This paper reviews a universal efficiency scaling process for turbomachines of different specific speed nq^ with “hydraulieally smooth” as well as “technically rough” surfaces. Relevant studies are based on model machine characteristics, measurements with individual machine components, and the laws of resistance of technically rough plates. A scaling formula has been derived from these, which is equally suitable for pumps and water turbines. Theoretical and experimental data are in good agreement. The investigations also provide information on the coefficient of equivalence in the smoothrough transition region and on the magnitude of the technical roughness characterizing hydraulieally smooth surfaces. Such data are important to provide information and guidelines for the clarification of questions concerning an economically justifiable extent of surface finishing with which friction losses can be kept within reasonable limits.Technical University, Darmstadt, Fed. Rep. of Germany