A 7,000 ton displacement tankship carries two slack tanks of alcohol with a S.G. of 0.8. Each tank is 50 ft. long and 30 ft. wide. What is the reduction in GM due to free surface with the vessel floating in sea water, S.G. is 1.026?
• Free surface effect (FSE) and how it reduces GM (metacentric height) • Formula for free surface moment of a rectangular tank and how S.G. of the liquid and seawater affect it • Relationship between displacement, FSM per tank, and total reduction in GM
• First, compute the free surface moment for ONE slack rectangular tank using its length and breadth; then account for TWO tanks. • Think about how to adjust the free surface moment because the liquid S.G. (0.8) is different from the surrounding seawater S.G. (1.026). • Once you find the total free surface moment for both tanks, consider how to convert that into a reduction in GM using the vessel’s displacement of 7,000 tons.
• Make sure you are using the free surface formula for a rectangular tank: check whether it uses breadth squared or length squared, and in what way. • Confirm that you correctly scale the free surface moment by the ratio of liquid S.G. to seawater S.G. before using it to find GM reduction. • Verify that your final GM reduction is in feet and that your units are consistent (tons, ft, ft², etc.) throughout the calculation.
No comments yet
Be the first to share your thoughts!