The liquid mud tanks on your vessel measure 40'L by 20'B by 8'D. The vessel's displacement is 996 T and the specific gravity of the mud is 1.7. What is the reduction in GM due to a pair of these tanks (p/s) being slack?
• Free surface effect and how it reduces GM (metacentric height) • Formula for free surface correction: virtual rise in G based on free surface moment / displacement • Effect of specific gravity on the free surface moment for a liquid cargo
• How do you calculate the free surface moment for a rectangular tank when it is slack? Think about the tank breadth and the liquid’s density. • Once you have the free surface moment for one tank, how will having port and starboard tanks both slack affect the total correction? • How does the vessel’s displacement (in tons) factor into turning that free surface moment into a reduction in GM?
• Be sure you are using breadth of the tank (not length or depth) in the free surface moment formula. • Confirm that you convert the tank’s free surface moment into ton-feet using the liquid’s specific gravity and proper units. • Check that you apply the correction for both tanks (p/s) and then divide by the vessel’s displacement (996 T) to find the GM reduction.
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