In this blog post, we’ll examine the causes of ship capsizing, as well as the design principles and key operational measures used to prevent it.
Why is ship stability important?
In international trade, ships are a vital means of transportation capable of carrying large volumes of cargo at once. Ultra-large oil tankers (VLCCs) and ultra-large crude carriers (ULCCs) can carry hundreds of thousands of metric tons of crude oil, while large cruise ships sail with thousands of passengers on board. Therefore, the scale of economic losses, loss of life, and environmental pollution resulting from the sinking or capsizing of a single ship is immense. Since rescue operations at sea are difficult and damage tends to escalate, ensuring a ship’s stability is of the utmost importance.
Forces Acting on a Vessel and Static Equilibrium
A vessel floating normally is subject to the forces of gravity and buoyancy; since these two forces are equal in magnitude but opposite in direction, they maintain static equilibrium. However, if the vessel heels due to waves, strong winds, a collision, or a sudden change in course, the point of application of buoyancy shifts and no longer lies on the same line as the center of gravity, resulting in a loss of static equilibrium.
In this heeled state, two possible outcomes arise depending on the direction of the moment. One is when a restoring moment acts to return the ship to its original attitude, thereby restoring stability; the other is when the moment continues to act in the direction of the heel, eventually leading to capsizing.
Restoring Moment and the Role of the Metacenter
When a ship heels, the center of buoyancy—the point of application of buoyancy—shifts. The point defined by the relationship between the line of action of buoyancy and the ship’s centerline in this heeled state is called the metacenter (M). The greater the GM (metacenter height)—the vertical distance between the metacenter (M) and the center of gravity (G)—the greater the restoring moment generated at the same angle of heel, thereby increasing the ship’s stability.
Conversely, as GM decreases, the restoring force weakens, making the ship prone to heeling even under small external forces; if GM becomes negative, the ship cannot recover its original attitude on its own, and the likelihood of capsizing increases significantly. Therefore, securing and maintaining an appropriate GM during both the design and operational phases is key to ensuring stability.
Practical Examples of Cargo Stowage and Design
Stowing cargo at a high position raises the center of gravity, which reduces the GM and weakens the ship’s righting capacity. In actual maritime accidents, there are numerous cases where the center of gravity has risen due to overloading or errors in cargo stowage, significantly compromising the ship’s stability. Therefore, the basic principle is to stow heavy cargo as low as possible in the hull to lower the center of gravity.
However, it is not always possible to freely adjust the height of the cargo. For example, when salvaging a sunken vessel, it may be necessary to lift the entire vessel onto a deck for transport. In such situations, since the cargo height cannot be lowered, stability is ensured through design measures—such as widening the hull—to relatively raise the metacenter. Specialized vessels, such as semi-submersible heavy-lift carriers, are designed by actively utilizing this principle.
Operational Countermeasures: Cargo Securing and the Free Surface Effect
Major factors that impair righting capacity include cargo shifting and the free surface effect—the sloshing of liquid inside tanks. When a ship heels and the cargo shifts to one side, an additional heeling moment is generated, which can increase the risk of capsizing. Therefore, on container ships and similar vessels, containers are securely fastened with lashing devices and strictly managed to prevent movement during navigation.
On ships carrying liquid cargo, the free surface effect—in which the liquid inside the tanks moves, reducing righting capacity—can occur. To minimize this, tanks are filled to an appropriate level in accordance with operational standards and, when necessary, divided into multiple compartments for management. In particular, LNG carriers and crude oil tankers strictly manage tank design, filling rates, and operational procedures in accordance with international safety standards.
Simple Preventive Measures and Conclusion
Ship safety is ensured when complex design technologies and basic operational principles are observed together. During the design phase, sufficient GM must be secured, and various physical factors—such as hull width and stability—must be comprehensively considered. During the operational phase, it is paramount to adhere to basic safety rules, including proper cargo loading, thorough cargo securing, tank management, and prevention of overloading.
Simply adhering to these basic principles can prevent many maritime accidents and capsizing incidents. Of course, it may be difficult to completely prevent capsizing under extreme natural conditions—such as typhoons or giant waves—or in exceptional accidents. Nevertheless, strictly adhering to the basic principles of ship design and safe navigation is the most effective way to reduce maritime disasters.
