🔍 Key Concepts

• Free surface effect in slack tanks and how it reduces stability • Relationship between displacement (ship gets heavier) and heeling moment from free surfaces • How moment = force × distance scales when the vessel’s displacement changes but tank dimensions stay the same

💭 Think About

• If the tank size and liquid depth stay the same, what happens to the free surface’s ability to create a heeling moment when the ship as a whole becomes heavier? • Think about GM correction for free surface (GMo vs GM): when displacement increases, does the free surface correction represent a larger, smaller, or same fraction of the ship’s overall stability? • Is free surface moment itself changing, or is the effect of that moment on stability changing as displacement changes?

✅ Before You Answer

• Distinguish clearly between free surface moment (FSM) and free surface correction (FSC) to GM; they are not the same thing • Assume the geometry of the tank and liquid level are unchanged; only the ship’s displacement changes • Recall that in most stability tables, FSM is a property of the tank and liquid, while its impact is normalized by the ship’s displacement