About Submarines
How They Dive And Surface

Buoyancy And Displacement
The Basic Principles
Illustration of marble experiment below.
Q: If you attempt to float a marble in a body of water, you discover it readily sinks to the bottom. If you place the marble in a small paper or plastic cup, it will float. Why?

A: The marble by its self has little surface area to offer resistance to the water (displacement), allowing the pull of gravity to easily pull the marble to the bottom. There is more surface area to the cup allowing the resistance of the water and cup to oppose gravity enough to keep the cup and marble afloat (Buoyancy). A submarine dives and surfaces by changing it's effective surface area. Most submarines use these principles (some use 'ballast release" much like the sandbags on a gas filled balloon) to submerge and surface. The submarine uses the diving planes to obtain the depth the submarine desires to operate at, and trim to maintain it.

Main Ballast Tanks

These graphics are for example only, and do not depict any particular submarine.
Picture of the side view of a sub showing Main Ballast Tanks and free flood areas.
Actual submarine weight is determined by the weight of the hull and all things within it. Auxiliary and Trim tanks are used to fine tune the total weight of the ship and the balancing of the ships weight fore and aft and port to starboard.

As supplies are used and as miscellaneous tanks are filled or emptied the Ballast Control Panel operator shifts water around, or floods in, or pumps out, water to maintain trim.
As sea water temperature changes, the water changes density too, further necessitating changes in trim.

A submarine actually has two hulls. The outer hull and pressure hull serve to give the ship its shape and protect the crew. as can be seen above and below, the pressure hull is much smaller than the actual ship diameter. If the area between the outer hull and pressure hull is filled with water, the ship becomes heavier and the ship becomes less buoyant and sinks.

If the water is kept out of the space between the two hulls, the ship is effectively larger and displaces more water. This makes the ship more buoyant and it rises.

The spaces between the two hulls are compartmentized into sections called Main Ballast Tanks (MBT's.) These tanks are open on the bottom and have valves at the top called Vents. In the sub pictured there are 6 Port and 6 Starboard MBT's.

Water may be flooded into the MBT's by opening the MBT Vents letting the air out. When the vents are closed air can be forced into the MBT's pushing the water out the openings in the bottom of the MBT's. This can be done at a controlled rate ("Normal Blow") or all at once ("Emergency Blow".)

Picture of the top view of a sub showing Main Ballast Tank Vents (MBT Vents)

For more info go here

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