How is the density of a submarine controlled?

The density of a Submarine never changes - what does change is its buoyant properties. The boat must maintain what is known as Neutral Buoyancy throughout different changes in water as it travels through the ocean. Temperature and Salinity changes are the 2 biggest factors in maintaining NB.

There are 3 primary methods for maintaining submerged operations:

1. Main Ballast Tanks

2. Trim Tanks

3. Dive Planes

The Main Ballast Tanks are nothing more than large sections between the outer hull and inner hull (submarines have 2 hulls, much like a thermos bottle), and the bottom of the tank has an open grate, where the top of the tank has a hydraulically activated valve. The pressure on the air inside the tank when surfaced is enormous - to dive the boat, the MBT valves are opened, and the sea pressure forces the air out of the tank through the open valve. Once the tanks are full of water, the valves are closed. As with all onboard systems, the MBT valves can be opened hydraulically or manually.

After submerging, the Chief of the Watch (the primary operator of the key diving and control systems for the boat) will bring in or purge water from several Trim Tanks on the boat. Trimming the boat gets the boat to Neutral Buoyancy, which is a submerged state in which the boat neither rises nor sinks. This is also used by Scuba Divers, and how they maintain their own depth control. The Chief of the Watch takes orders from the Diving Officer, who is in charge of keeping the boat at NB while submerged. The Diving Officer also gives orders to the Helmsman and Planesman (Helmsman steers, Planesman maintains depth) as necessary.

From there the Dive Planes take over for moving deep or shallow. Older WW2 era submarines used Bow Dive Planes, which were located at the bow of the boat. When the Teardrop Hull shape used today was developed, the Dive Planes were moved to the Fairwater, which is the superstructure that houses the periscopes and other masts. It is often incorrectly called a Conning Tower. As such they are referred to as Fairwater Planes.

Fairwater Planes have their problems though. In rough water, if the boat is at Periscope Depth, and is inadvertently sucked to the surface by a large enough wave, the boat can lose its depth control ability very quickly. If in a tactical situation, it makes it even more dicey if non-discovery is imperative. For this reason. the new Seawolf and Virginia class boats have gone back to Bow Planes, which give the boat depth control whether or not the sail is out of the water or not.

There are 2 primary methods of surfacing a boat - an 'Emergency Blow" in which compressed air stored in tanks is released rapidly into the Main Ballast Tanks to expel water and thus create Positive Buoyancy; and a Low Pressure Blow, using the boats' onboard Low Pressure Blower. This is only used near the surface at Periscope Depth, as it requires the Snorkel Mast to be raised. Utilizing the LPB saves vital compressed air, which is used for emergency systems and weapon ejection.