Why can a slack ballast tank weaken stability when the same tank pressed full would not?

Answer: Liquid mass shifts sideways

Liquid mass shifts sideways ✓ — Right. A partly filled tank has a free surface, so the liquid stays level and moves toward the low side as the ship heels. That shift acts like a virtual rise in the ship's center of gravity and reduces GM. This is why stability practice often prefers ballast tanks either full or empty, not casually slack.

Lower water level helps — No. A lower water level can mean less total ballast mass, but it also creates the free surface that lets liquid shift sideways. The danger is not simply how much water is inside; it is whether that water can move its center of mass during a roll. A controlled low tank can help, while a slack wide tank can hurt.

Air pocket cancels buoyancy — No. The air pocket inside a slack tank does not cancel the ocean's buoyancy outside the hull. The ship still displaces seawater and receives buoyant force from that displacement. The stability loss comes from internal liquid motion changing the effective weight distribution.