A submarine dives and surfaces by controlling its buoyancy, which is the upward force from water that pushes on the vessel. Unlike a surface ship, a submarine can change how much water it holds inside special ballast tanks. When the tanks take in seawater, the submarine becomes heavier and can sink.
When compressed air pushes water out of the tanks, the submarine becomes more buoyant and rises.
The key idea is that a submarine compares its weight with the buoyant force from the water it displaces. If weight is greater than buoyant force, it dives; if buoyant force is greater than weight, it surfaces. Ballast tanks do not make the submarine smaller or larger, but they change its total mass by adding or removing seawater.
Small adjustments with trim tanks and control surfaces help the crew hold a steady depth while moving underwater.
Understanding Ships and Submarines: How a Submarine Dives
Main ballast tanks are usually placed outside the strong inner hull where the crew live. This matters because seawater can enter those tanks without flooding rooms, machinery, or electronics. Openings near the bottom let water move in, while vents near the top let trapped air leave.
A tank will not fill quickly if its air cannot escape. This is the same reason liquid pours slowly from a bottle when air cannot get back inside. The crew monitor valves, tank levels, depth, and the angle of the submarine throughout this process.
Keeping a submarine level is as important as changing its depth. Water stored in trim tanks can be moved from the front toward the back, or in the opposite direction. This changes the balance of the vessel.
If the front is too heavy, the submarine points downward. If the back is too heavy, it points upward. Control surfaces called diving planes help steer the craft through water.
They work best while the submarine is moving, because flowing water creates a force on them. Ballast provides the basic balance, while the planes make fine changes during travel.
Water pressure increases greatly with depth. At a deep location, the air used to force water from a tank must have a pressure greater than the water outside. That requires stored compressed air, which is limited and carefully managed.
A rapid emergency rise may use a large amount of this air to clear tanks quickly. Near the surface, other air systems can help finish the job.
Engineers must build the tanks, valves, pipes, and hull to withstand pressure, corrosion from seawater, and repeated use. A small leak or stuck valve can affect balance, so submarines use backup systems and detailed checks.
Submarine buoyancy connects to several situations students may know. A loaded cargo ship sits lower in water than an empty one because it must displace more water to support its greater weight. A scuba diver changes buoyancy with a vest that holds more or less air.
Temperature and salt content matter too. Cold, salty seawater is denser than warm, fresh water, so it can provide a larger upward force for the same displaced volume. When learning this topic, separate three ideas clearly.
Buoyancy depends on displaced water. Weight depends on mass and gravity.
Water pressure depends mainly on depth. These ideas are related in submarines, but they are not the same thing.
Key Facts
- Buoyant force equals the weight of displaced water: F_b = rho_water g V_displaced.
- Weight equals mass times gravitational field strength: W = mg.
- A submarine dives when W > F_b.
- A submarine rises when F_b > W.
- A submarine is neutrally buoyant when W = F_b, so it can stay at a nearly constant depth.
- Flooding ballast tanks adds water and increases mass; blowing ballast tanks adds compressed air and pushes water out.
Vocabulary
- Buoyancy
- Buoyancy is the upward force a fluid exerts on an object placed in it.
- Ballast tank
- A ballast tank is a compartment that can be filled with seawater or air to change a submarine's buoyancy.
- Displacement
- Displacement is the volume of water pushed aside by a floating or submerged object.
- Neutral buoyancy
- Neutral buoyancy occurs when an object's weight equals the buoyant force acting on it.
- Compressed air
- Compressed air is air stored under high pressure and used in submarines to force water out of ballast tanks.
Common Mistakes to Avoid
- Thinking a submarine dives by pointing its nose downward only. The main cause of diving is increased weight from flooded ballast tanks, while control surfaces mostly help steer and adjust depth during motion.
- Confusing mass with volume. Flooding ballast tanks increases the submarine's mass, but the outside volume and the amount of displaced water stay nearly the same.
- Assuming air in ballast tanks pulls the submarine upward. Air does not pull upward; it replaces heavy water, lowering the submarine's total weight so buoyant force can lift it.
- Ignoring neutral buoyancy. A submarine does not always either sink or float upward; when weight and buoyant force balance, it can cruise at a constant depth.
Practice Questions
- 1 A submarine displaces 2,000 m^3 of seawater with density 1,025 kg/m^3. Using g = 9.8 m/s^2, calculate the buoyant force on the submarine.
- 2 A submarine has a buoyant force of 18,000,000 N. If its current mass is 1,820,000 kg, use W = mg with g = 9.8 m/s^2 to decide whether it will rise, sink, or be neutrally buoyant.
- 3 Explain why opening vents to let seawater into ballast tanks helps a submarine dive, even though the submarine's outer shape and volume do not change much.