A rational approach to the design of propulsors behind axisymmetric bodies

Abstract

In the context of "Lifting Line Methodology", this thesis presents a rational approach to Marine Screw Propeller design and its applications in combination with a "Stator" device for further performance improvement. The rational nature of the approach is relative to the Classical Lifting Line procedure and this is claimed by more realistic representation of the propeller slipstream tube which contracts in radial direction along the tube at downstream. Therefore, in accordance with the Lifting Line Methodology, the design procedure presented in this thesis involves the representation of the slipstream shape by a trailing vortex system. The deformation of this system is considered by means of the so-called "Free Slipstream Analysis Method" in which the slipstream tube is allowed to deform and to align with the direction of local velocity which is the sum of the inflow velocity and induced velocities due ,to the trailing vortices. This deformation is neglected in the Classical Lifting Lin~ approach. The necessary flow field data or the wake for the design is predicted by using a three-dimensional "Panel Method" for the outer potential flow, whilst a "Thin Shear Layer Method" is used for the inner boundary layer flow. The theoretical procedures in both methods neglect the effect of the free surface and therefore the implemented software for the flow prediction caters only for deeply submerged bodies. However, the overall design software is general and applicable to surface ships with an external feedback on the wake. Since the realistic information on the slipstream shape is one of the key parameter in the design of performance improvement devices, the proposed design methodology has been combined with a stator device behind the propeller and the hydrodynamic performance of the combined system has been analysed. The design analysis involved the torque balancing characteristics of the system and the effects of systematic variations of the key design parameters on the performance of torpedo shape bodies and surface ships at varying loading conditions. The ·overall conclusions from the thesis indicate that a more realistic representation of the slipstream shape presents a higher efficiency in comparison to the regular slipstream shape assumption, in particular for heavily loaded propellers. Moreover, this representation is essential for sound design of the stator devices as it will determine the radius of the stator. From the investigation on the stator it was found that the undesirable effect of the unbalanced propeller torque can be avoided by the stator. The efficiency of the system will increase with the increase in the number of stator blades and the distance between the stator and the propeller over a practical range of the design parameters. It is believed that the procedure and software tool provided in this thesis could provide the designer with capability for more sound propeller and the stator design for, partly, surface ships and for submerged ships in particular torpedos, Autonomous Underwater Vehicles (AUV) and submarines. Although the improvement gained by the present procedure will be accompanied by an increase in computer time, this is not expected to be a major problem considering the enormous power of existing computers. In fact, this has been the major source of encouragement for the recommendation in this thesis to improve the present procedure by using the "Lifting Surface Methodology" as the natural extension of the Lifting Line Methodology.