Voith Schneider Tugs
One of the most radical propulsion systems was developed by Austrian engineer Ernst Schneider in the 1930's.
The Voith Schneider Propeller consists of a very substantial rotor casing fitted flush to the the underside of the tugs hull, in a forward position below the wheelhouse. The propeller blades are flat straight fins mounted vertically to form a rotating cylinder, the entire cylinder rotates, with each blade also rotating on its own axis to provide variable pitch. Thrust is increased when the "angle of attack" (pitch) of each blade is increased. Complex internal fulcrums allows the resulting thrust to be vectored in any direction. See our interactive demonstration below.
Since the rotor casing itself remains stationary, in order to change direction, the tug merely changes the pitch of the appropriate blades. This is pre-set by the internal fulcrums and wheelhouse control, consisting of simple pitch levers and a steering wheel for thrust direction.
See our interactive demonstration below -
The skeg fitted at the opposing end of the vessel from the thrust gives excellent steering capabilities. (i.e. the principle is similar to driving a car in reverse). The centre of skeg acts as centre of pressure (COP) around which the tug rotates.
The skeg and the rotors act as braking appendages when in "Indirect Towing Mode", thus the Voith configuration is an excellent tug for escort work.
All of these innovations give a Voith Schneider tug very high manouverability with very fast response times to control inputs, making them ideal for harbour work, particularly in enclosed docks.
On the negative side, at higher speeds (above 8 kts), much of the thrust generated is needed by the tug itself to maintain position (in shiphandling). Damaged units can be costly to repair and because the units are mounted beneath the hull, the draft is significantly deeper than other propulsion types.
Steaming distances for escort work can be expensive as a Voith Tractor has a high Fuel consumption:speed ratio.
In confined areas and static water conditions the Voith is yet to be surpassed by other towage propulsion systems.
See below for a computer graphics video of a Voith Schneider harbour tug in action.