This paper presents a computational fluid dynamics (CFD) approach which can be used to investigate the hydrodynamic performances of various antifouling paints. Steady Reynolds - Averaged - Navier - Stokes (RANS) equations were solved for the flows around a flat plate at Reynolds number of 2,8x10(6) and 5,5x10(6). The surface roughness effects were modelled as downward shift of the velocity profile at the log - law region of the boundary layer. Granville's similarity law was used to extrapolate the results for a full scale ship. Results are in good agreement with the previous experimental and numerical studies.