Submarine Moving up and Down

How does a submarine ascend?
- Nancy (age 15)
Iowa

A submarine has big tanks on its side which hold either seawater or air. Submarines are made out of steel which is heavier than water, and have a lot of air inside, and float like normal ships when their hollow spaces are filled with air. However, when the big ballast tanks are filled with water, the submarine is more dense than water on average, and sinks. By allowing water to flow into the tanks or by pushing water out of the tanks, the submarine can control the difference between its weight and the buoyant force pushing upwards (the total weight of the water the submarine displaces).

The tanks don’t actually have pumps on them. They are filled with seawater just by opening ports on the side, and the water is pushed out by forcing compressed air in (the ports have to be on the bottom of the tank so that water may leave the tanks while the air stays inside). This process is called "blowing" the tanks.

Tom

(published on 10/22/2007)