The liquid mud tanks on your vessel measure 20'L by 18'B by 7'D. The vessel's displacement is 866 T and the specific gravity of the mud is 1.8. What is the reduction in GM due to 2 of these tanks being slack?

Correct Answer: Option D — 1.12 feet

Accounting for free surface effect is mandatory whenever tanks are partially filled. Computing the total inertia moments for two 20'x18' tanks holding 1.8 SG mud on an 866 T displacement vessel gives a total metacentric height reduction of 1.12 feet.