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.
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.