How does a submarine submerge?
Submarines stay on the surface by keeping their ballast tanks filled with air. To submerge, the submarine opens special valves at the top of the ballast tanks. When the valves open, air escapes out the top of the tanks as seawater enters the tank from the bottom. Since the seawater entering the tank is heavier than the air it replaces, the submarine becomes heavier and submerges.
A submarine has ballast tanks between the outer hull and the inner, pressure hull. These are compartments that can be filled either with air or seawater, depending on whether the submarine is on the surface or submerged. When the sub is on the surface, the ballast tanks are filled with air. When the order is given to dive, the submarines’s Diving Officer opens valves at the top of the ballast tanks and sea water enters them through floods at their base. The sub’s engines drive it forward during this process so that it descends in a controlled fashion. Also, hydroplanes mounted on the stern and the sail/forward hull further control the evolution just as an airplane’s control surfaces cause it to climb or dive. Once the sub is at its desired depth, the valves are closed so that no more seawater enters the tanks. Smaller tanks called trim tanks are then used to make the sub neutrally buoyant. Finally, when the sub is to surface, high pressure air is forced into the tanks and the seawater leaves through the lower floods. This air gives the sub buoyancy and it surfaces. Easy, right?
