How Does a Submarine Dive and Surface? Understanding Ballast Tanks, Buoyancy and Diving Planes

A submarine disappearing beneath the ocean looks almost effortless. A few minutes earlier, it was floating on the surface; now thousands of tonnes of steel are moving beneath the water.

But submarines do not simply "fill a tank and sink."

Diving and surfacing are carefully controlled engineering processes involving buoyancy, ballast tanks, trim systems, compressed air, control surfaces, sensors and automated control systems.

Understanding these principles provides the foundation for understanding almost every other submarine system.

Key idea: A submarine controls its underwater motion by managing the relationship between its weight, buoyancy, trim and hydrodynamic forces.

1. The Principle Behind Diving and Surfacing

The starting point is buoyancy.

According to Archimedes' principle, an object immersed in water experiences an upward force equal to the weight of the water it displaces.

For a submarine:

If buoyant force is greater than its weight, it tends to rise.
If buoyant force is less than its weight, it tends to sink.
If buoyant force approximately equals its weight, it can remain at neutral buoyancy.

A simplified relationship is:

Buoyant force = Weight of displaced water

This means that a submarine does not need to be "heavy enough to sink" in the ordinary sense. Instead, its effective buoyancy is carefully controlled.

A simple way to visualize it

Positive buoyancy

Buoyant force > Weight
→ Submarine tends to rise

Neutral buoyancy

Buoyant force ≈ Weight
→ Submarine can remain at depth

Negative buoyancy

Buoyant force < Weight
→ Submarine tends to descend

This principle is the foundation of submarine diving and surfacing.

2. What Are Main Ballast Tanks?

The main ballast tanks (MBTs) are among the most important systems involved in changing a submarine's buoyancy.

They are located outside or around the pressure hull in the submarine's outer structure, depending on the design.

Their fundamental purpose is to allow the submarine to change the relationship between its overall weight and the volume of water it displaces.

When a submarine is on the surface, the main ballast tanks contain a significant amount of air.

When the submarine is prepared to dive, seawater is allowed to enter the ballast tanks, replacing some of the air.

This changes the submarine's buoyancy characteristics.

Surface condition

The ballast tanks contain air and are relatively free of water.

The submarine has sufficient positive buoyancy to remain afloat.

Diving condition

The ballast tanks are flooded with seawater.

The submarine's buoyancy is reduced, allowing it to submerge.

Surfacing condition

Where the submarine's design and operating procedure call for it, compressed air is used to displace water from the ballast tanks.

The submarine's buoyancy increases and it rises toward the surface.

3. What Happens During a Dive?

The diving process can be understood in several stages.

Step 1: The Submarine Is on the Surface

At the surface, the submarine has positive buoyancy.

The main ballast tanks contain air, while water has been displaced from the relevant tank spaces.

The submarine floats because the upward buoyant force is sufficient to support its weight.

Step 2: Main Ballast Tanks Are Flooded

When the submarine is configured to dive, the main ballast tanks are allowed to flood with seawater through their designed flooding paths.

Air is displaced from the tanks through vents.

As water enters the ballast tanks, the submarine's effective buoyancy decreases.

The submarine begins transitioning from its surface condition toward its submerged condition.

Step 3: The Submarine Begins to Submerge

Once the submarine has developed the appropriate underwater condition, its motion can be controlled using its propulsion system and control surfaces.

This is where diving planes become particularly important.

Diving planes generate hydrodynamic forces as water flows over them.

By changing their angle, the submarine can control its pitch and therefore influence its trajectory through the water.

Step 4: The Submarine Establishes the Desired Depth

Once submerged, the submarine does not simply continue sinking.

Its ballast, trim and control systems are adjusted to establish the required underwater condition.

Sensors continuously provide information about parameters such as:

Depth
Pitch
Roll
Speed
Tank conditions

Control systems use this information to help maintain the desired state.

4. How Do Diving Planes Work?

Diving planes are control surfaces that help a submarine control its pitch while it is moving through the water.

Their operation is broadly analogous to the control surfaces of an aircraft, although the submarine operates in a completely different fluid environment and under different control requirements.

When water flows over the diving planes, changing their angle changes the hydrodynamic force acting on the submarine.

