The Research Of Hydrodynamic Derivatives And Maneuverability Forecast On Trimaran

As a new kind of high performance ship, trimaran has many advantages in stability,seakeeping performance and etc. And it draws many researchers attention. There are manyreferences about resistance and seakeeping of trimaran, but much less about maneuverability.As one of the most important performance, maneuverability is closely linked to the economyand the safety of ship. Half of the marine accidents are due to the maneuverability accordingto the statistics which not only could cause great losses of life and property but also dodamage to the environment. Therefore, IMO (International Marine Organization) and manycountries are formulating criteria of maneuverability. So the requirements of maneuverabilitywill become clearer and stricter. The prediction of maneuverability has to be more and moreaccurate than before during the construction and design of ships. With the wide application oftrimaran in military and civil area, the maneuverability requires to be investigated urgently.One of the principle methods of predicting ship maneuverability is the equations of shipmaneuvering motion method. The hydrodynamic derivatives in the equations of shipmaneuvering motion have to be determined in this method. Currently, the captive model testis recognized as the most reliable method to determine the hydrodynamic derivatives. PlanarMotion Mechanism (PMM) is an effective installation with comprehensive function in thecaptive model test. It can measure various velocity derivatives, angular velocity derivatives,acceleration derivatives and the coupling derivatives. But the model test has it owndeficiencies, such as high costs, long test period, inconvenient to design optimizingmaneuverability by changing hull form. Along with the rapid development of computerscience and technology, Computational Fluid Dynamics (CFD) technology has made itpossible to solve the problems of ship maneuverability. When using the CFD technology tosimulate the flow field of captive model test and to determine the hydrodynamic forces on themaneuvering ship model, it can overcome the shortcomings of the ship model test such ashigh costs and long test period, and it is also convenient to design optimizing maneuverabilityby correcting hull form and maneuvering devices. And when using the CFD technology tosimulate the flow field around the full scale ship, the scale effect can be avoided.In this paper, CFD method is applied to simulate towing tests of a wigley trimaran withvariable rudder angle. The viscous flow around the hull in pure sway and pure yaw test with PMM is also simulated. Hydrodynamic forces and hydrodynamic derivatives are calculated.The free surface effect is considered during the calculation. But the changes of ship floatingcondition are not taken into account. It is found that the lateral force on the hull and yawmoments present a simple harmonic variance with time. The amplitude of force and momenthas a linear relationship with the frequency of oscillation which indirectly verify that themethod of numerical simulating is appropriate for calculating hydrodynamic derivatives oftrimaran. It is also investigated that the influence of the location of the outrigger hulls on thehydrodynamic force. The center of the gravity of trimaran is predicted by first order responseKT equation and the hydrodynamic derivatives which is calculated. It is proved that thetransverse location of the outrigger hulls has a great influence on the turning performance ofthe trimaran. The turning performance is better if the distance between the hulls is smaller.The longitudinal location has little influence on the turning performance of the trimaran. Theturning performance will be better when the outrigger hulls are around the midship.