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How Does a Submarine Ballast Tank Work?

Published in Submarine Buoyancy Control 4 mins read

Submarine ballast tanks are fundamental to controlling a submarine's buoyancy, allowing it to dive below the surface and return safely upwards. These tanks work by manipulating the submarine's overall weight and displacement, thereby altering its density relative to the surrounding water.

The Science Behind Submarine Buoyancy

A submarine's ability to dive and surface relies on the principle of buoyancy, discovered by Archimedes. An object floats if it displaces a weight of fluid equal to its own weight.

  • Positive Buoyancy: When the submarine is lighter than the water it displaces, it floats.
  • Negative Buoyancy: When the submarine is heavier than the water it displaces, it sinks.
  • Neutral Buoyancy: When the submarine's weight is equal to the weight of the water it displaces, it remains suspended at a constant depth.

Ballast tanks enable the crew to achieve these states by controlling the amount of water (or air) inside them.

How Submarines Submerge

To dive, a submarine intentionally increases its weight, causing it to become negatively buoyant.

  1. Opening Vents and Valves: The crew opens large vents (known as "main vents") at the top of the main ballast tanks and flood valves at the bottom.
  2. Flooding the Tanks: Water from the sea is allowed to freely flow into the main ballast tanks by gravity, displacing the air originally contained within them.
  3. Increasing Density: As water fills the tanks, the submarine's total weight increases significantly, making its overall density greater than that of the surrounding seawater.
  4. Descent: With increased weight and negative buoyancy, the submarine begins to sink below the surface.
  5. Achieving Neutral Buoyancy (for operation): Once submerged to the desired depth, a small amount of water might be pumped out or taken in from smaller "trim tanks" to achieve neutral buoyancy, allowing the submarine to maintain its depth without expending too much energy.

How Submarines Surface

Surfacing is essentially the reverse process of diving, where the submarine decreases its weight to become positively buoyant.

To bring the submarine back to the surface, the crew shuts the vents in the top of the ballast tanks and releases compressed air into the tanks. This high-pressure air plays a crucial role in expelling the water.

  1. Closing Vents: The main vents at the top of the ballast tanks are sealed shut.
  2. Introducing Compressed Air: High-pressure air, stored in onboard air flasks, is released into the top of the ballast tanks.
  3. Expelling Water: The compressed air rapidly accumulates at the top of the tanks, forming an air pocket that exerts significant pressure on the water within. This air pocket pushes the water out through the bottom valves.
  4. Decreasing Density: As water is forced out and replaced by much lighter air, the submarine's overall weight decreases, and its buoyancy increases.
  5. Ascent: With increased buoyancy, the submarine becomes positively buoyant and begins to rise towards the surface. Once on the surface, the remaining water in the tanks is fully blown out by continued compressed air or pumps.

Key Components of the Ballast System

Understanding the different components helps illustrate the process:

  • Main Ballast Tanks (MBT): These are the primary tanks used for diving and surfacing, taking in or expelling large volumes of water. They are typically located between the inner and outer hulls.
  • Trim Tanks: Smaller tanks located at the bow and stern, used for fine-tuning buoyancy and adjusting the submarine's pitch (angle).
  • Compensation Tanks: Used to compensate for changes in weight due to fuel consumption, torpedo launch, etc., maintaining overall balance.
  • High-Pressure Air System: Contains compressed air cylinders used to force water out of the ballast tanks.
  • Vents and Valves: Critical for controlling the flow of water and air into and out of the tanks.

Summary of Ballast Tank Operation

The following table summarizes the key actions for submerging and surfacing:

Feature Submerging (Diving) Surfacing (Rising)
Primary Action Filling tanks with water Expelling water with compressed air
Buoyancy Effect Decreases (becomes negative/neutral) Increases (becomes positive)
Vent/Valve State Top vents open, bottom valves open Top vents shut, compressed air released, bottom valves open
Result Submarine sinks Submarine rises
Energy Source Gravity (water enters naturally) Stored high-pressure air

By mastering this delicate balance of water and air, submarine crews can precisely control their vessel's depth, enabling stealthy underwater operations and safe return to the surface.