During a descent

The diving planes can be positioned to produce a downward pitching moment.

The submarine's bow moves downward and the submarine follows a descending trajectory.

During an ascent

The planes can be positioned to produce an upward pitching moment.

The submarine's bow moves upward and the submarine follows an ascending trajectory.

An important distinction

Diving planes do not replace ballast tanks.

Ballast systems primarily influence the submarine's buoyancy condition.

Diving planes provide dynamic control of attitude and trajectory, particularly when the submarine has sufficient forward motion.

This distinction is important when trying to understand how modern submarines maneuver underwater.

5. What Is Neutral Buoyancy?

Once a submarine is underwater, one of the most useful operating conditions is neutral buoyancy.

In simplified terms:

Weight of submarine ≈ Buoyant force

When the two are balanced, the submarine does not have a strong tendency to rise or sink.

This allows the submarine to maintain depth using comparatively small control inputs.

Neutral buoyancy is particularly important because continuously forcing a strongly buoyant submarine downward or a strongly negatively buoyant submarine upward would be inefficient and would require greater control effort.

A properly controlled submarine can therefore operate underwater with a carefully managed balance between:

Weight + Buoyancy + Hydrodynamic Forces

6. What Is the Role of Trim Tanks?

Ballast and trim perform different functions.

Main ballast systems primarily deal with the submarine's overall buoyancy condition.

Trim systems are used to adjust the distribution of weight and buoyancy along the submarine.

Imagine a long submarine with slightly more weight toward the bow.

The bow would naturally tend to sit lower.

By transferring water between appropriate trim tanks, the distribution of mass can be adjusted.

This allows the submarine to maintain the desired longitudinal attitude.

Trim helps with:

Maintaining longitudinal balance
Controlling pitch tendency
Compensating for changes in loading
Managing changes caused by water transfer
Reducing unnecessary control-surface demands

Trim is therefore a fine-control mechanism rather than simply an alternative to the main ballast system.

7. How Does a Submarine Maintain Depth?

Once the submarine reaches its desired depth, maintaining that depth becomes a control problem.

Consider a simple example.

Suppose the desired depth is:

300 metres

A depth sensor reports:

305 metres

The control system now knows that the submarine is deeper than its desired depth.

Depending on the submarine's control philosophy and operating condition, appropriate control actions can be taken to reduce the depth error.

The process can be represented as:

Desired Depth

↓

Depth Controller

↓

Control Command

↓

Actuators / Control Surfaces

↓

Submarine Motion

↓

Depth Sensor

↓

Feedback

This is a closed-loop control system.

The same fundamental concept is found in many engineering systems, from industrial process control to aircraft autopilots.

8. How Does the Submarine Surface?

Surfacing is essentially the reverse of the buoyancy change used for diving, but it is not simply a mirror-image process.

The submarine must remain under control throughout the ascent.

For submarine designs that use compressed-air blowing of main ballast tanks, high-pressure air is introduced into the ballast tanks to displace water.

As water is expelled, the submarine becomes more buoyant.

The simplified sequence is:

Compressed air enters ballast tanks

↓

Water is displaced

↓

Effective buoyancy increases

↓

Submarine develops an upward tendency

↓

Control surfaces manage the ascent

↓

Submarine approaches the surface

The exact sequence, tank configuration and use of compressed air vary between submarine designs and operating conditions.

9. Why Is Compressed Air Important?

Compressed air is an extremely useful energy source in submarine engineering.

For ballast operations, stored compressed air can be used to displace water from ballast tanks when the submarine needs to increase buoyancy.

However, the submarine does not have an unlimited supply of compressed air.

That means compressed-air systems require:

High-pressure storage
Pressure regulation
Piping
Valves
Monitoring
Safety systems

Different pressure levels and air systems may serve different functions depending on the submarine design.

10. A Numerical Example: How Much Water Makes a Difference?

The physics becomes more interesting when we put some numbers into it.

Suppose we want to change buoyancy by approximately the weight of 1,000 kg.

