In this blog post, we will examine what ship stability is and its implications for design and safety, using the 1993 “Hwe-ri” ferry sinking accident in the West Sea as a starting point.

Accident Overview

On a coastal hill near Pajanggeum Port on Wido Island in Buan County, North Jeolla Province, stands a memorial tower approximately 7 meters high and 8 meters wide, engraved with the names of 292 people. These are the victims of the 1993 Ferry Ho sinking in the West Sea. Although no weather advisory had been issued at the time of departure, conditions were not ideal for sailing. When waves grew higher than expected during the voyage, the captain decided to turn back. The moment the ship turned, it was struck by severe swells, causing it to list heavily to one side and eventually capsize. This incident, which resulted in the loss of a large number of passengers and crew members, is remembered as a landmark case that raised public awareness of the dangers of maritime accidents.

Basic Concepts and Necessity of Stability

When a ship sails through waves and wind, it must withstand the motion of pitching and rolling. The property that allows a ship to return to its original position even after tilting to a certain degree is called stability. Stability is the most fundamental and crucial property determining a ship’s safety.

Understanding Buoyancy and Moment

Before explaining stability, it is necessary to briefly understand buoyancy and moment. An object submerged in a fluid receives a force from the fluid directed upward, opposite to the direction of gravity; this is called buoyancy. It is thanks to buoyancy that we can float in a swimming pool. A moment can be thought of as the action of a force that attempts to rotate an object.

How Stability Works

When a ship is in equilibrium and floating horizontally, the center of gravity and the point of application of buoyancy lie on the same line of action, so gravity and buoyancy balance each other out in opposite directions. If the ship heels to one side, the shape of the submerged portion changes, shifting the center of buoyancy, while the center of gravity remains relatively unchanged. As a result, gravity and buoyancy act along different lines of action, creating a rotational moment; this moment generates a restoring force that attempts to return the ship to its original horizontal state.
However, if the list increases further, the point of application of buoyancy shifts again and may eventually move to the opposite side of the center of gravity. From this point on, a rotational force that causes capsizing—rather than a restoring force—begins to act, causing the ship to list further or capsize. To simplify this process, initially the relative positions of buoyancy and gravity create a restoring force, but beyond a certain angle, those relative positions shift, creating a capsizing force.

The Relationship Between the Ferry Accident and Stability

When the Ferry encountered swell and listed to one side, the points of application for gravity and buoyancy reversed, causing it to lose its righting force, and it eventually sank. The fact that the cargo inside shifted to one side during the incident, causing the center of gravity to move toward the more inclined side, also accelerated the sinking. Therefore, it can be concluded that the interaction between the internal load distribution and the ship’s stability was decisive, rather than the external waves alone.

Balance and Practical Considerations in Stability Design

It is not always best to simply maximize stability. If stability is excessive, even a slight list will trigger an overly strong restoring force, shortening the period of rolling motion. This can cause discomfort to crew and passengers or result in damage to equipment and cargo.
Conversely, if stability is too low, there is a risk that the ship will capsize easily. Therefore, ships must be designed to have an appropriate level of stability, and safe operation is only possible when factors affecting stability—such as load distribution, cargo securing, and hull form—are comprehensively considered.