posted by Mike .
I have previously understood a ship's GM determined her role period. However I have just read that if weight on the centreline of the ship is moved laterally outwards to the sides of the ship this will slow the ships roll ie. increase its roll period. I understand the GM of the ship will not alter under these circumstances because as the weight is moved laterally the COG will not change.
GM is not changed, but the righting arm is. If you move G/2 to each side perpendiular to GM, so that G is unchanged, then heel the ship, then draw (or calculate the righting arm for each G/2 mass), the NET (one side is positive, one side is negative) righting arm is increased, due to the distances from each G/2 to the vertical BM line. One then has has a net increased righting arm, with a decreased mass (G/2). The ship will be not so tender, and have a longer roll period.
The idea is to make a vessel less tender so that it does not take on the shape of a high wave and roll over, but rolls more slowly. In practice, shifting fuel ballast or cargo in heavy seas is dangerous and is usually to be avoided.