On a vessel displacing 8,000 tons, what is the reduction in metacentric height due to free surface when a tank 45 feet long and 45 feet wide is partly filled with salt water?
• Free surface effect and its impact on metacentric height (GM) • Second moment of area of a rectangle: I = (b × l³) / 12 or (l × b³) / 12 (know which side is breadth) • Relationship: Loss of GM = Free Surface Moment / Displacement (in consistent units)
• How do you decide which dimension of the tank (45 ft × 45 ft) is used as the breadth in the free-surface moment calculation? • Once you compute the second moment of area (I), how do you convert that into a free-surface correction to GM for a vessel of 8,000 tons? • Are your units consistent when you combine tank dimensions, water density, and ship displacement in tons?
• Be sure you are using the correct formula for rectangular tank free surface moment and that you square/cube the right dimension. • Confirm that ship displacement is converted to the same unit system as your free surface moment (long tons vs cubic feet with 35 ft³/ton). • After computing the GM reduction, compare your result to the closest option (A–D) and check that it is in feet, not inches.
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