Azimuth Propulsors in Off-design Conditions
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- Institutt for marin teknikk 
While the use of azimuth propulsors such as azimuth thrusters and podded propulsors as main propulsion device on ships has been growing quite fast, the understanding of hydrodynamics related to propulsors in real operational conditions experienced by the propeller such as oblique inflow and waves has been limited. It is believed that hydrodynamic loads due to oblique inflow and waves can be partly responsible for mechanical components failure such as bearings, seals, and bevel gears, leading to propulsion system breakdown. The main goal of the current research is to provide sufficient information related to hydrodynamic loads, with particular focus on hydrodynamic side force and bending moment applied to the propeller shaft, to enable the designers to design the mechanical components of azimuthing thrusters and pods for loads of correct magnitude, such that future designs can ensure the reliability of the propulsion system even in extreme conditions such as high oblique inflow in cruising speed and in extreme wave conditions. A wide range of experiments for an azimuth thruster model was carried out under different conditions in the large towing tank (LxBxD=260x10x5) at the Marine Technology Centre in Trondheim, Norway. The tests were performed on the pulling and pushing modes of the thruster model in different oblique inflow angles in range of -40 deg to +40 deg at different advance coefficients in range of 0 to 1, in open water and for a twin pulling azimuth thruster in behind a ship hull . A novel shaft dynamometer capable of measuring six components of force and moment was designed to measure all the force and moment components on the propeller shaft. The ship hull nominal wake was also measured in the experiments at different propeller azimuth positions in the propeller plane in order to explain the physical phenomena involved in behind conditions associated with the propeller performance and propeller shaft bending loads. The ship hull wake and the thruster body wake were found to have a significant effect on the loads experienced by the propeller shaft in oblique inflow, so that the propeller at a negative oblique inflow experiences different loads than at a corresponding positive heading angle. Measurement was also made in ventilating conditions to allow comparison with the forces due to oblique inflow in open water conditions. In addition to the tests in steady azimuth angle (oblique inflow), azimuth angles were changed dynamically with time in order to find the dynamic effect on the transient or unsteady loads. Transient loads were also investigated by changing the propeller RPM with time (accelerating and decelerating conditions). The transient operations were found to have significant effect on the shaft bending loads when dynamically changing the propeller azimuth angle. In addition to the tests in calm-water, propeller and shaft loads were also measured in regular waves at different sea states in straight a head condition, in both head and following sea, in order to investigate the effect of hull motions on the periodic propeller loads. Larger bending loads are found in head sea conditions compared with the following sea conditions. In general, it is found that the hydrodynamic side force and bending moment applied to the propeller shaft in both oblique inflow and waves are of significant magnitude, much larger than the crude estimate of these force components considered by the propeller shaft specialists addressed in the ITTC (2005) report. The hydrodynamic side forces and bending loads are found to be important to consider in the mechanical design layout and for dimensioning of components. Finally, this research evaluates various computational methods to calculate propeller performance and hydrodynamic side force and bending moment applied to the azimuth propulsor shaft in oblique inflow. A computationally efficient model called blade element momentum theory (BEMT) , capable of taking into account the wake from ship hull and from the thruster body, was developed to predict not only propeller torque and thrust but also hydrodynamic side force and bending moment applied to the propeller shaft in oblique inflow both in open water and in behind conditions. RANS calculations are used to compute the loads on the propeller and also the nominal wake velocity from the thruster body in order to be used in the BEMT model as inflow to the propeller like in a hybrid method. A panel method based on potential flow theory, implemented in the commercial software AKPA was also used to predict the load on the propeller. The asymmetry observed in the hydrodynamic loads with respect to positive and negative heading angles is captured fairly well by the BEMT model and the RANS calculations, as an effect of ship hull wake and the thruster body wake. All the methods are found to be able to predict the hydrodynamic loads in oblique inflow with reasonable accuracy.