A submarine is a specialized vessel engineered to operate independently beneath the ocean’s surface. Its primary function is to navigate covertly in the three-dimensional underwater environment, relying on precise control over buoyancy and movement. The vessel transitions seamlessly between floating on the surface and remaining suspended at great depths. Understanding how a submarine functions requires examining the physical principles that govern its vertical stability, forward thrust, and directional control.
Achieving Neutral Buoyancy and Submergence
The ability of a submarine to dive and maintain a specific depth relies on manipulating its overall density relative to the surrounding seawater, a concept governed by Archimedes’ Principle. To transition from a positively buoyant state (floating) to a negatively buoyant state (sinking), the submarine uses large Main Ballast Tanks (MBTs) located between its inner pressure hull and outer shell. These tanks are flooded with seawater, significantly increasing the vessel’s total mass without changing its overall volume, thereby increasing its average density.
Once submerged, the goal is to achieve neutral buoyancy where the submarine’s weight exactly equals the upward buoyant force of the displaced water, allowing it to hover. Because seawater density changes with temperature and salinity, and the submarine’s weight changes as supplies are consumed, fine adjustments are necessary. Smaller Depth Control Tanks (DCTs) manage these minor weight fluctuations by pumping small amounts of water in or out of the hull.
Beyond depth control, the submarine must maintain a level attitude, known as “trim,” to prevent the bow or stern from pitching. Trim tanks, positioned far forward and aft, shift water internally along the vessel’s length to counteract weight shifts. Surfacing is accomplished by using powerful compressors to force high-pressure air into the MBTs, rapidly expelling the water and replacing it with air, which reduces the submarine’s density and restores positive buoyancy.
Generating Underwater Movement (Propulsion)
Propulsion systems must generate forward thrust while remaining exceptionally quiet to maintain the submarine’s operational stealth. Most large, long-range submarines use a nuclear reactor to produce heat, which creates steam to drive turbines connected to the propeller shaft. This power source grants the vessel virtually unlimited range and high sustained submerged speed, since it does not require air for operation.
Alternatively, smaller submarines designed for coastal operations typically use a diesel-electric system, running diesel generators near the surface to charge massive battery banks. When operating submerged, diesel-electric submarines switch to silent electric motors powered by the batteries, which makes them extremely quiet at slow speeds but limits their submerged endurance to a few days.
The physical creation of thrust often involves either a large, highly-skewed propeller or a pump-jet propulsor. Traditional propellers are designed with backward-swept blades to reduce the phenomenon of cavitation, which occurs when the blades move so fast that they create vapor bubbles that collapse noisily.
The pump-jet propulsor, utilized on modern nuclear submarines, enhances stealth by encasing the rotor blades within a cylindrical duct. This design smooths the water flow entering the rotor and uses a stator to straighten the flow exiting the duct, suppressing the noise generated by cavitation.
Controlling Direction and Pitch Underwater
Once a submarine is moving forward, its direction and depth are dynamically controlled using hydrodynamic surfaces that function much like the wings and control surfaces of an aircraft. The primary control surfaces are the vertical rudder, which steers the vessel left and right (yaw), and the horizontal stern planes, which adjust the angle of the bow up or down (pitch). These planes deflect the flow of water generated by the forward movement, creating lift or depression forces to change the vessel’s orientation.
Stern planes are positioned at the very rear of the hull, often directly in the flow of the propulsor wash to maximize their effectiveness for rapid depth changes. A second set of planes, known as bow planes or sail planes, are located near the front of the vessel and are used for fine-tuning depth control, especially when operating at shallow depths. Because these control surfaces rely entirely on the force of water flowing over them, they are only effective when the submarine is in motion.
An alternative configuration, known as an X-shaped stern, replaces the traditional cross-shaped rudder and stern planes with four diagonally-oriented control surfaces. Each of these four planes can be controlled independently, allowing them to collectively manage both pitch and yaw simultaneously. This integrated design provides superior maneuverability, particularly at lower speeds.