Your vessel displaces 728 tons and measures 138'L by 31'B. What is the reduction in GM due to free surface if the fish hold (36'L by 29'B by 9'D) is filled with 3.6 feet of water? (Each foot of water weighs 29.8 tons)
• Free surface effect (FSE) and how it reduces GM (metacentric height) • Formula for free surface moment (FSM) and its relation to tank breadth and length • How to convert a moment into a reduction in GM using vessel displacement
• First, figure out the water volume in the fish hold from its dimensions and water depth. How does that relate to the given weight per foot of water? • Recall the formula for free surface moment in a slack tank: which dimensions (L, B, D, or water depth) actually enter the FSE formula? • Once you have the free surface moment, how do you turn that into a GM reduction using the vessel’s displacement of 728 tons?
• Be sure you’re using the correct breadth of the tank in the free surface formula, not the vessel’s overall breadth unless they’re the same. • Confirm that you’ve used displacement in tons, not weight of water in the tank, in the GM reduction step. • Double-check unit consistency so your final GM reduction is in feet, matching the answer choices.
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