In this blog post, we’ll explore the principle by which submarines rise or sink underwater through buoyancy control, and how that principle resembles other phenomena found in nature.
If you drop a rubber duck into a bathtub, it will immediately float back to the surface. However, if we were to get into the bathtub, our bodies would sink to the bottom. Just like this, when objects enter water, they either float to the surface or sink to the bottom. But there is one object that can sink underwater or resurface depending on the situation: the submarine. A submarine can adjust its depth underwater without relying on engine power. How does a submarine control its depth underwater?
First, let’s examine the differences between objects that float and those that sink. To understand the principles of a submarine, we need to briefly look at its history. The first submarine is believed to have been built by the 17th-century Dutch inventor Cornelius Drebbel. His submarine was simply a small wooden vessel propelled by human power, but over the following centuries, submarines evolved for military purposes and became equipped with complex technology. At the heart of this development was an understanding of an object’s buoyancy and gravity.
Gravity acts equally on objects submerged in water. However, since an object floating on the water’s surface is at rest, we can see that the net force acting on it is zero. This is because another force is acting in the opposite direction of gravity—and that force is buoyancy. Buoyancy is a principle first discovered by the ancient Greek scientist Archimedes; it occurs as a reaction to the water displaced by an object submerged in water. Its magnitude is equal to the weight of the water displaced, and the volume of water displaced is equal to the volume of the object submerged in the water. In other words, an object floating on water displaces an amount of water equal to its own weight, creating a state of equilibrium between gravity and buoyancy.
An object sinks into water because its weight exceeds the weight of the water displaced by its volume. Therefore, whether an object floats or sinks in water depends on whether its mass per unit volume—that is, its density—is greater or less than that of water. Submarines utilize this property to adjust their density and move underwater.
So, how does a submarine adjust its density underwater? It uses buoyancy tanks to control its density. Buoyancy tanks are structural components of a submarine consisting of large empty spaces that can be filled with air or water. When a submarine submerges, it fills the buoyancy tanks with seawater to increase its density. When the submarine surfaces, compressed air that has been prepared in advance is injected into the buoyancy tanks, and the external valves of the tanks are opened. This causes the seawater in the buoyancy tanks to be expelled, and the tanks are filled with air, reducing the submarine’s total mass. However, since the volume occupied by the submarine underwater remains the same, the submarine’s density also decreases. As a result, the submarine’s average density becomes lower than that of seawater, allowing it to rise to the surface.
The buoyancy tanks also serve to maintain a balance between the submarine’s weight and buoyancy, enabling it to move at a constant depth underwater. At this point, the submarine’s density matches that of seawater, and the buoyancy at this state is called neutral buoyancy. In a state of neutral buoyancy, the submarine can maintain its depth while conserving fuel, making it efficient for long-duration submerged operations.
This principle of submarines is similar to that of a fish’s swim bladder. Fish use an organ called a swim bladder, which can hold air, to regulate their depth underwater. Therefore, submarines ultimately regulate their depth by balancing gravity and buoyancy underwater, using the same principle as fish.
This capability of submarines plays a crucial role not only in military operations but also in scientific exploration. Marine researchers use submarines to explore deep-sea environments, survey seabed resources, and contribute to the protection of marine ecosystems. This is greatly helping humanity gain a deeper understanding of the Earth and the oceans.
