Feature 3 | PROPELLERS AND THRUSTERS Driving out of recession
A desire to optimise propulsion efficiency is indicative in new propulsion systems unveiled during Nor-Shipping, writes Patrik Wheater
T
here is nothing quite like a recession to spur creative thinking and technological advancement in the
marine industry, especially when the focus is placed firmly on delivering energy-efficient systems capable of reducing a shipowner’s bottom line for little or no extra outlay. Indeed the drive to optimise propulsion
efficiency was indicative in the number of new – and not so new – propulsion systems being unveiled during the recent Nor-Shipping exhibition. Voith Turbo the German manufacturer
famous for the ‘Viennese whirl’, aka the Voith Schneider Propeller, is about to relaunch a product it ceased to produce more than 30 years ago. Te Voith Radial Propeller (VRP) was a
360deg azimuthing thruster introduced in the 1970s, but, despite its potential success – there are several still in operation – unit production was stopped by the end of that decade. However, following the outcome of an extensive research project in to the hydrodynamics of steerable thrusters, Voith Turbo has been able to redesign an underwater mountable VRP for optimal dynamic positioning and hull-thruster interaction. The system is being marketed towards the offshore segment. According to Voith Turbo, optimum
thruster-hull interaction is of the utmost importance to the efficiency of propellers in offshore vessel application, specifically mobile drilling units and semi-submersible rigs. Indeed, some operators of these vessels have reported substantial energy losses – up to 50% thrust losses during offshore operations, in some instances. But while current technological practice to limit these losses dictates tilting the nozzle of the thruster downwards by four degrees, new research suggests greater reductions can be achieved by tilting the gear axis further to an angle of eight degrees. (Voith Turbo operates its own special gear machinery, so it is possible to tilt the propeller and nozzle axis in identical fashion.) It was found that an axis tilt of zero degrees,
losses could be as much as 45% of the generated thrust but by increasing the downward tilt, it was found that thrust losses reduced linearly
58 Voith Radial Propeller.
and removed underwater with the vessel afloat. Voith Turbo will deliver the first ship set of two VPR units for installation to a semi-submersible rig intended to be launched in 2011. Like Voith Turbo engineers, competing
counterparts at Rolls Royce have also been working to improve the energy efficiency of its propulsion systems. One is the use of hybrid propulsion, something the company has been promoting for many years and is now gaining full acceptance by ship operators; another is the development of individual products; but even mature products can be improved given the company’s up to date research facilities and use of the latest computational methods. For example, by redesigning the CP propeller
hub Rolls Royce has been able to optimise hydrodynamic and mechanical efficiency. Te results of this research is now manifest in the new S3 series of Kamewa waterjets, which can deliver increased thrust for the same power and less weight. Both are showing actual gains in service. Combining individual product
Promas to the fore.
up to a seven degree axis tilt. At an axis tilt of four degrees, losses equalled 28% but with an eight degree axis tilt thrust losses were significantly less at just 5% of generated thrust. Interestingly, greater tilt angles do not appear to lead to further improvements in efficiency. Based on the results of its research and the
service experience of those 1970s-built units still in operation, Voith Turbo is confident the new VPR 42-55 will be better placed to stand the test of time. A 5.5MW prototype unit, with a bollard pull of 100tonnes at dynamic positioning, is already in production at the German manufacturer’s Heidenheim works. Based on an L-drive arrangement with a
4200mm diameter fixed pitch propeller, it incorporates a special98 deg gear that transfers the vertical input shaſt to the propeller axes that tilted at eight degrees. It can be installed
improvements in optimised systems can be another route to saving fuel and reducing total emissions for a given amount of useful work done. Promas is a good illustration of this, applied to the stern of merchant vessels. In this Rolls-Royce system, Promas is an integration of the conventional propeller and rudder configuration of a typical single or twin screw vessel. A special hubcap is fitted to the propeller which smooths the flow on to a bulb that forms part of the rudder, while the spade rudder itself has a special shape with a twisted leading edge. The resulting improvement in efficiency is made up from several components. One is the reduced loss in the hub region of the propeller. Te second is that the shape of the rudder converts some of the swirl energy in the propeller slipstream, which is normally lost, into additional forward thrust. Te third component is that the shape of the rudder gives a much higher side force for a given rudder angle in the ±5 degree range normally used for course corrections. Te best results are achieved on blunt single screw vessels with a block coefficient greater
The Naval Architect July/August 2009
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