Trans RINA, Vol 153, Part A1, Intl J Maritime Eng, Jan-Mar 2011


difference in hydrodynamic forces between pushing and pulling thrusters is extensively discussed in [2].


The main motivation for the study is the increasing popularity of azimuthing propulsors as main propulsion devices for a variety of ship types. In a study [3], problems


with propeller shaft bearings have identified as one of the most significant causes


been of


mechanical failure. In addition to the bearing problems, there have also been some problems with bevel gears in the azimuth thrusters when used as main propulsion device. A detailed study of the bearing forces and moments due to the rotation of the propeller and the azimuthing of


sufficient information for mechanical design engineering such as the development of optimum bearing systems. Therefore, it


loads on the propeller shaft


is important to know the hydrodynamic under all operating


conditions. It has been found that the shaft side forces and bending moments in open water conditions at high oblique inflows and high advance velocities are large - so large that the shaft bearings experience three times larger load than when only the propeller weight is considered [2]. The hydrodynamic loads might be even higher when the thrusters work in the wake of a ship hull and in waves, which is the investigation of this paper. Further, the lack of knowledge about the hydrodynamic imposed propeller shaft forces has been identified for more than thirty years - for both conventional rudder-propeller and steerable propulsion systems [4]. The importance of the hydrodynamic imposed


bending loads for the


conventional rudder-propeller system has been addressed in [5].


One objective of this paper is to study the hull wake effect on the propeller performance and shaft bending loads in calm water at different thruster azimuth angles and ship velocities. There are a large number of publications on azimuth thrusters and podded propulsors, but they mainly focus on the propeller torque and thrust in calm water without the presence of the ship hull (see for instance


[6], [7]). Islam and his colleagues at


Memorial University have done a set of experiments to determine the hydrodynamic forces acting on podded propulsors. A difference from the study reported here is that Islam et.al. measured the six-degree of freedom forces on the entire thruster only, not on the propeller shaft as well. They have also investigated the effect of hub taper angle, pod-strut configuration and pod gap and pod–strut geometry on podded propulsor


performance


in open water [8], static and dynamic azimuthing conditions ([9], [10]). Pengfei Liu has performed a parallel development of numerical prediction of podded propulsor using a panel method ([11]). Investigation of the effect of the ship hull wake on the propeller performance has a long history in ship propulsion technology for the conventional rudder-propeller system [1]. However, it is quite new when it comes to azimuthing propulsors, since the wake experienced by an azimuthing


propeller is A-10 strongly dependent Figure 1: Coordinate system for measurements on the thrusters test set-up. on the


Descriptions of the measurement system for the propeller forces are found in ([2], [13]). The thruster configuration and local coordinate system for the local forces and moments on the propeller are presented in Figure 1. The main dimensions and form parameters are listed in Table 1 for the ship hull and in Table 2 for the propeller. The


local force


azimuth angle. In the current study, propeller shaft loads in calm water in the presence of a ship hull have been measured at the same conditions as without the hull. The measurements without the hull are presented in detail in [2], with only comparisons shown here. By such a comparative study, one can get more insight into the ship hull wake’s influence on the propeller performance and shaft side force and moment components.


the pod unit is required to provide


In addition to oblique inflow, wave loading has been cited as one of the primary reasons for damage to propulsion thrusters [12]. It is also believed that wave loading is a contributing factor in fatigue damage to the aft tail shaft bearing in conventionally shafted propellers. These are found frequently and are very costly damages. For that reason, this study investigated the influence of waves and ship motion on the propeller performance and shaft bending loads. Some tests have been performed in waves to find the effect of waves and ship motion on propeller performance and the shaft side forces and bending moments. Ship motion actually causes another velocity component into the propeller disc in addition to the wave particle and hull wake velocities are superimposed on the ship motion effects and might change the propeller hydrodynamic loads significantly.


2.


MODEL-HULL THRUSTER CONFIGURATION


A 5.4 meter long model of an oil spill vessel was fitted with two azimuth thrusters as the main propulsion device. The twin tractor azimuth thruster assembly was fitted to be


aligned with of the flow at the heaviest


displacement. The port thruster was equipped with instrumentation to measure forces and moments in six degrees


freedom on the propeller shaft. On the


starboard thruster, only propeller thrust and torque was measured. It is the measurements on the port thruster that is presented in this paper


©2011: The Royal Institution of Naval Architects


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