Exam Question
What do you understand by 'Free surface effect' and how does it affect the ship's stability? (16)
Class 4 (4th Engineer)
SC&S
📅 Sep 2025
Reference Answer
### 1. Definition and Concept of Free Surface Effect
The Free Surface Effect (FSE) is a phenomenon that occurs when a ship with a partially filled tank (a 'slack' tank) is heeled by an external force (e.g., wind, waves). The liquid within the tank, under the influence of gravity, will seek to remain level, causing its surface to stay parallel to the vessel's waterline. This results in a transverse shift of a wedge of liquid from the high side to the low side of the tank. This movement of mass within the vessel causes a reduction in the ship's initial stability and its righting capability.
### 2. Mechanism of Stability Loss
When a ship heels to a small angle (θ), the following sequence occurs:
The Free Surface Effect (FSE) is a phenomenon that occurs when a ship with a partially filled tank (a 'slack' tank) is heeled by an external force (e.g., wind, waves). The liquid within the tank, under the influence of gravity, will seek to remain level, causing its surface to stay parallel to the vessel's waterline. This results in a transverse shift of a wedge of liquid from the high side to the low side of the tank. This movement of mass within the vessel causes a reduction in the ship's initial stability and its righting capability.
### 2. Mechanism of Stability Loss
When a ship heels to a small angle (θ), the following sequence occurs:
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