Mathematical Model for the Manoeuvring Ship Motion in Shallow Water (3rd Report) : Manoeuvrability of a Twin-propeller Twin rudder ship

Abstract

Turning and course keeping qualities significantly change according to the water depth. In the previous reports, the author presented the predicting techniques of the manoeuvring ship motion both in deep and shallow water for a single-propeller single-rudder ship. Meanwhile, the investigation into the manoeuvrability and its predicting method for a twin-propeller twin-rudder ship are also desired, because such kind of ships are widely adopted to wide-beam or blant ships as well as high-speed ships. However, it may be said that the inadequate experimantal or theoretical researches can be seen for these ships. In shallow water, the turning qua1liy of some wide-beam and twin-propeller twin-rudder ship are unexpectedly improved, though the conventional ships usually become the worse in the shallower water depth. In this report, the manoeuvrability and its predicting techniques for a twin-propeller twin-rudder ship are investigated by means of conducting the captive model tests and digital simulations with different two twin-propeller twin-rudder ships. Through the present study, the following conclusions are obtained. 1) The presented methematical model based on MMG's model is available for the manoeuvring prediction of a twin-propeller twin-rudder ship both in deep and shallow water. 2) The mathematical model and the measured hydrodynamic data show that the mechanism of shallow water effects on rudder force is quite different from those on hull damping force. The rudder force depends upon the hull resistance and propulsive performance, while the hull damping force upon the effective aspect ratio of hull. 3) As the turning quality is produced by the balance of yaw moments which are induced by two forces as mentioned above, the stronger hull damping force makes the worse turning quality in shallow water, and the stronger rudder force the better one like the presented wide-beam ship. 4) One of the important factors to improve the turning quality in shallow water is the reduction of propulsive effeciency. For this purpose, additional ship stopping devives such as spoiler, fin or water parashot may be usefull.