Quasi-steady determination of dynamic forces acting on tug during escort operations

The general increase in size of merchant ships requires the use of tugs with higher performances in terms of versatility, efficiency, towing capability and powering. Escort operations are carried out by special tugs through a towline connected aft of the assisted ship in order to control course and speed in potentially dangerous areas, so reducing the risk of grounding and collision. Steering and braking forces are developed by the escort tug which works as a large “remote rudder”. The “escort tug” service notation is assigned by Classification Society through full-scale tests or by computer model simulations able to evaluate the maximum steering force developed 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, and also the compliance with specific intact stability criteria. A case study for a tractor-tug is presented.