How a Submarine Dives and Surfaces

Have you ever wondered how a submarine dives and surfaces? A submarine dives and surfaces using a principle called buoyancy. Inside a submarine, there are special tanks called ballast tanks that can be filled with water or air. These allow the submarine to float on the surface or submerge. To find out how a submarine sinks or surfaces and to learn about buoyancy, try this simple experiment.

You will need:

a plastic 2-liter bottle and cap
a straw or hose
a tub of water

A submarine dives and surfaces using ballast tanks (located between the pressure hull and the outer hull), bottles of compressed air, seawater and diving planes.

Put the cap on the bottle with no water inside and try to get the bottle to stay under water. Because the bottle is full of air, the bottle weighs less than the water it displaces. If pushed under the water and then released, the bottle will float to the surface. This is called positive buoyancy.

When a submarine surfaces, the main ballast tanks are full of air. This provides the buoyancy necessary to keep the submarine afloat. At the top of each ballast tank is a vent that is closed while the submarine is on the surface. At the bottom center line of each ballast tank is a flood port that is always open to the sea. As long as each vent is shut, the air pressure keeps water from filling up in the tanks.

The submarine dives when the vents are opened, which releases air trapped in the ballast tanks and allows the seawater to enter through the flood ports. As the main ballast tanks fill with water, the submarine becomes heavier than the water it displaces. This causes the submarine to submerge or sink. The diving planes angle the submarine downward so that it can dive faster.

Fill the bottle completely with water, and make sure there is no air inside. After capping the bottle, put it in the tub of water. The bottle will sink because it weighs more than the water it displaces. This is called negative buoyancy.

Once the submarine is fully submerged, water from a special ballast tank (called a negative tank) is blown back to sea using compressed air. The submarine can then float under water because it weighs the same amount as the water it displaces. This is called neutral buoyancy.

With the bottle full of water, capped and under water, remove the cap. Note: Don't take the bottle above the surface. Next, put the straw or hose in the mouth of the bottle. Then slowly blow air into the bottle. Once you have put some air into the bottle, put the cap back on to see if the bottle sinks or floats. To achieve neutral buoyancy, you must get just the right amount of air and water inside the bottle. If you can do this, the bottle should float under the surface of the water.

For the submarine to surface, the vents for the main ballast tanks are shut. Compressed air is forced back into the tanks. As the air expands, it forces the seawater back out through the flood ports, causing the submarine to weigh less than the displaced water. Positive buoyancy results, and forces the submarine to rise and eventually surface. To make the bow of the submarine rise first, water is blown out of the forward ballast tank (called the bow buoyancy tank). The diving planes also angle the submarine upward so that it rises quickly.