Current industry practice to control roll motions in floating production storage and offloading (FPSO) vessels is based on using large-width bilge keels. This paper details an experimental study involving a range of bilge keel widths from 0% to 20% of half beam of a FPSO with rectangular geometry. Both free decay and forced oscillation tests were conducted on the range of geometries at different amplitudes and frequencies. The results show that, for given amplitude of roll motion, the damping coefficient increases with increasing bilge keel size up to a certain point and then declines. Numerical simulations using the free surface random vortex method were performed on rectangular cross sections with bilge keels, which show good agreement with the experimental results. Simulations extended up to even larger keel widths indicate that the same trend for damping coefficient as a function of keel size is found. An examination of the simulation results suggests a likely explanation for this behavior. A simplified formulation for damping coefficient is developed as a function of bilge keel width and roll amplitude.