This paper addresses the effects of hull roughness on pro-pulsion performance of ships and demonstrates the importance of taking full scale roughness effects into account when designing propulsion devices. The investigation of the hull roughness effect was performed numerically using SHIPFLOW with the built in roughness model based on the assumption that hull surface roughness is uniformly distributed and can be characterized by the equivalent sand roughness.

The ship investigated is a SSPA VLCC with three typical energy saving devices (ESDs), which include a duct, a standard pre-swirl stator (PSS) and two SSPA generic ESDs (GKDM and GKDF). As an initial validation study, numerical simulation and model tests were carried out for the bare hull with two surface conditions: smooth and rough surface. The results from numerical simulation were validated against towing tank tests and clearly indicates a gradual change of flow characteristics/propulsion performances with hull roughness growth: thickening of boundary layer, increase of resistance and propulsion properties (T, Q and RPM). Following the model scale study, full scale simulations have been performed. The results from full scale simulations confirm the trend in increase of EHP and DHP as roughness grows, but even much faster in full scale compared to model scale.  

This paper will further focus on combined hull roughness and scale effects in the design of propeller/ESD and prediction of the performance of a ship. A quite interesting finding is that the roughness is not always affecting in negative direction. The propeller can be operating in more favorable conditions with higher angle of attack due to the thickening of the boundary layer with the increase of hull roughness. This can directly lead to the improvement of propulsive efficiency and in turn result in further power reduction with the use of ESDs.     

This paper will discuss additional steps needed to take into account of hull roughness in design optimization process of propeller and ESDs and present design methodology for the successful development of propellers and ESDs performing well in actual operational conditions.