 gallon bucket filled with water

A set for each group:

  • 24 inches of flexible tubing with a 3/8 inch outside diameter
  • 16 oz. plastic soda or water bottle with cap
  • Approximately 8 oz. of ballast weight
  • Electrical tape


1. Divide the class into groups small enough to allow each student to participate in the construction and observation of the submersible ballast system.

2. Drill a 3/8 inch hole in the bottle cap and the bottom of the bottle. Safety Note : Caution must be exercised when drilling the hole. For greatest safety, teachers should  perform this step ahead of time, supplying students with a pre-drilled cap.

2. Tape the ballast weight, to the bottom of the bottle.

3. Place the flexible tubing in the hole in the cap and insert it about one inch.

4. Tape the tubing in place.

5. Screw the cap onto the top of the bottle.

6. Place the bottle in the 5 gallon bucket and make sure your thumb is over the other end of the tubing.

7. Now, remove your thumb and watch as the bottle fills with water. You should feel air rushing out of the tubing.

8. When the bottle has sunk to the bottom, blow into the tubing, and watch the bottle come back to the surface.


Have your students discuss or write a summary of the outcome of the activity:

  • What did you observe happen?
  • Why do you have to place your thumb over the end of the plastic tubing to keep the bottle afloat?
  • What would be the advantages and disadvantages to an active ballast system?


Build Your Own Underwater Robot and Other Wet Projects, by Harry Bohm and Vickie Jensen. Published by Westcoast Words, Vancouver, B.C., Canada. ISBN 0-9681610-0-6.

Explore more online Sea Perch lessons and explore the Sea Perch Website.

Read our Feature story about the participation of four Massachusetts schools in the 2010 Sea Perch program.

One Response to “Lesson: Ballast Systems in Submarines”

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