Manoeuvring and seakeeping aspects of pod-driven ships

An existing coupled non-linear six-degree-of-freedom model, which combines manoeuvring and seakeeping, is being enhanced for the simulation of motions of azimuthing pod-driven vessels. The equations of motions and modified numerical model for calculation of pod-induced propulsive and hull forces are presented. The modified numerical code has been verified using an extensive amount of experimental data for both conventional and pod-driven roll-on roll-off passenger ship/ferry (ROPAX) hull forms. Comparisons have been made between conventional and podded control using zigzag and pull-out manoeuvring tests and significant motion amplitudes in waves, with the aim of investigating the directional stability and course-keeping ability of pod-driven ships, as well as the effect of large pod-induced heel angles to the turning and ship motions in waves. The results showed satisfactory agreement with experiments for the enhanced model. In the light of this investigation, the importance of hydrodynamic optimization for the azimuthing pod-driven ship design to eliminate any stability and control problems caused by design modifications has been demonstrated by the use of numerical simulations. Finally the efficiency of the azimuthing podded drives, in terms of overall controllability and seakeeping characteristics of ships, is discussed and conclusions are drawn.