DNV DNVGL-CG-0155 PDF

General

Escorting is different from conventional towing in many ways. Conventional towing takes place in smaller harbours at low speeds (2-3 knots). The tug provides a towing force to the assisted vessel that is mainly a function of the tug’s thruster capacity in terms of pure power. Escorting, on the other hand, takes place at higher speeds (6-10 knots). The towing force is not explicitly dependent on the thruster capacity, but more a function of the hydrodynamic flow field around the tug’s hull. See Figure 1.

As the hydrodynamic forces acting on the tug's hull increase approximately with the square of the speed, the steering ability increases more than proportionally with the speed. Escort service is therefore normally undertaken in the speed range of 8 to 10 knots.

The notation Tug(Escort (FS,t,v)) means that the escort tug is capable of providing a continuous transverse steering force of FS tonnes to the towing wire used to control the assisted vessel while proceeding at a forward speed of v knots. Furthermore, it can manoeuvre from a specific operational position in the aft of the assisted vessel to a corresponding position on the other side in t seconds. Several force-speed combinations are possible in the class notation. The force and time measurements have to be performed following the hereby prescribed test procedure, and the vessel shall show outstanding dynamic stability to meet the requirements of the class notation.