When I was still studying in Naval Architecture department of Pattimura University, I was told that the rolling period of a ship should be between 7 and 8 seconds. This is to ensure that passengers and crews will not suffer from sea sick if the rolling period is less than seven. It means the centre of gravity of the ship is too low creating a big righting moment that is responsible for restoring the ship into its upright position. If the rolling period is higher than 8 seconds than it will be considered not safe for the stability of the ship herself.
Sometimes if the cargo is not evenly distributed, the crews have to balance the ship by filling the ballast tanks with sea water. The ballast tanks are usually placed in the double bottom of the ship. The ballast condition improves the vertical and longitudinal center of gravity thus making the ship more stable during her voyage in the middle of the sea.
Back to the discussion of the rolling period. I remembered that there was other explanation from a book entitled Buoyancy, Stability and Steering of Ship written by Dutch Naval Architect Drs. A.R. Bakker, If I am not wrong, which said that the rolling period of a ship can be expressed in a non dimensional period number which is between seven and fourteen. I wish that I could get that book again. I have forgotten how to calculate the non-dimensional rolling period of Kemp stated in that book but I believe some naval architects still read the book as a reference material especially in the Netherlands. by Charles Roring in Manokwari of West Papua - Indonesia