Qualitative Aspects of Dynamic Behaviours of Trimaran in Astern Seas

For identifying possible danger of a trimaran running in following and quartering waves, numerical simulation in the time domain was systematically conducted. Here a surge-sway-yaw-roll manoeuvring model with a PD autopilot was used. The manoeuvring forces, the roll restoring moment and the wave-exciting forces are calculated for the trimaran with hydrodynamic interaction among three hulls taken into account. As a result, as well as harmonic motions and stable surf-riding, repeated broaching within one wavelength at higher speed and sub-harmonic yaw motions with large amplitude at lower speed are found. This repeated broaching could be chaotic via period doubling. These phenomena do not result in capsizing thanks to large restoring arm due to side-hulls but are threats to safe manoeuvring. They can be prevented by increasing the magnitude of differential gain for the auto pilot.