Calculations on the Oil Film Between a Propeller Shaft and the Aft Sterntube Bearing

The propulsion plant of a vessel generally consists of a propeller, a main engine and a shaft transferring the power. The shaft is supported by a set of bearings. One bearing in particular, the one directly forward of the propeller, is of special interest in this study. The correct functioning of this bearing is vital for the propulsion plant to remain operating. As a result of the different operating conditions of the propulsion plant particularly this bearing is subjected to varying loads. It also has to accommodate a certain slope mismatch with respect to the shaft. If a bearing functions correctly or not depends on the presence of an oil film between the bearing and the shaft. Also the thickness of the film and the generated pressures within it are deciding whether the loading is acceptable or not. As such a detailed analysis of the oil film between the shaft and the bearing presents a scientifically supported method to evaluate the acceptability of the loading of the bearing. Another advantage is that the method makes it possible to verify the correct functioning of the bearing in varying loads related to off design conditions. This study presents a general method to calculate the minimum oil thickness within a cylindrical bearing given such external parameters as load, slope mismatch, shaft speed and the oil viscosity. First a part of the bearing theory is presented as well as the relations governing the oil flow through the bearing. The method to solve the set of equations used is the finite difference method. Some adaptations of the formulae to enable the use of the method are presented. Also is explained what assumptions and boundary conditions are applied. In succession to the theoretical part the method is validated with a comparison to a set of experimental data. The practical significance of the use of the method is demonstrated by a practical case. Finally some conclusions are presented.