A submarine controls its depth by changing its average density compared with seawater. In normal operation, ballast tanks can be flooded with water to dive or filled with air to rise. A ballast blow system is the emergency method that uses stored high-pressure air to force water out of the ballast tanks quickly.
This matters because a submarine may need to reach the surface fast if it loses power, has flooding, or faces another serious hazard.
During an emergency blow, valves open from compressed air cylinders into the main ballast tanks. The incoming air expands into the tanks and pushes ballast water out through flood ports, making the submarine more buoyant. As the submarine rises, outside water pressure decreases, so the air in the ballast tanks expands even more and increases the upward force.
The result is a rapid, steep ascent with large streams of bubbles and water leaving the submarine.
Understanding Ships and Submarines: The Ballast Blow System
The emergency blow system works against the enormous pressure of the ocean. At a great depth, seawater presses hard on every opening in a ballast tank. The stored air must have a pressure higher than the surrounding water before it can drive water out.
This is why submarines carry air in strong cylinders and use pressure reducing equipment before sending it to the tanks. The crew cannot simply release all air without control. Valves, pipes, and tank connections must deliver enough air fast enough while avoiding damage from a sudden pressure surge.
A useful idea is that the submarine's outer shape keeps displacing nearly the same volume of water during most of the ascent. What changes quickly is its total mass. Water leaving the ballast tanks reduces that mass, while the displaced water remains nearly unchanged.
The upward buoyant force can then exceed the submarine's weight. At first, the rise may be difficult because deep water pressure compresses the air trapped in the tanks. Higher up, the pressure falls and the trapped air takes up more space.
It pushes out more water without requiring as much additional stored air. This effect can make the ascent accelerate.
A fast ascent is not automatically safe. If the submarine rises too steeply, its bow can point upward and the crew may struggle to control depth. Near the surface, the vessel can break through with considerable speed.
This is called broaching. Waves, rough weather, or nearby ships create extra danger at that point. The crew must secure equipment, account for people in different compartments, and monitor the hull during the event.
They also need to preserve enough high pressure air for other safety needs. An emergency blow is therefore a serious action, not the usual way to surface.
Students can connect this system to a plastic bottle held underwater. The deeper the bottle goes, the more the water presses on it. A sealed bottle with air inside tends to rise because it displaces water while having relatively little mass.
A submarine uses the same buoyancy principle, though its equipment is far more complex. Pay attention to the difference between pressure, force, mass, volume, and density. Pressure describes how strongly water pushes on an area.
Force is the total push. The key change during a blow is the loss of water mass from the tanks. The expanding air helps produce that loss, while the changing pressure with depth explains why the rise can become stronger on the way up.
Key Facts
- Buoyant force equals the weight of displaced water: F_b = rho_water V_displaced g.
- An object rises when its buoyant force is greater than its weight: F_b > mg.
- Average density determines floating or sinking: rho_avg = m / V.
- A submarine dives when ballast tanks fill with seawater, increasing its mass and average density.
- A submarine surfaces when compressed air forces seawater out of ballast tanks, decreasing its mass and average density.
- Water pressure increases with depth: P = P_0 + rho g h.
Vocabulary
- Ballast tank
- A compartment that can be filled with seawater or air to control a ship or submarine's buoyancy.
- Ballast blow
- The process of releasing compressed air into ballast tanks to force water out and increase buoyancy.
- Buoyant force
- The upward force exerted by a fluid on an object, equal to the weight of the fluid displaced.
- Compressed air cylinder
- A strong storage tank that holds air at high pressure for use in systems such as emergency ballast blowing.
- Flood port
- An opening that allows seawater to enter or leave a ballast tank.
Common Mistakes to Avoid
- Thinking the submarine rises because the air makes it lighter than air. The submarine rises because removing ballast water lowers its average density relative to seawater.
- Forgetting that pressure increases with depth. Deep water pushes harder on the submarine, so the compressed air must be at a pressure high enough to drive water out.
- Assuming buoyant force changes only because the submarine's volume changes. In a ballast blow, the main change is the submarine's mass, since water is expelled while the outside volume is nearly the same.
- Treating an emergency blow like a slow normal surfacing. Emergency blowing uses high-pressure air rapidly and can cause a steep, fast ascent with large bubble streams.
Practice Questions
- 1 A submarine displaces 4.0 x 10^6 kg of seawater when submerged. What is the buoyant force on it? Use g = 9.8 m/s^2.
- 2 A ballast tank contains 25,000 kg of seawater. If an emergency blow expels 80 percent of this water, how much mass is removed from the submarine?
- 3 Explain why compressed air becomes more effective at helping a submarine rise as the submarine gets closer to the surface.