Escort tug design alternatives and a comparison of their hydrodynamic performance. Discussion

Escort tugs can generate forces for steering and braking a disabled tanker which are greater than the bollard pull delivered by the propulsion system. This is achieved by using a hull shape and appendages that can generate very high forces at yaw angles up to 45 degrees, combined with an azimuthing propulsion system to resist the resulting yaw moments generated from the hydrodynamic forces. This mode of operation is known as indirect steering or braking and results in a high degree of interaction between the flow around the hull and the flow due to the propellers. Model experiments are the most promising approach for studying these complex interactions, but there is very little model data published on the subject. Successful escort tug designs have resulted from different combinations of appendages and propulsion systems but there is no published data that compares the predicted performance of different alternatives. This paper attempts to fill this gap by presenting the hydrodynamic performance of three different combinations of hull shape, appendages and propulsion system, and uses the results to discuss the effectiveness of alternative design solutions for escort tugs with specified steering force requirements.