Tractor Tug
Propulsion forward, tow point aft: Voith Schneider and forward azimuth variants, the IS Code transverse thrust coefficient, and escort application.
A tractor tug carries its propulsion forward of amidships and tows over the stern, the reverse of the azimuth stern drive arrangement. The propulsors may be Voith Schneider vertical axis units or forward mounted azimuth drives, and both produce the same tow point geometry.
That geometry is recognised numerically in the 2008 IS Code. Part B paragraph 2.8.2.1 sets the transverse thrust coefficient in the self tripping heeling lever at 0.90 divided by one plus the ratio of the longitudinal towing point distance to Load Line length for azimuth units at a single point along the length, floored at 0.7 for a tractor tug towing over the bow and at 0.5 for one towing over the stern. The Code therefore assigns a different exposure to the two ways of working the same hull.
The full article will cover the Voith Water Tractor and forward azimuth variants, the aft skeg and its role in lateral stability, the tow point and winch arrangement, the escort application, and where a tractor is preferred to an azimuth stern drive tug.