ABSTRACT

Turning tests in waves are simulated by the coupling between slender-ship theory and modular maneuvering model in the framework of two timescale method. At each time step, the second-order horizontal steady forces and yaw moment are estimated by the far-field method utilizing the Kochin function as a function of incident-wave frequency and amplitude, relative heading angle between incident wave and ship, and forward speed, and their magnitudes are added as the wave effect in the maneuvering motion equations. Accordingly, the maneuvering-motion simulation provides the instantaneous forward speed of a ship and the heading angle to the seakeeping analysis. In order to keep the accuracy of the simulation model, calm water hull derivatives are taken from the captive tests. A comparison between simulations and measurements indicates the practical reliability of the proposed model. Improvement in the estimation of wave-induced steady forces and yaw moment is crucial, particularly in short waves, while the model provides better estimation in longer waves. Sensitivity study in short waves also implies that the steady sway force is the largest contributor to the phenomenon that the ship drifts to the direction of incident waves.

INTRODUCTION

As required by IMO (International Maritime Organization), maneuverability of a full scale ship is to be evaluated in a calm weather condition through the so-called sea trial. One of the tests is to measure the steady turning circle with the maximum design rudder angle until completing at least two turning circles. Then, the maneuverability is judged by the tactical diameter, advance, transfer, and other information. However, oceangoing ships are expected to sail the real seas, with ocean current, wind, and wave. Compared to the ocean current, the wind and wave are known to be more complex in nature, especially the latter one. In certain conditions, the steady forces, which are of second order in the wave amplitude, can be significantly large to drift the ship from its course, or at least to impair the course stability. Therefore, it is unsafe to judge the ship maneuverability in waves based only on the trials performed in calm sea.

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