What Happens when the Ship Rolls?
To understand this, please closely refer to the attached picture. Let us assume that the ship rolls over to the starboard side. As mentioned earlier, the center of buoyancy “B” shifts to “B1.” This is because of the change in the water line or the underwater area. As the new center of Buoyancy “B1” is not in line with the center of gravity “G”, let us draw a vertical line from “B1” to the original center line and call the point of intersection the metacenter “M.”
The force of Buoyancy and Gravity are equal in magnitude, but opposite in direction. At this position of heel, they are not acting in the same line, but parallel to each other. Thus two equal forces acting opposite to each other constitute a “Couple.” This couple tends to produce a rotational force, which brings the ship back upright.
Once the ship becomes upright, the force of Gravity and the force of Buoyancy, which are equal in magnitude and opposite to each other act on the same straight line and thus “B1” comes back to “B.”
GM is called the Metacentric Height of the ship and GZ is called the Righting Moment.
Here are some typical values for metacentric height for some various types of ships:
The same effect happens when the ship inclines to the port side, too. These effects happen when the ship inclines for small angles of approximately 7 to 10 degrees. The Metacenter “M” does not change for these small angles, and the righting arm is considered to be straight. At higher angles, the position of “B1” will tend to be on an arc. Also the above explanation is valid when the metacentric height is positive. The effects when the metacentric height is negative will be discussed in future articles.