Initial and steady turning characteristics of KCS in regular waves

Abstract

A ship encounters wave loads in actual seas, it is necessary to estimate the ship's manoeuvring performance in waves for its safe operations. In this study, turning characteristics of a KCS model ship in regular waves were investigated by free-running model tests. 35 degrees port and starboard turning circle tests in regular waves were carried out in KRISO Ocean Engineering Basin with variations of regular wave heights, lengths, directions and model approach speeds. Propeller revolution rate was mostly fixed at the constant value corresponding to the full-scale speed of 16 knots in calm water. Ship's turning behaviours in waves can be divided into initial and steady turn phases. Firstly, during initial turns, the ship's safety which is related to avoiding a fixed front obstacle is focused on. 'DMINw' is defined as the minimum distance between the fixed front obstacle and the ship initial trajectory. And safety index 'STw' is proposed, which is formulated by using the ratio between DMINw in calm water and in waves. The ship becomes safer in head waves than calm water due to small advances. In starboard beam waves, when the ship turns to starboard, that becomes dangerous due to small transfers. In port beam waves, the ship which turns to starboard is mostly safe. But, in short port beam waves, yaw angle is slowly increased and the advance is relatively large. It is hypothesized that the wave drift yaw moments prevent the ship from turning to starboard. Secondly, drifting distances and angles in steady turns are quantified. The drifting distances increase with decreasing wave lengths, when the wave lengths are varied from 0.5 to 1.0 ship length. The drifting distances are approximately proportional to the wave steepness. Finally, approximate formula of STw is proposed. That is formulated as the function of ship advance and transfer in calm water and waves only. DMINw can be simply obtained if the initial trajectory is approximated as the ellipsoidal quadrant. Based on several parametric simulations, it was confirmed that STw is proportional to the cube of the transfer in waves and is inversely proportional to the advance in waves. STw is formulated so that it can be applied to arbitrary ships.