Underwater Dream Machine
Students prepare a diving tank and a submarine and make the submarine neutrally
buoyant in the tank.
Students will be able to:
- copy of the "Build Your Own Submarine" student handout
- 2-liter plastic soda bottle, rinsed, label removed, and cut to
- package of 3-ounce drinking cups, plastic preferred
- large needle
- large paper clips
- kitchen utility scissors
- waterproof markers
- water, room temperature
- dishwashing liquid
- for ballast: paper clips and metal washers
are important to military operations and to undersea exploration because they
can function entirely underwater. Military submarines are usually much larger
than scientific submarines, and they often carry weapons. Scientific submarines
carry a small number of crew members and are usually designed for specific
exploration missions. The submarine featured in this program is a recreational
submarine used to transport passengers to underwater wrecks.
Submarines usually have two hulls, an outer and inner, and both are often made
of steel. How deep a submarine is able to dive is limited by the strength of
the hulls. The ballast is the part of the submarine that is between the hulls.
Ballasts are filled with air or water: filling the ballast with water sinks a
submarine, filling it with air causes a submarine to float. Neutral buoyancy
occurs when the submarine's density is about equal to the density of the
In this activity, students will make a diving tank and a submarine and then
investigate how weight and buoyant forces apply to the submarine.
ballast: Something that gives stability. In a submarine, it is the space
between the hulls that is filled with either water or air.
buoyancy: The upward force a fluid puts on an object less dense than
hull: The main body of a boat or casing of a submarine.
negative buoyancy: A boat or submarine has negative buoyancy if it
weighs more than water.
neutral buoyancy: A boat or submarine has neutral buoyancy if it weighs
about the same as water.
positive buoyancy: A boat or submarine has positive buoyancy if it
weighs less than water.
submarine: A ship that can operate completely underwater.
Before class, prepare diving tanks for each team by cutting around the
shoulder of a soda bottle so that the remaining base is tall and
straight-sided. For safety, use kitchen utility scissors. This activity
involves working with these "tanks" filled with water. Work in a wet lab,
outdoors, or place them in plastic dishpans.
Organize students into teams and distribute copies of the "Build Your Own
Submarine" student handout. Demonstrate for students the construction of a
submarine (see the student handout for an illustration of the final
Poke two holes with a large needle on opposite sides of the drinking cup's
Bend a large paper clip into a U-shape and attach to the cup through the
Neatly cut a bean-shape hole in the side of the cup about a quarter of an
inch (6.35 millimeters) from the bottom. The submarine will be easier to
control if the edge of the hole nearest the cup bottom is straight and parallel
to the cup bottom.
Supervise students as they build their submarines and prepare their diving
tanks. (If students have difficulty making their submarines neutrally buoyant,
add some dishwashing liquid to the diving tank. This will reduce the surface
tension, making the size of the escaping bubbles smaller and thus making it
easier to achieve neutral buoyancy.)
Once students have gotten their submarines to be neutrally buoyant, conduct
a class discussion about buoyancy using the scenarios listed in the questions
section of the student handout.
As an extension, change the density of the water and repeat the experiment.
First have students predict what might happen, then retest the submarine in
dissolved sugar or salt water.
A submarine rises because the weight of water pushing up on the submarine,
known as the buoyant force, is greater than the downward force, the weight of
the submarine. If the submarine weighs more than the buoyant force, it sinks.
If it weighs less than the buoyant force, it will rise. If the buoyant force
and the weight are about equal it will drift (either on or beneath the
surface). At this point, the submarine is neutrally buoyant. That is, there is
no tendency for it to rise or sink so the submarine should remain at about
whatever level it was placed.
Divers use this same principle when they add or let air out of their buoyancy
vests in order to control their position in the water. Bony fishes use a swim
bladder to maintain neutral buoyancy. Like the diver's buoyancy vest, this
saves energy that would otherwise be spent maintaining vertical position.
Student Handout Questions
You are piloting a new research submarine among stony towers rising from the
center of the Mid-Pacific Rise. Your guest scientist points to a two-meter-tall
tower and asks if you can break it off to take back to the surface. Although
the mechanical arm can do that, and you have a forward sample basket big enough
to hold the piece, you are doubtful for two reasons. What are they? The
first reason is that the pilot can't know whether it can be lifted safely
because she doesn't know the weight of the sample. If it is more than the
weight of the sub's releasable iron pellets, taking it threatens the sub's
ability to surface. The second reason concerns where the sample is situated;
placing a large weight near the front end of a submarine will cause the sub to
be off-balance and could result in it pitching forward into an unsafe head-down
On the forward surface of your submarine are six large floodlight bulbs.
These bulbs are like regular light bulbs, except for their large size. Imagine
that something bumps the floodlight rack, breaking the bulbs. How will this
affect your submarine? What will you have to do? Because they contain air
but don't have much weight, the bulbs add to the buoyancy of the sub. If the
bulbs break, that buoyancy is lost. To remain neutrally buoyant, the sub will
have to drop some iron pellet ballast.
NOVA—Underwater Dream Machine
Find articles, interviews, slide shows, and resources in this companion Web
site to the program.
How Submarines Work
Discusses many aspects of submarines, including diving and surfacing, life
support, power supply, and navigation.
Reviews the parts of a sub and explains the differences between military and
Submarines: How They Work
Addresses the concept of buoyancy, the relationship between pressure and
volume, and methods of propulsion.
DK Eyewitness Guides: Force and Motion
by Peter Lafferty. Dorling Kindersely, 1992.
Discusses the principles behind several different kinds of force and motion,
DK Eyewitness Guides: Submarine
by Neil Mallard. Dorling Kindersley, 2003.
Describes how submarines work, their role in wars, and how submarines are
detected in the water.
Submarines (History Series)
by J.J. Tall. Barrons, 1998.
Shows submarine development, with emphasis on the vessels of WWI, WWII, and
modern nuclear submarines.
The "Build Your Own Submarine" activity aligns with the following National
Science Education Standards (see
Science Standard B
Motions and forces
Science Standard B
Motions and forces
Classroom Activity Author
James Sammons taught middle and high school science in Rhode Island for 30
years. His teaching practices have been recognized by the National Science
Teachers Association, the Soil Conservation Service, and the National
Association of Geoscience Teachers. This activity originally appeared in a
slightly different form on NOVA's "Hitler's Lost Sub" Web site.