For describing the manoeuvring motion of a ship, the so-called M. M. G. mathematical model is now established. The M. M. G. model is a modular type hydrodynamic model which is proven to be useful in various aspect of manoeuvring motion. However, this model is originally constructed for the single-propeller single-rudder ship. Now, a lot of multi-propeller multi-rudder ships are in use, but few things are known about the manoeuvring performance of the multi-propeller multi-rudder ship. In this report, a twin-propeller twin-rudder ship is chosen as one of the most popular multi-propeller multi-rudder ships, and the authors try to clarify the manoeuvring performance of that kind of ship. Firstly, the various parameters included in the mathematical model for manoeuvring motion are investigated experimentally for a twin-propeller twin-rudder ship. As a result, it is found that generally speaking the M. M. G. model, which was originally constructed for a single-propeller single-rudder ship, can be applied to a twin-propeller twin-rudder ship except for the mathematical expression of the propeller wake. Then, the authors try to modify the mathematical model of the propeller wake so that the amended model may describe well the characteristics of the propeller wake of the twin-propeller twin-rudder ship. Finally, the predicted manoeuvring performance of the twin-propeller twin-rudder ship is compared with the results of free-sailing model tests in order to examine the validity of the newly-proposed mathematical model.