Exam Question

The correct options describing a situation that causes a virtual rise in the center of gravity are **(a) filling a partially filled tank** and **(c) emptying a partially filled tank**. Both scenarios inherently involve a 'slack tank', which is the direct cause of the Free Surface Effect (FSE). For the purpose of a single-answer justification, we will analyze this phenomenon in detail.
### 1. Definition: Virtual Rise in Center of Gravity (G)
A virtual rise in the center of gravity is not a physical shift in the ship's calculated center of gravity (KG). Instead, it is an apparent or effective rise, denoted as GGv, caused by the movement of liquid within a partially filled (slack) tank as the vessel heels. This effect reduces the ship's initial metacentric height (GM) to an effective value (GMeff) and consequently reduces the righting lever (GZ) at all angles of heel, thereby impairing stability.
As stated in the reference material: *"The reduction of stability caused by the liquids in slack tanks is known as a free-surface effect. This adverse effect on the stability is referred to as a 'loss in GM' or as a 'virtual rise in the vertical centre of gravity KG'"*.