The righting arm GZ is best described as which of the following?

• A. The horizontal distance between the line of action of buoyancy and the weight through G.
• B. The vertical distance between deck and keel.
• C. The distance from center of gravity to the hull.
• D. The angle between true north and magnetic north.

Answer: (A)

The key idea is that the righting arm represents the horizontal separation between the line of action of buoyancy and the weight through the ship’s center of gravity. When the hull heels, the buoyant force shifts to act through the new center of buoyancy, while the weight continues to act straight down through the center of gravity. The horizontal distance between these two vertical force lines is the righting arm, GZ. That distance, times the weight, gives the restoring moment that acts to return the vessel to upright.

This ties directly to stability: for small heel angles, GZ is roughly the metacentric height (GM) times sin(heel angle). The larger the GZ, the stronger the righting moment and the steadiness of the vessel.

The other options describe unrelated distances or angles (vertical deck-to-keel distance, CG-to-hull distance, or north/south orientation), none of which define the horizontal force-arm that produces the righting moment.