Directional Stability and Control of Ships in Waves

Abstract

The stability and oscillatory motions of ships (automatically steered and unsteered) in the horizontal plane were examined on a digital computer for the case of regular following seas. Available hydrodynamic data for Series 60 hull forms were used. Analysis of directional stability was made for the case of zero encounter frequency (i.e., the ship runs at high speeds equal to the wave celerity). The ship (which is hydrodynamically stable without automatic control in calm water) is directionally unstable in following seas except for the small region near the ascending node of the waves. Addition of automatic control can give the ship directional stability when it is located on the wave trough, but not when it is located on the wave crest. At relatively high frequency (i.e., at low speeds in following seas), the rudder and control system are almost incapable of reducing oscillatory motion. Violent rudder activity in following seas can be decreased by reducing the yaw-rate-gain control constant and by increasing the rudder-response-time constant.