The required force is approximately:

F = mg

where:

m = 1,000 kg
g = 9.81 m/s²

Therefore:

F ≈ 9,810 N

If seawater density is approximately:

ρ = 1,025 kg/m³

then the volume of seawater corresponding to approximately 1,000 kg is:

V = m / ρ

V ≈ 1,000 / 1,025

V ≈ 0.98 m³

So, approximately 1 cubic metre of seawater—about 1,000 litres—corresponds to roughly 1 tonne of mass.

This is a simplified illustration rather than an operational submarine calculation, but it demonstrates an important principle:

A relatively small change in the quantity of water associated with a large submarine can represent a significant change in force and mass distribution.

That is why accurate instrumentation and careful control of ballast and trim systems are so important.

11. Diving vs Surfacing

Parameter	Diving	Surfacing
Ballast condition	Tanks flooded as required	Water displaced from tanks as required
Air in ballast tanks	Reduced relative to surface condition	Increased where tanks are blown
Effective buoyancy	Decreases	Increases
Vertical tendency	Downward	Upward
Control surfaces	Used to manage descent	Used to manage ascent
Trim	Adjusted as required	Adjusted as required
Depth monitoring	Continuous	Continuous
Control objective	Reach and maintain desired depth	Reach the surface safely

The table is deliberately simplified. Actual submarine procedures vary by class and operating condition.

12. What Happens Once the Submarine Reaches Depth?

Reaching a particular depth is only one part of the problem.

The submarine must then stay there.

This requires continuous monitoring and control.

Sensors can provide information about:

Depth
Pressure
Pitch
Roll
Speed
Tank conditions

The control system processes this information and provides commands to appropriate actuators.

This creates a continuous feedback loop.

Even when the submarine appears to be "stationary" at a particular depth, its control systems are actively managing small changes in the underwater environment.

13. Why Tank-Level Measurement Matters

This brings us directly to another important subject on Submarine Technicals:

How is the amount of water inside a submarine tank measured?

If the submarine is relying on ballast or trim systems, knowing the condition of those tanks is extremely important.

Depending on the application and submarine design, tank condition can be determined using appropriate instrumentation based on principles such as:

Hydrostatic pressure
Differential pressure
Other liquid-level measurement techniques

For a hydrostatic measurement system:

P = ρgh

where pressure changes with liquid height.

Accurate tank instrumentation therefore converts a physical quantity—pressure—into useful information about the tank condition.

This information can then be displayed to operators or used by control systems.

Read next: How Water Level in Submarine Tanks Is Monitored

14. Why Can't a Submarine Just Fill Its Tanks Completely?

This is a common misconception.

Submarine buoyancy is not simply an empty tank = float / full tank = sink problem.

A submarine needs to manage several variables simultaneously:

Overall buoyancy
Longitudinal trim
Pitch
Depth
Speed
Hydrodynamic forces
Internal loading
Water density
Tank conditions

The objective is controlled operation, not simply maximum sinking or maximum floating.

15. Common Misconceptions About Submarine Diving

Myth 1: "Submarines just fill their tanks with water and sink."

Not quite.

Flooding the main ballast tanks changes the submarine's buoyancy condition, but controlled diving also involves trim, propulsion and control surfaces.

Myth 2: "The diving planes make the submarine sink."

The diving planes primarily control the submarine's pitch and trajectory while moving through the water.

They are not the primary mechanism for changing the submarine's overall buoyancy.

Myth 3: "A submarine remains underwater by constantly fighting gravity."

Not necessarily.

A submarine can establish a condition close to neutral buoyancy, allowing it to remain underwater without continuously generating a large downward or upward force.

Myth 4: "Surfacing simply means blowing every ballast tank."

Actual ballast and surfacing procedures are more sophisticated and depend on the submarine's design and operating condition.

16. The Engineering Systems Behind a Simple Dive

What appears to be a simple action—"dive"—actually involves many systems working together.

Ballast System

Controls the submarine's buoyancy condition.

Trim System

Controls the distribution of weight and buoyancy.

Hydraulic or Electric Actuation

Moves control surfaces and operates valves and mechanisms, depending on design.

Compressed-Air System

Can be used to displace water from ballast tanks during appropriate operations.

Depth Measurement

Provides information about the submarine's vertical position.

Motion Control System

Coordinates control of the submarine's movement and attitude.

Power System

Provides energy for pumps, valves, sensors, control systems and other equipment.

