Course Stability of a Ship Towing System

This paper proposes a numerical model for course stability of a towed ship coupled to tow ship through towline using both nonlinear and linear approaches. Captive model tests were conducted at the towing tank to obtain hydrodynamic derivatives of both tow and towed ships. Effects of unstable towed ship (2B) and stable towed ship (2Bs), towline, tow points and rudder autopilot parameters at tow ship were treated in the numerical analysis. The simulation results demonstrated better course stability of 2Bs compared to 2B. A longer towline and shifting the tow point towards the bow both for 2B and 2Bs made the towing system more course stable, which is indicated through attenuation in slewing motion. Lateral motion of the tow ship increased gradually with shifting the tow point forward of the ship's centre of gravity. This degrades the course stability of the towing system. The rudder action enforced by the autopilot improved the course stability. In general, the increase of the tow ship's dimension was still insufficient to improve the course stability of the ship towing system.