Marine propeller roughness penalties
The main objective of the project is to investigate the influence of surface roughness of marine propeller blades on propulsive power. The work has involved studies in the concept and practice of surface roughness measurement and characterisation as well as application of boundary layer theory for the analysis propeller-ship hull flow interaction of propeller flow and propellar-ship hull flow interaction. From extensive measurements of the surface topography of in-service propellers, a standard measurement procedure using different commercially available propeller-surveying instruments is described. A development of turbulent boundary layer procedures has been made to determine sufficiently accurately the increment of drag coefficient of propeller blade sections due to propeller blade surface roughness. The roughness function used for this integral boundary layer analysis is derived using, principally, Musker's experimental data. In addition, an experimental determination of the roughness function of a replicated propeller surface using a rotor apparatus has been carried out and described in detail. The turbulent boundary layer procedures require a knowledge of the surface variation of pressure over the propeller blade. For this purpose a program based on Riegels method has been used to give the velocity distribution for a given propeller section geometry. This is used with the boundary layer procedures for developing a complete program "PROFNESS" to calculate the increment of drag coefficient of the blade section. Results from different propellers analysed indicate that the power penalty is proportional to the relative blade roughness to the 1/3 power. An investigation has been made to compare the increment of frictional coefficient for a flat plate and propeller section profiles. It is shown that a "rough" flat plane calculation is quite adequate for such work.' The use of a flat plate analogue as a reference to calculate the skin friction resistance of both propeller and hull surfaces is considered. It is shown that the proposed solution of flat plate momentum integral equations provides a valid, simple and practical solution to the problem of predicting the hull and propeller roughness drag penalties. It also provides, particularly for ship hull resistance, a strong support for the ITTC Correlation Line, not only, and importantly, in regard to its slope, but also its level. For shipowners and operators who may not wish to access advanced computer programs, a simplified method has been proposed to calculate the propeller roughness penalties. There is a good agreement between the two simplified and detailed propeller analysis methods. The propeller roughness penalties, which can be obtained from either the simplified or the more rigorous method, can be related to the Rubert Propeller Comparator Gauges in order to quantify the benefits and justify the cost of the blade surface roughness. Analytical procedures have been included which can be used to calculate the combined effects on ship performance of propeller blade and ship hull surface roughnesses.