Submarines

Someone wrote to ask: How do they make a submarine go up and down?

Submarines on the surface of the water, and all surface ships and boats, float because of "positive bouyancy". They weigh less than the volume of water they would displace if fully submerged.

Submarines have large tanks built into their bodies called "ballast tanks". To go down, or sink, the submarine opens these tanks and allows them to fill with water. This makes the submarine heavier, so it sinks. To return to the surface, they fill the tanks with pressurized air and flush the water out, making the submarine lighter and more bouyant, thus it rises.

As a submarine goes deeper, the water pressure outside the hull rises. This causes the Submarine's hull to compress, which decreases displacement. Water density also increases, although it does not compensate for the hull's compression. A submarine under the surface is somewhat unstable.

Depth Control Tanks are smaller tanks that are able to withstand higher pressure, and are usually located either near the submarines center of gravity, or spaced along the body. These tanks can be filled or emptied more rapidly and are used for fine controlling the submarines depth when submerged.

Forward and aft trim tanks are used to keep a submarine stable. Pumps move water between these tanks to change weight distribution.