Performance and effectiveness of omni-directional stern drive tugs

Shiphandling tugs are a critical aspect of ship manoeuvring in confined waters. Knowledge of their performance and effective use in various situations provide necessary guidance for the marine pilot and tug master to improve shiphandling techniques. Shiphandling simulators are a well accepted tool for training of marine pilots, development of port operational procedures and port development studies. Realistic simulation of shiphandling manoeuvres requires not only a reliable hydrodynamic model of the ship, but also precise representation of the forces available from the assisting tugs. In the past, the limited capabilities of tugs meant that force prediction and evaluation of operating procedures was a relatively simple matter. However, the push for expanding and more efficient port operations has led to a new generation of shiphandling tugs being introduced into major Australian ports during the 1980's. These tugs are of greater displacement, power and manoeuvrability compared with their twin screw predecessors and represent a significant technological step. Increased manoeuvrability of the so called omni-directional stern drive tug has been achieved through the fitting of azimuthing thrusters, which are capable of producing thrust in any direction. The enhanced manoeuvrability of the omni-directional stern drive tug and increased power has enabled significant increases to be made in the efficiency of shiphandling operations, particularly in the ability to assist at speed. However, to fully realise the potential of these complex vessels a more thorough understanding of their hydrodynamic aspects is required. Further, with increased speed greater consideration needs to be given to the interaction of tug forces with those produced by the ship's rudder, propeller and hull. To investigate the performance and effectiveness of omni-directional stern drive tugs, a series of physical model tests has been performed. Hydrodynamic forces acting on the hull and thrusters have been measured, with particular attention being paid to hydrodynamic interactions between thrusters and the thrusters and hull. Measurements were initially made of forces acting on a bare tug hull and a single thruster in open water. Thruster-thruster interaction was determined from measurement of forces acting on twin thrusters in open water and thruster-hull interaction was determined from measurement of total forces and those acting on the thrusters of a self-propelled free-running model. Data derived from the model tests has been used to construct a mathematical model which may be solved using equilibrium or, in the time domain, as may be desired for real time simulation. Quasi-steady predictions of tug forces have been used to identify general performance, optimum operating techniques and the influence of tug forces on ship motions in confined waters. In cooperation with Australian towage operators, practical findings from this investigation have been detailed in a manual to assist tug masters and marine pilots. In addition to harbour tugs, this work may also provide insight into the manoeuvring of escort tugs and other vessels with omni-directional propulsion.