Voith’s established technology has become ‘future-proof’, writes Stevie Knight.
A recent update to the well-established Voith Schneider Propeller is resulting in useful pairings with innovative, but demanding vessel designs. Take Østensjø Rederi’s new CSOVs: DP2 vessels which are to serve as motherships in the fast-developing wind commissioning and maintenance field.
So, what tipped the choice in favour of Voith’s technology? Firstly, “the ship can still operate in much higher waves because the thrust can be adjusted quickly”, says Dirk Juergens, Voith’s vice president of marine R&D. Further, “on a DP2 ship, the worst case single failure has to be considered… If this should occur and a driveline fails, a VSP-driven vessel can hold its position most safely”. Again, “the reason is the fast pitch adjustment” he explains.
However, while the (almost) century-old technology has seen incremental changes along the way, this last update is arguably the most significant: an electrical version, the eVSP, is gaining a “remarkable” amount of interest says Juergens.
Interestingly, the story really began with a different technology. More than a decade ago, the Voith Inline Thruster (VIT) was fitted with a permanent magnet motor. “Up till then a typical thruster had an electrical induction motor running to a gear-linked propeller, but a rim drive is quite different,” says Juergens’s colleague, Michael Rommel. The rotor has a series of permanent magnets around its perimeter, so it doesn’t need a power supply, resulting in a more straightforward, more robust construction. Further, a rim drive results in lower radial and axial stresses and much quieter running.
The question was, could the advantages cross over to other propulsors? A considered look at the well-known Voith Schneider Propeller suggested there were enough similarities to make it worthwhile, despite a sizeable step up in scale: the largest VSP is a 4MW model.
There was a certain impetus as several challenges “had already been overcome” on the VIT, says Juergens. For example, the VIT’s drive is adequately cooled by seawater without additional systems; partly as PM motors operate at lower temperatures than induction motors. Moreover, it was shown that the solid-state electrical components could be encapsulated in resin, preventing water ingress.
Perhaps most convincingly, he explains that the VIT had achieved a high degree of efficiency “not only at the nominal RPM but especially in the partial load ranges”.
However, translating the electrical design to the VSP entailed significant changes, including dispensing with the huge, almost iconic circular bevel gear… a necessary step to clear the ground for “a very large diameter, high torque and relatively low RPM permanent magnet motor” he explains.
But to understand why it makes such a good match, you have to dig deep into the design of the VSP. At its heart are vertical blades that circle round on their base, changing their angle of attack to suit both the size and direction of thrust, with no need for a rudder. If you look at the path of a single blade, it is somewhat reminiscent of the way a cyclist will angle a pedal to gain maximum traction at the right moment.
Interestingly, as this thrust force is set at right angles to the axis of rotation it’s liberated from much of the hydrodynamic inertia inherent in standard propeller design. Therefore, it can neatly switch direction “in seconds” says Rommel, just by changing the blade’s orientation. But there are no two ways about it; this still involves high load transfers.
This is where the characteristics of a synchronous motor come in. It works by exciting a number of stator windings in turn, causing the magnets on the rotor to chase after a moving field: as a result, the rotor spins at a synchronous speed – one that won’t drop away even given abrupt load changes. In short, it simply remains locked to the excitation. Despite some control fade-out at very high RPM, the resulting load-independent rotation makes it particularly attractive for the eVSP as it avoids any drop in power output, even when meeting dynamic forces arising from a sharp change of direction.
Further, while standard propellers need to rotate faster for a given thrust, on a VSP the force is generated by varying the pitch of the blades, not by speed alone, allowing the PM motor to work well within its sweet spot. Further, the eVSP’s large diameter allows for rapid, high-torque pick up “and so we can vary the RPM very quickly”, says Juergens. But efficiency “is the most important criterion” adding that “compared to an asynchronous motor, the efficiency advantages come to about 2% across the full load range – but importantly, it’s 5% for partial loads”. This is particularly relevant for the offshore and renewables support sector.
Finally, compared with ducted screw propellers in model tests, an average advantage of 15% for the eVSP was measured with regard to power requirements in transit.
There are installation advantages as well, explains Rommel. Changing the rotor RPM requires a variable frequency drive (the flip side of that synchronous ‘lock’ on speed is that you can’t control a PM motor by merely dropping the voltage). However, removing a number of mechanical components more than makes up for it, allowing for “a compact build” he adds. Moreover “completely dispensing with the gears means the PM motor is very easily accessible from within the ship”.
It’s worth noting the mechanical components also benefit from an overall efficiency rise of 4% in total over a standard VSP, mostly due to fewer bearings – and of course, the absence of gears. It all helps to reduce noise and vibration: trials on a testbed corresponding to a blade diameter of 1.2m have shown it’s exceptionally quiet, making it a potential solution “for research vessels, mine-countermeasure vessels (MCMVs) and passenger ships”, says Juergens. This alone should make those 120 people (crew and technicians) stationed onboard Østensjø Rederi’s CSOVs more comfortable, but it’s also combined with the way the eVSP can reduce rolling motion by up to 70%, both during transit and also while stationary in the field.
However, there are further applications for the eVSPs onboard the ships of the future: these CSOVs, like other vessels, are being prepared for zero-emission running. The arising issue is that there’s quite a drop in volumetric energy density of many of the green energy carriers, including hydrogen and batteries. “As a result, they require comparably better efficiency, not just at higher RPM but across the entire speed range,” says Rommel. Again, this calls out to PM technology such as the eVSP.
But there’s another aspect: the ships of the future will likely utilise alternative energy sources such as photovoltaics, wind or fuel cells, so the likelihood is that they won’t want to lose efficiency to unnecessary energy conversions. Likewise, an eventual move to remote or unmanned ships will require electrical, rather than mechanical propulsion control: it’s more straightforward (and the solutions more robust) if the mechanical elements are kept to a minimum.
Despite all this, the eVSP remains suitable for retrofitting in place of a mechanical model or azimuth thruster. Further, as it is power-source agnostic “you can choose where the energy for the eVSP will come from – including battery, hydrogen or other future fuels”, says Juergens, adding that likewise, “you can switch your power source in the future”.