Non-linear roll motions of a frigate-type trimaran

Two nonlinear roll damping models using a roll decrement and an energy method are considered. The objective is to use experimental hydrodynamics to validate the numerical prediction of nonlinear roll motions in waves. Reasonably good fits are obtained with 1 DOF mathematical model of the roll motion augmented by a viscous roll damping term. Variability of side hull longitudinal stagger, transverse spacing and speed on roll damping is examined. Forced roll experiments are employed to study the effect of roll excitation frequency on roll response and nonlinear roll damping. Screening of parametric roll resonance is carriedout at wave encounter frequencies generating nonlinear coupled roll-pitch motions. A dynamic stability study follows and the unstable regions are identified on the InceStrutt diagram. This investigation reveals that trimaran vessels, which normally benefit from high transverse stability, can in fact encounter seaways that can cause significantly large or resonant roll motions and also wave trapping. The nonlinear roll damping models were in good agreement with the experimental analysis for the frequency domain roll response and time domain parametric roll resonance.