Simple Simulation Model for Manoeuvring Ship Motion of Twin-propeller and Single-Rudder Ships

Twin-propeller and single-rudder ships are frequently used for medium high-speed ships such as container ships and passenger ferries. However, the maneuvering simulation for these ships is not easy because the interaction forces between propellers and rudder become very much complicated. In this paper, the authors have investigated a simple analysis and simulation technique for maneuvering ship motion of twin propeller and rudder ships. For this purpose, captive model tests have been carried out, where the propeller thrusts and rudder normal force as well as ship hull forces are obtained for various parameters including propeller rps and rudder angle. Although the propeller thrusts are slightly different in each other for port and starboard sides and this makes the additional yaw moment, the thrust is assumed to be a single propeller’s one by means of combining the both sided thrusts as if the ship is a single propeller and rudder ship. In this analysis, the propeller diameter is assumed to be √2 times larger and propeller rps 1/√2 times, so that the thrust coefficient KT becomes the average thrust coefficient of twin propellers. This assumption can make very simple and easier since the conventional mathematical model for single propeller and rudder ships can be applied. In order to validate the above method, free-running model tests have been carried out and compared with the simulated results. As the result, it is found that this simple technique becomes very useful for the simulation of twin-propeller and single-rudder ships.