Abstract
This paper deals with critical wind velocity of a RoRo passenger ship equipped with side thrusters. Two different methods were developed to identify critical wind velocity. One is based on static equilibrium of forces and moment acting on a ship. Wind forces and moment were calculated by Isherwood empirical method. The other is numerical maneuvering simulation of the ship in a port. In the existing studies, critical wind velocity was usually expressed by a graph called a control boundary that explains the relationship between wind-to-ship speed ratio and relative wind angle. Also other studies indicated that the critical wind velocity is approximately proportional to the square root of the rudder angle and of the ratio of effective rudder area to above-water lateral area. Some features are common to these studies. They usually considered ship’s speed or effective rudder area to calculate critical wind velocity and they dealt with equilibrium equations of steady motion of a ship. However modern ships, such as RoRo passenger ships, became to be equipped with side thrusters. Also it is common that their motion is very unsteady or their speed is nearly zero in port area. Therefore there are some limitations in calculating their critical wind velocity using only the existing methods when they are operated in a port. New method taking the effect of side thrusters into consideration should be developed. In this paper, wind forces and moment were calculated in slow ship speed or almost zero speed and they are compared with forces and moment acting on a ship caused by side thrusters or rudder angle to identify critical wind velocity. Furthermore numerical maneuvering simulation in a port was also developed to confirm the calculation results, because the actual motion of a ship in a port is unsteady. This paper may offer a more efficient way to estimate critical wind velocity of a ship equipped with side thrusters and can also be applied to a basis for investigating the cause of aground accident in a port due to string wind.
