During what condition does the pocketing phenomenon primarily occur?

The pocketing phenomenon primarily occurs when the tank is only partially filled, because this condition allows for fluid movement within the tank, which can significantly affect the stability of the vessel. When a liquid tank is partially filled, the liquid can shift and create a concentration of weight on one side of the tank, leading to dynamic forces that can change the center of gravity and thereby influence the vessel's stability.

As the vessel rolls or pitches due to waves or maneuvering, the liquid can create a counteracting moment that affects how the vessel behaves. The fluid can move towards the low side during a roll, causing a sudden shift in weight that may lead to increased heeling. This dynamic response can be critical in understanding how stability is impacted under different operational scenarios.

In contrast, a full liquid tank doesn't experience the same shifting effect because the liquid is held firmly in place, thus maintaining a more stable condition. When at large angles of inclination, while there may be other stability concerns, the pocketing effect is not as pronounced with fully occupied tanks. Similarly, operating in shallow waters may involve different stability considerations but does not specifically relate to the pocketing phenomenon.