How do bilge keels reduce a ship’s rolling motion?

Bilge keels are a pair of long, narrow fins that project from the hull of a ship, typically positioned where the bottom of the hull meets the side. In the context of naval architecture, they function as a passive damping mechanism to check or reduce a ship's rolling motion.123

When a ship rolls, these keels act as resistance surfaces that move through the water. By creating hydrodynamic damping forces, they oppose the rolling motion, effectively absorbing energy and helping to stabilize the vessel. Because they are designed to be placed along the natural lines of flow, bilge keels add minimal resistance to the ship's movement in calm water. While they are a common and effective method for roll reduction, they are generally considered less powerful than active measures, such as mechanically rotated antiroll fins or specialized anti-rolling tanks.23