🔍 Key Concepts

• Free surface effect (FSE) reduces GM by an amount equal to the free surface correction (FSC), which depends on the tank’s moment of inertia and the ship’s displacement. • For a rectangular tank with breadth across the ship and length fore‑and‑aft, the second moment of area about the centerline involves the breadth cubed. • When there are multiple tanks at the same level, their individual free surface moments are added together before dividing by the vessel’s displacement.

💭 Think About

• Identify which tank dimension is the breadth across ship and which is the length fore‑and‑aft, then recall the standard rectangular‑tank formula used in stability for free surface moment. • Calculate the free surface moment for one tank, then account for the fact there are two identical tanks created by the shaft alley. • Use the vessel’s displacement (5,000 tons) to convert total free surface moment into a reduction in GM and compare with the answer choices.

✅ Before You Answer

• Be sure you cube the breadth (transverse dimension), not the length, in the moment of inertia formula. • Confirm you double the result for one tank, since there are two separate tanks at the same level. • Verify that you divide the total free surface moment by the correct displacement (5,000 tons) and keep all units in feet and tons so the final result is in feet of GM reduction.