This study describes the development of a time domain simulation to assess the dynamic behaviour of a locally designed passenger ferry in damaged and beam sea condition. The ship is a high-speed passenger ferry that has a peculiar design of high L/B ratio of more than 10. This means that it can be categorised as a slender body ship. The slender body of the ship with long compartments is very sensitive to damage stability and rolling in beam waves. The mathematical modelling for motions in beam waves has taken into consideration the progressive flooding as well as effect of water accumulation. Damage stability experiment has been done to validate the simulation results. The experiment was conducted using image processing technique. Experimental results have been found to be quite close to simulation results. Parametric investigation is carried out to identify the effect of a number of key parameters on damage stability assessment. These include the wave height and loading condition of the ship. The results of parametric investigation are presented and discussed.