Your vessel displaces 477 tons and measures 116'L x 31'B. You ship a large wave on the after deck. What is the reduction in GM due to free surface before the water drains overboard, if the after deck measures 54'L x 31'B and the weight of the water is 51.5 tons?

Correct Answer: Option D — 7.25 feet

A large wave shipped on deck introduces dangerous free surface effects that immediately degrade stability margins. Using the standard deck tank free surface formula for 51.5 tons of water on a 54'x31' area gives a 7.25 feet reduction in GM.