An Auxiliary Thruster for Free-Running Model Ship Test

Abstract

The free-running model ship test is an important measure to investigate into ship performance and response in various situations. Its basis is the Froude’s similarity law. Since Reynolds number remains quite different between model ships and full-scale ones, phenomena concerning viscosity cannot be similar. One of the most specific features related to the viscosity is the friction resistance and thus propeller load that differs to a large extent between model ships and full-scale ones. This difference affects torque and thrust responses in waves and wind, and also rudder effectiveness or manoeuvrability. The authors have developed a prototype of auxiliary thruster that assists free-running model ships’ propeller. The auxiliary thruster can control its forward force and adjusts the model ship propeller load to arbitrarily time varying target values. The prototype consists of a duct fan, a load cell mounting the duct fan on it, an amplifier for the duct fan, and a PC. The PC controls the auxiliary thrust, or forward force generated by the duct fan, using the model ship speed and the force data measured by the load cell. This report presents the prototype of auxiliary thruster and its trial test applied to a free-running model ship to study the effect of propeller load on manoeuvrability. The trial test clarified how the auxiliary thrust or the propeller load affects the model ship responses to steering. Theoretical calculations simulating the effect of propeller load corresponding to the trial test confirmed these effects. Applicability of the auxiliary thruster to other free-running model tests is also discussed.