| Podded propulsor is a new type of ship propulsion device developed in recent years, which offers improved hydrodynamic performance, saved space, increased payload capability and improved battle effectiveness. Therefore, it could make the advantage of electric power propulsion bring into play enough and has wide market application prospect and great military application value. In this dissertation, systemically researches were carried out on the hydrodynamic performances of podded propulsors with the use of numeric calculations and model experiments, which mainly included a numeric computation method research, numeric calculations and model experiments for steady and unsteady hydrodynamic performances and the propulsion prognosis for a vessel driven by podded propulsors.Based on potential flow theory, a numerical method was proposed for predicting the steady and unsteady hydrodynamic performances of puller type, pusher type and twin screw type podded propulsors. In this method, the hydrodynamic forces on propeller blades were calculated with a vortex lattice method, while the hydrodynamic forces on the pod and strut were calculated with the Hess-Smith method for non-lifting bodies. Hydrodynamic interactions between the propeller between the pod and strut were treated via iterative calculations. Based on this method, a computation program was designed, written and tested, and then was used to investigate the hydrodynamic characteristics of podded propulsors.The open water performance of a propeller without pod was calculated at first to verify the accuracy of the present method. The numerical results agreed very well with the experimental data. Then the steady performances of the three types of propulsors were calculated and compared with the experimental results obtained from tests carried out in a cavitation tunnel. The distribution of induced velocity at the propeller disk caused by pod and the loading distribution on propeller blades was numerically discussed. The results show that the present method can finely predict the performance of puller type and pusher type podded propulsors, but noticeable discrepancy is found for twin screw type podded propulsors. Pod blockage effection could bring on additional hydrodynamic loads on the propeller of a puller or pusher type podded propulsor, this effection is more obvious for a puller type one. Furthermore, a strong wake area exists in front or rear of the pod and struts.For improving the precision of the numeric calculation method for the hydrodynamic performance of a podded propulsor, a new wake model of its propeller was developed. To investigate the influences of several key parameters of the wake model on the predicted propeller performance, a great deal of numerical computation was carried out by changing the value of these parameters. With the wake geometry calculated by a simplified wake alignment method, the interrelation of the hydrodynamic pitch with the geometrical pitch and the inflow pitch was analyzed so that a model for the interrelation was derived, and then the coefficients in the model were determined. In this way, a new trailing vortex wake model for podded propulsor was developed. Real case calculation was carried out to verify the validity and applicability of this new wake model, in which two different podded propellers were used. Then comparison with the test data and computation results by using the old wake model was also preceded. It is shown that the precision of this new wake model is well than that of the old wake model, while applicability of this new wake model is also good.The influence of the propeller loads, nominal wake, effective wake induced by the pod and strut, and their components on the unsteady hydrodynamic performances of podded propulsors was investigated systemically by using the numeric calculation method developed in this dissertation. The results show that for the effective wake induced by the pod and strut, the axial and tangential components are mainly induced by the strut, while the radial component is mainly induced by the gondola, and the wake components are not eudipleural due to the influence of the rotating flow behind the propeller. The axial and circumferential wake components will weaken when the loads on the propeller decrease, whereas the radial wake component will vary little. Thrust and torque on the blade will increase obviously while it rotates through the strong wake field areas. The unsteady forces and moments on the propeller consist of blade frequency components mainly. It is feasible to neglect the effects of the radial and circumferential components of the wake induced by the pod when the steady hydrodynamic performance of a podded propulsor is investigated, and to neglect the effect of the radial one for unsteady performance. The nominal pod wake can be obtained easily, but some errors will appear when the design or performance prediction of a podded propulsor is dealed with based on this. The fluctuation amplitudes of the unsteady hydrodynamic forces and moments depend upon the ratio between the hull wake and pod wake, while the characteristics of the unsteady forces will vary noticeably when the pod wake is neglected.Model experimental study was carried out systemically on the steady hydrodynamic performances of puller, pusher, and twin screw type podded propulsors. The test instrumentation was designed and manufactured according to the design of the experiment scheme, then the test method was established and several model experimental researches were finished. The analysis to the test results show that all the experimental results indicate an increase in propeller efficiency with pod as compared with the case of a propeller without pod. However, it is possible that the maximum efficiency of the whole podded propulsor can be lower than that of a single propeller when the resistance of pod is taken into account. The hydrodynamic efficiency of a podded propulsor correlates with the interaction of the propeller, pod and strut, and this correlation is stronger for a twin screw type podded propulsor. Therefor, it is necessary to consider the geometries of the pod, strut and other components and the distribution of the hydrodynamic loads on the front and rear propellers in the design of a twin screw type podded propulsor.For one vessel driven by two podded propulsors, at first, the ship model resistance test was carried out to obtain the effective power curves, and then a method based on podded propulsor design charts was used to design the podded propulsor of this ship. Adjacent to the open water test for this propulsor, the test data was modified to obtain the hydrodynamic performance characteristic of this propulsor. Finally, the calculations of propulsion prediction and cavitation examination were undertaken based on the data as mention above. Prognosis results show that the performance of the podded propulsor designed in this dissertation fulfils the design requirements. |
