Study of ship turning in irregular waves

This paper presents the turning test results of ships having a rudder angle of +/- 35. in short-crested irregular waves using ship models of a tanker (KVLCC2) and a container ship (KCS). The tests were performed in head waves at the time of approaching with the significant wave height 4.5 m for KVLCC2 and 3.0 m for KCS at full-scale. Turning indices such as the advance A D and tactical diameter D T, and drifting parameters during turning such as the drifting distance H D and drifting direction mu(D) are used to characterize the ship turning in waves. With a decrease in the approach speed of the ships sailing in the same wave condition, A D decreases but D T does not change significantly. With a decrease in the approach speed, both H D and mu(D) increase significantly, and the tendency of the ship drifting to the rudder execution point in space becomes remarkable. Although a variation in the turning trajectories which may be introduced due to the slowly varying second-order wave forces acting on the ship models is observed, its influence on turning is negligible in view of practical purposes. In addition, theoretical formulas for H D and mu(D) were derived on the assumptions of small rudder angle, small maneuvering motions and small wave-induced steady forces. The calculation results using these formulas roughly agree with the turning test results. The formulas newly derived are useful for a better understanding of the wave-induced drift motion of ships during turning.