Human Operators

Monitor the system, assess conditions and intervene when required.

This is a good example of a fundamental principle of submarine engineering:

The submarine is not a collection of independent machines. It is an integrated system.

17. Why Diving and Surfacing Require Precision

A submarine is a very large and heavy machine operating in a three-dimensional environment.

The crew and control systems must manage:

Vertical position

Horizontal position

Pitch

Roll

Yaw

Speed

Buoyancy

Trim

These parameters interact with each other.

For example, changing the submarine's pitch can alter its depth trajectory. Changing speed can alter the hydrodynamic forces acting on the control surfaces. Changes in tank conditions can affect both buoyancy and trim.

This is why submarine motion control is a fascinating application of fluid mechanics, control engineering, mechanical engineering, electrical engineering and instrumentation.

18. The Complete Process in One Diagram

The entire concept can be simplified as:

SURFACE

Positive buoyancy

↓

Ballast tanks flooded

↓

Reduced buoyancy

↓

DIVE

Diving planes control trajectory

↓

DESIRED DEPTH

Buoyancy + trim adjusted

↓

NEUTRAL BUOYANCY

Depth continuously monitored

↓

SURFACING

Compressed air used as appropriate to displace ballast water

↓

Increased buoyancy

↓

ASCENT

Control surfaces manage trajectory

↓

SURFACE

The exact implementation varies significantly between submarine classes, but the underlying engineering principles remain rooted in buoyancy, fluid mechanics and control.

19. Key Takeaways

If you remember only five things from this article, remember these:

Submarines dive by changing their buoyancy condition.
Main ballast tanks are fundamental to changing buoyancy.
Trim tanks help control the distribution of weight and buoyancy.
Diving planes control pitch and trajectory while the submarine is moving.
Sensors and control systems continuously monitor and manage the submarine's underwater state.

The most important concept is that diving and surfacing are controlled engineering processes, not simply sinking and floating.

Frequently Asked Questions

How does a submarine dive?

A submarine dives by changing its buoyancy condition, primarily through its ballast system. Seawater is admitted into the main ballast tanks, reducing the submarine's positive buoyancy. Once submerged, control surfaces and propulsion help control its trajectory.

How does a submarine surface?

For submarine designs that use compressed-air ballast blowing, compressed air is introduced into appropriate ballast tanks to displace water. This increases buoyancy and produces an upward tendency. Control surfaces and other systems manage the ascent.

What are main ballast tanks?

Main ballast tanks are tanks used to change a submarine's buoyancy condition. They can be flooded with seawater and, under appropriate operating conditions, have water displaced using compressed air.

What are trim tanks?

Trim tanks are used to adjust the submarine's longitudinal balance and help maintain the desired trim.

Do diving planes make a submarine sink?

Not directly. Diving planes generate hydrodynamic forces that control pitch and trajectory while the submarine is moving. The submarine's buoyancy condition is primarily managed through its ballast and other buoyancy-control systems.

How does a submarine maintain depth?

It combines buoyancy and trim management with depth measurement, control surfaces, propulsion and control systems. The exact architecture varies between submarine classes.

Why is compressed air used in submarines?

Compressed air has many applications. One important application is displacing water from ballast tanks during appropriate buoyancy-control operations.

How much water can change a submarine's buoyancy?

The exact effect depends on the submarine's displacement, tank arrangement, water density and operating condition. As a simple physical reference, approximately 1 cubic metre of seawater has a mass of roughly 1,025 kg.

Conclusion

A submarine's ability to disappear beneath the ocean is the result of a carefully coordinated interaction between physics and engineering.

Ballast tanks control the submarine's buoyancy condition.

Trim systems maintain balance.

Diving planes control pitch and trajectory.

Propulsion provides movement.

Sensors measure depth and other parameters.

Control systems process this information.

Compressed-air systems can provide the force needed to displace ballast water during appropriate operations.

Together, these systems allow a submarine to transition from the surface to the underwater environment, establish a controlled depth, maneuver through the water and eventually return to the surface.

What looks like a simple movement is actually a remarkable example of fluid mechanics, control engineering, instrumentation, mechanical engineering and human expertise working together.

It isn't magic. It's engineering.