Thrusters modelling for escort Tug capability predictions

The growing attention in reducing the risk of collisions or groundings during the approach to a harbour or while navigating in narrow fairways, makes the escort activities of paramount importance for a safe navigation in different scenarios. To this end, since early-design stage, Classification Societies prescribe to evaluate the maximum steering and braking force the tug is capable to provide. The analysis is usually performed adopting simplified methods, grouping all propulsive devices together to solve the equilibrium on horizontal plane. In this work, an advanced equilibrium resolution method is proposed considering each propulsive device as an independent actuator. This method allows to simulate the behaviour of a steerable thruster, by means of a 4-quadrant modelling of a ducted propeller, considering cross-flow effect as well. The adoption of such enhanced method makes the estimate of escort performances more reliable. To this aim, the performance prediction obtained with simplified and advanced method are here compared for a reference escort tug.