Z-drive escort tug manoeuvrability modelling: From model-scale to full-scale validation

A deep insight into the manoeuvring characterisation of a wider class of Azimuthal Stern Drive Escort tugs (ASD) is undertaken with the scope of defining a real-time parametric manoeuvring simulation code for the free-sailing and towing operations. To this end, given a simple set of tug design parameters (main dimensions, skeg size, azimuthal size, propeller pitch, choice of main engines, etc.), a physics-based 4-DOF mathematical prediction tool for the dynamic behaviour of the ASD tug has been developed in calm seas. To prove the suitability of the modelling, two independent validation processes have been provided in the present paper. The first successfully demonstrates the ability of the mathematical formulations to reproduce model-scale Escort-towing operations carried out on the same parent hull which was tested in towing tank (RM2812). The second validation test envisages simulator’s capability of describing the full scale free-sailing performance of a different but compatible hull, having dimensions, propulsion, skeg characteristics significantly different with respect to the ”parent design” used to principally develop the code. The verification involved the measurements purposely performed during sea trials onto two sister-ship ASD new-buildings (RM3213).