Energy Saving Devices are considered as a solution to improve propulsive efficiency in marine vessels. However, performance prediction of ESD presents many challenges; both for full-scale CFD simulation and for empirical methods used to scale-up the power prediction in model test. This paper evaluates practice of power gain prediction for the well-known KVLCC2 with an energy saving device. A duct with pre-swirl stator was designed and studied in a workshop with full-scale CFD simulation using different commercial codes. Model scale tests were performed, and full-scale power gain was predicted using ITTC recommendations. The comparison between different CFD codes and numerical techniques which was used to setup a self-propulsion simulation showed that reliable power prediction with CFD requires further improvement. On the other hand, the Reynolds number scaling of the small appendages with complicates flow regime, put forward many questions on model test prediction practices for ESD efficacy. Despite these issues, combining the existing tools, full-scale CFD and model test, should be the base for ESD design and could be confirmed only with sea-trial. Today, especially in industry, automated mesh generation are used to decrease the cost of setup and meshing in CFD simulations. Most of these methods are developed for resistance prediction in vessels. This paper discusses the considerations in evaluating the quality of simulation method for the purpose of ESD power gain estimation.