How submarines achieve stability
Submarines operate in a dynamic environment, where they have to balance the forces of buoyancy, gravity, hydrostatic pressure, and hydrodynamic forces. Submarine stability can be divided into two types: static and dynamic. Static stability refers to the tendency of a submarine to return to its original position after being disturbed by an external force, such as a wave or a current. Dynamic stability refers to the ability of a submarine to maintain a desired course and speed without excessive control inputs or deviations. Both types of stability depend on the shape, mass distribution, and control systems of the submarine. One of the main factors that affect submarine stability is the center of buoyancy (CB), which is the point where the resultant force of buoyancy acts. The center of gravity (CG) is the point where the resultant force of gravity acts. The metacentric height (GM) is the distance between the CB and the metacenter (M), which is the point where the line of action of buoyancy intersects the vertical axis of the submarine when it is tilted slightly. The GM is a measure of static stability: a positive GM means that the submarine is stable, a negative GM means that it is unstable, and a zero GM means that it is neutrally stable. Another factor that affects submarine stability is the weight distribution along the longitudinal axis of the submarine. The longitudinal center of buoyancy (LCB) is the point where the resultant force of buoyancy acts along this axis. The longitudinal center of gravity (LCG) is the point where the resultant force of gravity acts along this axis. The distance between the LCB and the LCG is called the longitudinal moment arm (LMA). The LMA affects the trim of the submarine, which is the angle between the horizontal plane and the longitudinal axis of the submarine. A positive LMA means that the submarine has a bow-up trim, a negative LMA means that it has a bow-down trim, and a zero LMA means that it has no trim. The control surfaces of a submarine are used to adjust its stability, trim, and maneuverability. The main control surfaces are the diving planes, which are located near the bow and stern of the submarine, and the rudder, which is located at the end of the propeller shaft. The diving planes are used to control the depth and pitch of the submarine, while the rudder is used to control its yaw and heading. The angle of attack of these control surfaces determines the amount and direction of lift and drag forces that they generate. These forces can be used to counteract or enhance the forces of buoyancy, gravity, and hydrodynamic forces. Another component that plays a role in submarine stability is the ballast tanks, which are compartments that can be filled with water or air to change the overall density and buoyancy of the submarine. By adjusting the amount and distribution of water and air in these tanks, submariners can control their depth, trim, and stability. For example, to submerge, submariners fill their ballast tanks with water to increase their density and decrease their buoyancy. To surface, they blow out water from their ballast tanks with compressed air to decrease their density and increase their buoyancy. A submarine surfaces by using its ballast tanks and its diving planes to create an upward force that overcomes its downward force. The upward force consists of two components: buoyant force and lift force. The buoyant force is equal to the weight of water displaced by the submarine's hull. The lift force is generated by tilting the diving planes upward, which creates a difference in pressure between the upper and lower surfaces of these planes. The downward force consists of two components: gravitational force and drag force. The gravitational force is equal to the weight of the submarine's hull plus any cargo or crew on board. The drag force is caused by friction between the water and the surface of the submarine's hull. To surface, submariners first increase their buoyant force by blowing out water from their ballast tanks with compressed air. This reduces their density and increases their volume, which displaces more water and creates more upward force. Then they tilt their diving planes upward, which creates more lift force by increasing pressure on their lower surfaces and decreasing pressure on their upper surfaces. These two actions combined create an upward force that exceeds their downward force, causing them to rise toward the surface. Submarine stability is essential for safe and efficient operation under water. It requires careful design, calculation, and testing of various parameters and factors that influence it. It also requires constant monitoring and adjustment by the crew and the control systems during operation. Submarine stability is a challenging but rewarding field of study for naval engineers and submariners alike.