Here we analyze the hydraulic performance of an ancient Spanish horizontal watermill. Previous studies of similar devices have focused on qualitative descriptions of their technical functioning, providing efficiency curves based on two-dimensional analytical approximations. In contrast, here we perform three-dimensional computational fluid dynamics (CFD) simulations that allow us to obtain quantitative values for both the hydraulic torque and the power. The results here found reveal how previous studies clearly overestimated the efficiency of these devices. Finally, we make use of the capabilities of CFDs by investigating the performance of a modified blade profile. The new design here proposed successfully increases the energy efficiency (up to 44%) in comparison with the classical one. (C) 2009 Elsevier Ltd. All rights reserved.