The results of an experimental analysis of the velocity field around a ship model in forced roll motion are presented. Tests, carried out at the INSEAN towing tank n° 2, aim to achieve a better physical understanding of the fluid dynamics and a certified experimental data set for CFD validation. Results describe the evolution of the flow field in two cross sections along the bilge keels during damped roll motion. A PIV algorithm based on window deformation is used to improve the estimate of the vorticity. The mean unsteady flow field is obtained by ensemble averaging of time histories. The hydrodynamic damping effectiveness of the bilge keels is highlighted by the generation of strong vortical structures that characterize the transfer of energy between the hull and the surrounding flow field.