Cycloidal rudder breaks new ground

After nearly 75 years of producing and marketing its highly successful cycloidal propulsion system, Voith Schneider in Germany has extended its range by developing the Voith Cycloidal Rudder (VCR). This is basically a modified propeller system based on the company?s vertical axis cycloidal propeller but with only two blades instead of the normal four to six. It is used in conjunction with standard propellers.

As with the Voith-Schneider Propeller (VSP), the VCR has a rotor casing with a vertical axis rotation and the two rudder blades lie parallel to the axis of the rotor casing. Detailed model scale tests and evaluations were completed last year and, in September, the first full scale tests were carried out in the North Sea onboard a former buoy tender, the 40m Aurora. Voith says these trials confirmed the results of the initial findings with the vessel performing well during all manoeuvres.

The new propulsion and manoeuvring system is targeted at ships that have to operate with maximum manoeuvrability over the entire speed range. It has two operating modes, a passive one in which the VCR act as a conventional rudder and an active one in which it functions in the same way as a VSP to ensure a high degree of manoeuvrability at slow speeds.

Active mode of

operation

In the active mode of

operation

, the controllable 360 deg. thrust is generated just as with a conventional VSP. The uniform rotation of the rotor casing is superimposed by an oscillating movement of both blades around their own axis. The blades? oscillating movement determines the magnitude of thrust while the angle of attack between 0 deg. and 360 deg. controls the steering direction. An identical thrust can therefore be generated in all directions.

This mode of

operation

is suited for low-speed

operation

of vessels, such as when berthing, where transverse forces of the conventional rudder may be insufficient because of reduced inflow speed. In emergency situations, ie breakdown of main propulsion, the VCR offers a secondary propulsion mode and gives a get-you-home capability.

Passive mode

In passive mode the rotor casing only performs partial rotations in both directions. Thus the locked rudder blades are adjusted relative to the inflow and generate transverse forces for steering. It is therefore identical to a conventional ship?s rudder and is used when the vessel is operating at cruising speed. Conventional rudders are however designed to produce sufficient lateral forces with small inflow forces and, at high speeds, the rudder area is oversized and produces additional drag resistance. The passive mode for VCR however is only used for high-speed

operation

and therefore the rudder area is much smaller, minimising drag losses.

Flexibility is the key

The dual mode of

operation

of the Voith Cycloidal Rudder provides a number of important properties and application possibilities that are useful for many maritime situations in both the commercial and naval sectors and contributes to increased ship safety.

Some of the applications include high speed container ships which have to operate at reduced speed in confined sea spaces such as estuaries or the Suez and Panama canals. Others are cruise ships that can be navigated into port with the minimum of tug assistance or a ferry that has to achieve high speed transit passages but requires maximum manoeuvrability when docking. Turning on the spot is possible and there is no need for side thrusters to be fitted.

One other advantage claimed is that it stabilises vessels. This aspect is not widely known since, unlike fin stabilisers, VCR can dampen roll movements without the vessel having to be travelling ahead at speed. The thrust direction can be varied quickly to counteract any rolling motion.