Your vessel displaces 747 tons and measures 136'L by 34'B. You ship a large wave on the after deck. What is the reduction to GM due to free surface before the water drains overboard, if the after deck measures 56'L x 34'B and the weight of the water is 58.6 tons?
• Free surface effect (FSE) and how it reduces metacentric height (GM) • Using the free surface moment of inertia of a rectangular tank/area: I = (L × B³) / 12 (or similar form) • How virtual rise of the center of gravity (GG₁) relates to FSE: GG₁ = (w × FSM) / Δ
• How do you treat shipped water on deck for stability calculations – similar to a slack tank having free surface? • Given the deck dimensions (56' × 34'), what is the free surface moment of inertia of that water area? How does breadth affect the result? • Once you have GG₁ from the free surface, how does that change GM if initial GM is not given explicitly?
• Be sure you are using the correct dimensions (only the flooded deck area) to compute the free surface moment of inertia, not the full vessel length. • Check that you are using displacement in consistent units (tons vs long tons vs weight in tons) when applying GG₁ = (w × FSM) / Δ. • Confirm the relationship: reduction of GM ≈ virtual rise in G (GG₁) caused solely by the free surface effect in this scenario.
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