The additional resistance due to roughness is studied by means of CFD simulations. The KVLCC2 hull at full-scale Reynolds number is considered as a test case. A wall function formulation is used to model the rough wall turbulent boundary layer, where the roughness function is based on data from towing flat plates coated with paint of similar roughness as for the full-scale vessel. The additional resistance for coatings with various roughness heights is studied, with roughness heights ranging from less than 10 μm to more than 60 μm. Also, the potential in low-cost reduction of frictional resistance is investigated. High-quality paint coating (with low roughness) can be applied at given locations where the skin friction is high, while using cheaper coating and application procedures (resulting in larger surface roughness) at other locations where skin friction is of less importance.