Inside a submarine there are containers called ballast tanks. If these are full of air, the submarine will float.
Even though it is made of steel, the average density of the submarine is less than that of water. By pumping water
into the ballast tanks, the submarine can sink. This is because when its ballast tanks fill with water, the submarine
has a greater density than water.
Submarines are ships that can operate both under and on top of the water. One of the first submersible
vessels was built around 1620 by a Dutchman named Cornelius van Drebbel.
We don't know that much about Drebbel's vessel, but diaries and books written at the time tell us his sub was really
just a rowboat covered with a waterproof leather skin. Apparently 12 people with oars moved the vessel through
the water. It could submerge to about 4.5 metres and go up to 8 kilometres before it needed to surface. It must
have had some type of portholes to let in the light because one passenger wrote that people could see well enough
underwater to read.
Submarines have changed a lot since Drebbel's day. Today some submarines are 200 metres long and carry a crew of
over 150! Nuclear powered submarines can stay underwater for months at a time.
How It Works
Submarines are designed for use at great depths. Their rigid, double-walled hulls allow the crew to
live and work normally underwater for as long as air and power supplies last. Submarines are steered by turning
a rudder left and right. A propeller moves the sub through the water--pushing against the water and creating a
When an object is underwater, it pushes aside (or "displaces") an amount of water equal to its volume.
Buoyancy is the upward force of water pushing against the submarine.
An object floats if it displaces enough water to support its weight. Subs don't sink because their metal shell
(or "hull") surrounds a volume weighing less than an equal amount of water.
Subs can sink, rise, and float underwater (maintain "neutral buoyancy"). Subs do all this by adjusting
the amount of water and air in their ballast tanks. When the tanks are full of air, the sub weighs less than the
volume of water it displaces and it floats. When the ballast tanks are flooded with water, the sub weighs more
than the water it displaces, and it sinks.
To rise again, the sub reduces its weight by pushing compressed air into the ballast tanks. The air forces the
sea water out, and the sub goes up toward the surface. To move beneath the surface and to hover, the amount of
water in a submarine's ballast tanks is made equal to the weight of the water it is displacing.
A submersible, called Alvin, was used to recover a hydrogen bomb accidentally dropped from an air force
bomber back in 1966.
Japan has a submersible called Kaiko that can dive over 11 kilometres. In 1994, Kaiko went down to the Mariana
Trench, the deepest spot in the ocean!
While the largest submarines stretch up to almost 200 metres, the smallest working submarine, the Water Beetle,
is only 2.7 metres long! It can go down to 30 metres and stay underwater for four hours.
A deep-diving submarine used to explore the ocean is called a submersible. Submersibles are usually smaller than
submarines. They are often equipped with external cameras, manipulating arms, and special lights. Submersibles
are built to do specific jobs, not for long-distance travel. We use them to help us recover "black box"
flight recorders from wrecked airplanes, bury cables in the sea floor, investigate ancient shipwrecks, map the
ocean floor, look for signs of undersea earthquakes, study marine life, repair damaged offshore oil wells, take
rock samples of the ocean floor, and study ocean currents.
Explorer Submarine Specs
How Subs work