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Beneath the Sea
Teaching Guide
BLowing Ballast

Submarines and deep dive submersibles like Alvin depend upon ballast tanks to help control their vertical position within the water column. When the tanks are filled with seawater, the submarine's weight is greater than the buoyant force that pushes upward on its hull. As a result of this imbalance, the submarine dives. When the seawater is pumped from the tanks, the submarine becomes lighter. This shift in the balance of forces pushes the submarine to the water's surface.

In this activity, you'll have the opportunity to build a model of a submersible. Within its clear plastic hull, a balloon will serve as a type of ballast tank. By varying the inflation of this balloon, you will be able to control the buoyancy of the submersible.


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This activity page will offer:

  • A hands-on experience in buoyancy
  • An observation of a working ballast tank
  • An operational definition of blowing ballast


  • 1-liter clear plastic beverage container
  • Large balloon
  • Rubber band
  • Fishing weights
  • Tape
  • Aquarium tubing (rubber hose or a chain of plastic straws with flexible necks will also work)
  • Large tub or tank


  1. Stretch a balloon by inflating and deflating it about a dozen times. If necessary, pull and stretch out the balloon to make it more flexible.
  2. Obtain a length of plastic aquarium tubing. If you do not have tubing, you can construct such a tube by connecting straws. If you are using straws, secure them with waterproof, sturdy tape, and make sure to place a bend in the straw as shown above.
  3. Place the mouth of the balloon over one end of the straw/tubing. Use a rubber band to secure the balloon to the straw/tube end. Make sure that the rubber band does not squeeze off the air passage.
  4. Insert the balloon into the 1-liter clear plastic beverage container.
  5. Place the container in a large tank or tub of water. Let the container fill with water. As it fills with water, the container should sink. If it does not sink, add several fishing weights until the water-filled container settles to the bottom of the tank.
  6. Make a prediction. Suppose you blew a small puff of air into the balloon. How would that change the buoyancy of your "submersible"?
  7. Suppose you inflated the balloon to a greater volume? Would that offset the sub's weight?


  1. Why was it necessary to "pre-stretch" the balloon?
  2. Why was it important to keep the air passageway unblocked?
  3. What was the purpose of the fishing weights?
  4. What happened when you blew into the open end of the straw/tube?
  5. Consider the balance of forces that are responsible for the surfacing and diving of your classroom submersible. How can you apply what you've learned to Alvin's operation?


A Biological Connection

Some species of seaweed have tiny air bladders that line their stem-like parts. Think about it. What survival advantage might these sacs of air offer?

Historical Fiction
Historical fiction is a writing style that is based upon the blend of historical events and fictional characters or exchanges. Can you image what it would have been like to be lowered down in a bathysphere, like William Beebe? In 1930, this four-foot-in-diameter steel sphere was the first to dive below 600 feet, eventually reaching an unprecedented depth of 3028 feet. How would it have felt to be the first person to see the deep ocean? With your instructor's approval, research the bathysphere and then write your own short story based upon these dives. Color the history with a fictional dialogue between yourself and the support crew at the surface.

Spy Stories
Learn about "Project Jennifer" and the CIA's secret operation to raise a Soviet sub from the seafloor. Using a modified research vessel called the Glomar Explorer, the agency tried to lift the hull into the secret compartment of this huge ship. Check it out at


Marine Operations at WHOI
Learn more about Alvin and the other research vessels at the Woods Hole Oceanographic Institution.

Submarine Construction
The basic construction of a submarine and its ballast tanks are illustrated at this site.

William Beebe - Going Deeper
Read this biography of William Beebe on The American Experience Web site.


The activities in this guide were contributed by Michael DiSpezio, a Massachusetts-based science writer and author of "Critical Thinking Puzzles" and "Awesome Experiments in Light & Sound" (Sterling Publishing Co., NY).

Academic Advisors for this Guide:

Corrine Lowen, Science Department, Wayland Public Schools, Wayland, MA
Suzanne Panico, Science Department, Fenway High School, Boston, MA
Anne E. Jones, Science Department, Wayland Middle School, Wayland, MA

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