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Twin-propeller ships are widely used for from high-speed coastguard ships to low-speed wide beam cargo ships. One of the authors firstly proposed the simulation method for twin propeller and twin rudder ships by means of applying MMG’s mathematical model. After then, some experimental researches~ have been done. However, the manoeuvring prediction for these ships is not easy because propeller and rudder forces are different between port and starboard sides, which introduce the complicated yaw moment. Moreover, its mechanism is very complicated and the hydrodynamic data are also limited comparing with conventional single propeller and single rudder ships. In this paper, the authors have investigated again with the experimental data and propose the equivalent single propeller and single rudder model for twin propeller and twin rudder ships. However, the characteristic of thrust coefficient as well as propeller load factor should not be changed by the use of above equivalent model since they strongly affect on the characteristic of rudder force. In order to realize this, the propeller diameter is assumed to be 2 times larger, propeller rps 2 1 times larger, and rudder area 2 times larger, respectively. This assumption can be also applied to the twin propeller and single rudder ship. This procedure makes very simple and easier prediction since the usual mathematical model as well as the hydrodynamic database for single propeller and single rudder ships can be used. In order to validate the above method, simulated results are compared with those of the original mathematical model as well as the free-running model tests. As the result, it is found that this simple mathematical model becomes very useful for the simulation of twin-propeller ships.

Equivalent Simple Mathematical Model for the Manoeuvrability of Twin-propeller Ships under the same propeller-rps