Abstract
In the last decade a growing interest can be observed in research aimed at the survivability of ships. Ship survivability is associated with larger amplitude motions and combined motions, such as capsizing (heave-roll) and broaching (sway-yaw-roll). Since large amplitude motions are inherently nonlinear, assessment of the seakeeping behaviour by the commonly used linear approach can be problematic. Complex effects, such as the viscosity dominated roll damping, submerged geometry variations and the coupling effects in the horizontal plane are disregarded. This may result in limited accuracy of these methods. The current research project is aimed at quantifying the importance of the nonlinearities in the hydrodynamic reaction forces due to large amplitudes and nonlinear coupling effects at forward speed between simultaneously performed motions by means of forced oscillation tests. A segmented model of the Royal Netherlands Navy M-frigate was mounted to a newly developed 6-DOF forced oscillator in the towing tank of the Ship Hydrodynamics Laboratory at the Delft University of Technology. Each of the seven segments was fitted with a 6-component force transducer. The new oscillator renders the possibility of performing both large amplitude and arbitrary motions in six degrees of freedom. In this paper the results of this research will be presented, analysed, and the possible importance of nonlinear effects quantified. For now the scope will be limited to combined sway-yaw, heave-pitch and heave-roll motions. Significant nonlinearities, mostly related with submerged geometry variations, unsteady lift and viscous forces, will be quantified and qualified.