Evaluation Method of Course Stability by means of Slewing Motion in a Towed Condition

An azimuthing podded drive system is established as one of the most promising technologies for the improvement of propulsive efficiency and maneuverability. The system is also useful for fishing vessels to control the power of generators during the draw in fishing gears for a long time in low speed. Although，the drawback to a fishing vessel with this system is that it will be a worry to the course stability in the voyage.A large number of studies have been made on maneuverability of conventional ships. What seems to be lacking，however，is the course stability of the fishing vessel with the azimuthing podded drive system.In this paper，a new evaluation method of course stability is proposed on the basis of experimental study that is conducted in a circulating water channel on survey of slewing motion in at owed condition. As a towing point moves to forward in a constant speed, the amplitude of slewing motion (a) decreases and its period (T) is getting longer. Accordingly，the index of evaluation is represented by the point of towing from the center of gravity of ship and the value of the amplitude divided by the period (2πa/T). The results are checked that compared with the course stability criterions by means of derivatives of hydrodynamic forces and moments. By using the present method， newly fishing vessels can be designed to achieve the improvement of propulsive efficiency and maneuverability