On the Calculation of the Forces Developed by escort Tugs

Authorities and harbour operators demand tugs to assist ever-larger ship in more severe weather conditions in confined waters. The escort of such ships is carried out by specially designed tugs connected to the assisted ship by a towline in order to control course and speed of the assisted ship when comes to sail in potentially dangerous situations, and so the risk of grounding and collision is strongly reduced. In order to achieve the "escort tug" service notation it is necessary to predict the maximum steering forces applied on the assisted ship at different speeds. The paper deals with a method for the prediction of the escort capabilities of a tug through the quasi-steady solution of the dynamic equilibrium.