Concept and operation mode of the advanced electronic control system of the azimuth propellers in tugs

Abstract

Although the azimuth thruster concept is an innovation dating back more than fifty years ago, nowadays it still remains a novelty to many ship owners and operators. The basic idea behind an azimuth thruster is that the propeller can rotate 360 degrees round a vertical axis providing omni-directional controlled thrust. This means superior manoeuvrability for vessels equipped with azimuth thrusters. It also eliminates the need for a rudder and a reverse gear (i.e. the azimuth thruster itself also functions as a reduction gear ). Unlike the cicloidal propellers where Voith Turbo Marine is the only manufacturer -from that the generic name of Voith propellers-, the azimuth thrusters are manufactured by different firms and have been referred to under different names e.g. Z-drives, rudder propellers, rotable propulsion units, omnidirectional thrusters, although the brand name of a manufacturer like Shottel, Ulstein Rolls-Royce, Steerprop, Niigata, Duckpeller etc. is often used as a generic label for all azimuth thrusters. The system consists of several different devices: azimuth thrusters, steering and control unit levers and shaft lines. As a whole, they might well be more accurately referred to as a "propulsion and steering system". In Spain, the tugs with this type of propellers are the tractor Z (its propellers are mounted in the forepart of the tug´s hull, the same as the tractor Voith) and the Azimuth Stern Drive (ASD) whose propellers are located aft, the same as conventional tugs. The remote control of these propellers from the bridge (speed and thrust direction) is electronic and each manufacturer has his own characteristic model, although the control system and the arrangement of the propellers to achieve the expected thrust are very similar. In this article, the remote electronic control devices in the bridge from the tug azimuth propellers are analysed in general, especially their performance and operation mode, without making any distinction between those placed forward (tractor Z) or aft (ASD), since their basis is similar and the only difference is the horizontal propeller position and consequently the position of the remote controls from the bridge in order to achieve the expected thrust direction.