into the 21st century through clever use of design software. Jon Excell reports.
Almost 80 years ago Viennese inventor Ernst Schneider had a very good idea; an idea that had the potential to make ships more manoeuvrable than had ever been possible with the conventional screw propeller.
Schneider’s vision was a weird and wonderful system that combined propulsion and steering in a single unit and allowed thrust of varying magnitude to be generated in any direction.
He joined forces with German engineering leviathan Voith, and in 1929 they developed the Voith-Schneider Propeller. Today, whether it’s used to power tugboats, help Royal Navy minesweepers pick their way through treacherous seas, or assist Woolwich ferry passengers across the Thames, Schneider’s invention is the system of choice on a huge number of craft.
The motion of a boat equipped with a Voith-Schneider unit is created by a propeller comprising a number of blades that project downwards beneath the hull. As well as rotating about a vertical axis, these are also capable of restricted movement (oscillation) about their own axes.
Varying the magnitude of the blade’s oscillation as it rotates controls the amount of thrust, while by controlling the point at which different blades are at different angles, the direction of the thrust can also be controlled. So, through a series of relatively simple mechanical linkages, the speed and direction of the boat can be changed smoothly and quickly without the need for a rudder.
It is also possible, of course, to vary the magnitude of the thrust by changing the speed of the central axis, although on most Voith-Schneider systems this rotational speed is fixed.
Until recently all knowledge of the hydrodynamic behaviour of the propeller was based on extensive model testing. However, the adoption of CFD software by propeller manufacturer Voith has enabled its design team to improve its behaviour substantially.
Led by mechanical engineer Michael Palm, the designers used Comet, a CFD (Computational Fluid Dynamics) tool from CD Adapco, to gain better understanding of the complex flow patterns around the propeller. With CFD they could predict the hydrodynamic loads on the blades and improve the efficiency and strength of the propeller. This was brought about primarily by altering both the blade profile and the oscillatory motion of the blade.
As well as the calculations for the propeller itself, the software was also used to model the flow around ship structures and significantly improve the interaction between the propeller and the hull. For instance, through visualising the computed flow field around ships, Palm’s team could ascertain that certain combinations of hull shape and the Voith-Schneider Propeller led to massive vortex generation at both the bow and the stern of the ship hull. This would clearly have a serious impact on performance.
Armed with this knowledge, the team can now make slight changes in hull shape to improve the performance of the ship considerably.
While the overall efficiency of the propeller has been improved in many different ways, perhaps the most significant enhancement, said Palm, is a six per cent improvement in bollard pull. This is a fairly abstract design criterion traditionally used in the selection of propellers for tugboats. It is a measurement of the theoretical thrust achieved at zero speed and full engine RPM.
Although this state cannot actually be achieved (mainly because propellers accelerate water as they spin so they never really see water at zero speed), bollard pull is nevertheless a neat way of comparing the towing abilities of different propellers.
Musing further on the propeller’s advantages, Palm said the ability of the Voith-Schneider to vary the thrust steplessly in both magnitude and direction enables it to outperform conventional screw propellers in terms of manoeuvrability.
‘If you’re moving forward in a boat equipped with a normal screw propeller and want to stop immediately, you have to reverse the thrust direction,’ he explained. To do this the propeller must be rotated through 180 degrees, and during this rotation, which takes around 30 seconds, the propeller generates a certain amount of thrust to the sides, making exact manoeuvres extremely difficult. It is precisely because the Voith-Schneider doesn’t subject boats to these unwanted side thrusts that it is so ideal in, for example, minesweeping operations.
‘With a Voith-Schneider Propeller, claimed Palm, ‘it is possible to perform the same manoeuvre in just six seconds, bringing the craft to a halt within one ship length. If we want to inverse thrust to stop the ship we simply reduce the blade angle, compress it to zero and then expand it again to the other direction.’
Palm said that the improvements made possible through the use of CFD have made the Voith-Schneider Propeller competitive with conventional propellers over a wider range of applications. However, he admitted that there is still plenty of scope for improvement and hopes to continue using CFD to improve the propeller’s behaviour. Future projects, he said, are likely to concentrate on improving hull-propeller interaction using optimisation algorithms.