Shiphandling tugs are a critical aspect of ship manoeuvring in confined waters.udKnowledge of their performance and effective use in various situations provideudnecessary guidance for the marine pilot and tug master to improve shiphandlingudtechniques. Shiphandling simulators are a well accepted tool for training of marineudpilots, development of port operational procedures and port development studies.udRealistic simulation of shiphandling manoeuvres requires not only a reliableudhydrodynamic model of the ship, but also precise representation of the forcesudavailable from the assisting tugs.udIn the past, the limited capabilities of tugs meant that force prediction and evaluationudof operating procedures was a relatively simple matter. However, the push forudexpanding and more efficient port operations has led to a new generation ofudshiphandling tugs being introduced into major Australian ports during the 1980's.udThese tugs are of greater displacement, power and manoeuvrability compared withudtheir twin screw predecessors and represent a significant technological step.udIncreased manoeuvrability of the so called omni-directional stern drive tug has beenudachieved through the fitting of azimuthing thrusters, which are capable of producingudthrust in any direction. The enhanced manoeuvrability of the omni-directional sternuddrive tug and increased power has enabled significant increases to be made in theudefficiency of shiphandling operations, particularly in the ability to assist at speed.udHowever, to fully realise the potential of these complex vessels a more thoroughudunderstanding of their hydrodynamic aspects is required. Further, with increasedudspeed greater consideration needs to be given to the interaction of tug forces withudthose produced by the ship's rudder, propeller and hull.udTo investigate the performance and effectiveness of omni-directional stern drive tugs,uda series of physical model tests has been performed. Hydrodynamic forces acting onudthe hull and thrusters have been measured, with particular attention being paid toudhydrodynamic interactions between thrusters and the thrusters and hull.udMeasurements were initially made of forces acting on a bare tug hull and a singleudthruster in open water. Thruster-thruster interaction was determined fromudmeasurement of forces acting on twin thrusters in open water and thruster-hulludinteraction was determined from measurement of total forces and those acting on theudthrusters of a self-propelled free-running model.udData derived from the model tests has been used to construct a mathematical modeludwhich may be solved using equilibrium or, in the time domain, as may be desired forudreal time simulation. Quasi-steady predictions of tug forces have been used toudidentify general performance, optimum operating techniques and the influence of tugudforces on ship motions in confined waters. In cooperation with Australian towageudoperators, practical findings from this investigation have been detailed in a manual toudassist tug masters and marine pilots.udIn addition to harbour tugs, this work may also provide insight into the manoeuvringudof escort tugs and other vessels with omni-directional propulsion.
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