GM, KG and Free Surface Effect: A Plain-English Stability Refresher

GM — metacentric height — is the core single-number indicator of a vessel's intact stability, but treating GM alone as the whole picture is how "stable on paper" loading conditions turn out not to be stable in practice.

Free Surface Effect: The Most Common Gap

Slack tanks — partially filled, allowing liquid to move freely as the vessel rolls — reduce effective GM below the value calculated for a fully pressed-up or fully empty tank. This is the single most common way a loading condition that looks fine in the stability calculation turns out to behave differently once the ship is actually at sea. Minimising the number of slack tanks, or pressing them up/emptying them where operationally possible, is a genuine stability decision, not just good tank management practice.

Damage Stability Is a Different Calculation

Intact stability and damage stability aren't the same thing, and most real casualties — as opposed to paper exercises — happen in the damage stability domain: residual GM and freeboard after flooding, not the vessel's stability before anything has gone wrong. A vessel with excellent intact stability numbers can still have a dangerously thin damage-stability margin depending on compartmentation and where flooding occurs.