The bilge keel, which is a widely used tool for passively reducing the roll motion of ships, generates and releases vortices periodically when the ship rolls, leading to dissipation of the roll kinetic energy and rise in the roll damping of the ship. The initial heel angle, the height and installation angle of the bilge keel, as well as roll frequency, all affect the vortex evolution at the bilge, causing differences in the roll damping to varying degrees. The two-dimensional roll decay of a typical Panamax ship model is simulated based on the free surface random vortex method (FSRVM). A new way of vortex generation is introduced in the current numerical model, and validated against experiments. The additional roll damping induced by bilge keel of different heights and installation angles is compared. In addition, the effects of bilge-keel configurations on roll motion under regular waves are also discussed. Based on the simulation, the characteristics and mechanism of bilge keel with different parameters on roll damping reduction are discussed. The preferred bilge-keel installation angle and the existence of the bilge-keel critical height are also reported.