Michael Palm and Dr. Dirk Jürgens, VOITH Turbo Marine, Heidenheim, Germany

The Voith-Schneider Propeller is a unique propulsion system allowing the control of thrust in magnitude and direction smoothly, precisely and quickly. Since its invention 75 years ago, all knowledge about the hydrodynamics of the Voith-Schneider Propeller was based mainly on extensive model testing. The introduction of CFD as a standard design tool has opened new horizons in propeller development.

On a Voith-Schneider Propeller the blades project below the ship hull and rotate about a vertical axis, having an oscillatory motion about its own axis superimposed on this uniform motion. The blade's oscillating movement determines the magnitude of thrust through variation of the amplitude, the phase correlation determining the thrust direction between 0 and 360 degrees. Fig. 1 shows a tug boat equipped with two Voith-Schneider Propellers. Fig. 2 shows a close-up of the two propellers and the guard plate, which protects the blades and also reduces the effects of tip vortices.

The flow solver Comet is used at VOITH to determine the hydrodynamic loads in an early design stage. This data forms the basis for all the following structural dimensioning. Fig. 3 shows the calculated pressure distribution on the propeller blades and on the guard plate for the configuration from Fig. 2. These pressure distributions are transferred directly to a finite element structural code and serve as input loads. In the past, the loads were determined by using simplified theory and correlations from experimental data, leading to some uncertainty. The ability to predict time-dependent load on the structure with sufficient accuracy is crucial for designing a propeller that will have the expected life at a minimum cost.

Furthermore, by the application of CFD, VOITH has gained more insight into the complex flow patterns around the Voith-Schneider Propeller. Thus we were able to further increase the efficiency of the propeller, for example by changing the blade profile or by changing the oscillatory motion of the blades. As an example, the bollard pull has been improved by 6%. These improvements have made the Voith-Schneider Propeller competitive with conventional propellers over a wider range of applications.

Alongside calculations regarding the propeller itself, the flow around ship structures is also an important issue in order to optimize the ship hulls equipped with Voith-Schneider Propellers. Fig. 4 shows the pressure distribution as well as the free surface shape of a double-ended ferry, computed for a customer in the course of propulsion optimization.

From experience on another ship hull it was known from model tests that the resistance values were quite poor. After visualizing the computed flow field, the reason for the bad performance became quite obvious, see Fig. 5. Massive vortex generation occurs at the bow as well as at the stern of the ship hull. With that knowledge, only slight changes in the hull shape were necessary to improve the resistance of the ship considerably. For comparison, the streamlines around the modified hull are shown in Fig. 6. The vortices are no longer present and subsequent tests in a towing tank as well as full-scale measurements have confirmed the improvements predicted using Comet.

As a conclusion, it can be said that the use of CFD considerably helped to improve the Voith-Schneider Propeller. However, there is still potential left to be activated when the model's complexity is increased, e.g. by considering the hull-propeller interaction or by using optimization algorithms. This will be the subject of our future research and development, with CFD analysis using Comet remaining one of our major tools.