The present paper proposes a simplified formula for predicting the frictional resistance increase ΔC_F due to painted ship-hull surface roughness. A linear fitting is made on the roughness function prepared by several experimental data. A linear approximation is also applied to the local skin friction formula including roughness effect. Further, an approximate integration of the momentum integral equation for a rough flat plate using the above assumptions leads to a following simple formula, [numerical formula] where R_n represents Reyno1ds number and R_k the roughness Reynolds number. It is also derived that ΔC_F is proportional to the square of the C_F value for the smooth flat plate of same length.
