Throughout the years, many types of Energy Saving Devices (ESDs) have been introduced to the naval industry, each claiming to achieve several percent of power reduction. Although their commercial success is undeniable, there has always been much controversy regarding the assumed working principles of ESDs. Therefore, the basis of this paper is a comprehensive explanation of the hydrodynamic working mechanism of an Energy Saving Device known as the Blade efficiency improving Stator Duct (BSD). Using knowledge of this working principle, an alternative way of analysing propulsion efficiency dedicated to pre-swirl devices has been developed. The working mechanism of the BSD and the assessment of its performance is subsequently demonstrated at model scale by means of RANS simulations. Another topic of discussion regarding ESDs are the scaling procedures used in model testing to predict the full scale performance, which possibly lead to exaggerated claims. The scale effect on the performance of the BSD is therefore addressed in the final part of this paper, also based on numerical simulations.