When the submarine is traveling on the surface, it is operated like any other boat with one major exception; it has flood ports along the bottom that are open to the sea. The sea is prevented from filling the ballast tanks because they are closed at the top, much like holding a drinking glass upside down under water. There is no place for the air to escape because vents along the top of the tank are kept closed. The surfaced submarine maintains positive buoyancy by riding on a bubble captured in the ballast tanks.

Illustration showing cross sections with and without water in ballast tanks.

Diagram A represents a submarine on the surface. The main ballast tanks are filled with air and the center of gravity (G) is above the center of buoyancy (B). The water displaced (the area within the heavy line) equals the weight of the submarine. Diagram B represents a submerged submarine. The ballast tanks are filled with water and the centers of gravity and buoyancy are reversed. The displaced water, the area inside the pressure hull, has decreased and the weight of the displaced water now is the same as or less than the weight of the submarine.

The submarine achieves a dive by opening the vents and allowing the air to escape and water to fill the ballast tanks. The bow planes are lowered and angled down to drive the submarine below the surface. Once submerged, the boat is trimmed, or balanced, by pumping water between trim tanks. Over-all trim is the process of attaining neutral buoyancy; final trim is the establishment of fore and aft equilibrium, or zero bubble, with neutral buoyancy. The submerged submarine is operated at neutral buoyancy and control of the vessel is obtained by the rudder and the bow and stern diving planes. A zero bubble should be maintained without excessive angles on the diving planes. The primary effect of the bow planes is on the depth, and the primary effect of the stern planes is on the angle of the submarine.

Negative buoyancy for a quick dive is achieved by flooding the negative tank which is located at the keel slightly forward of the center of the boat, adding a slight down angle to the submarine. The negative tank is blown free of water once submerged.

Pressure on the hull increases with depth. The total pressure that the hull must be able to withstand is measured by the difference between interior hull pressure and the external pressure at a given depth. When the maximum depth is passed, the pressure hull will be crushed.

Diving and Surfacing Phraseology

Diving Procedures

Surfacing Procedures


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