How does a submarine control its depth in water?

Answer 1

The method by which a modern submarine controls its depth is dynamic. It involves the use of the planes on the submarine, and the trim tanks inside the pressure hull. Let's take a look and see what's up.

Submarines have ballast tanks outside the pressure hull. These tanks have valves in the top called vents, and they have openings in the bottom. When the vents are closed and the tanks are filled with air, the submarine has positive buoyancy and it floats. When the vents are open, water pressure from below (through the openings in the ballast tanks) forces the air out of the top, and the tanks fill with water. With the buoyancy of the air in the ballast tanks gone, the submarine sinks. This is fairly simple and obvious. But there is more to the story.

Inside the submarine are other tanks called trim tanks. A logical setup is to have a set of them up forward and a set back aft. These tanks can be filled with seawater or can be pumped dry, and this is normally done with the submarine's trim pump, which is usually controlled from the ballast control panel. You can see what's coming now if we tell you one more thing. The boat has sets of planes that act as hydrofoils when the craft is moving through the water.

Picture a submarine with a set of planes in the stern (along with the rudder) and a set of planes on the sail. The sail is the superstructure somewhere in the "middle" of the submarine, and it houses the parts of the periscopes that are outside the pressure hull. With the submarine under way and submerged, water is moving over the planes, and the planes can act to push the boat up and down. It's like flying through the water with little "wings" out there to maneuver with. Obviously, the stern planes can effect a dramatic change in the way we "point" the submarine. Tip the stern planes up, and water pressure drives the stern of the boat down and points the bow up. Tip the stern planes down, and the opposite happens. Let's ignore the stern planes for now as we usually keep them at zero degrees unless we need to change depth rapidly. So what about the planes on the sail?

The sail planes are sometimes called the fairwater planes. They are located near the longitudinal center of gravity/buoyancy of the boat. That translates into their being able to act to push the whole boat down in the water when they are tipped down, or their being able to push the whole boat up in the water when they are tipped up. This assumes the boat is trimmed properly and is neutrally buoyant. It is the trim tanks that trim the boat so it is not bow heavy or stern heavy, and we can pump to both the forward and aft trim tanks to achieve neutral buoyancy.

Control of depth of a submarine that is under way, at least one with fairwater planes along with the stern planes, is done through a combination of the pumping of water into or out of the trim tanks and the use of the fairwater planes. The situation differs very little when we consider a boat with bow planes and stern planes. The ballast control panel operator will be working the hardest to keep the boat trim and make holding depth easy for the planesman.
Hovering is a precise maneuver, and is typically used by boats in preparation for surfacing through the ice, or for vertical missile launch ops where a stationary platform is required.

Hovering requires precise trim ballast tank control. Trim tanks are used to maintain Neutral Buoyancy while submerged, as well as to compensate for buoyancy and variable ballast changes when moving through depths or different temperature zones in the ocean. Once the boat is at an all-stop bell, the Ballast Control Panel (BCP) operator (the Chief of the Watch) will bring in or pump out the required amount of water from the trim tanks to hover. This can be tricky when you're trying to surface under the ice through a polynya (thin ice pond) as currents can move you when you move from a hovering/stationary position at depth to a slow ascent. It's not uncommon to get pushed into thick ice while making the transition.

For vertical launch missile ops, the boat must re-hover after a launch to regain a stable platform.