Energy saving devices

Rudders and Propellers Working together Further efficiencies are available from propeller/rudder combinations which are recent arrivals on the propulsion scene. Possibly the two most well-known are the Rolls-Royce Marine Promas system and Wärtsilä’s Energopac but other companies working alone or in tandem with others have similar systems on the market, one example of this co-operation is that between German propeller maker MMG and Dutch rudder maker Van der Velden. Typical efficiency savings of between 2% and 9% are claimed with the wide range due to different vessel types.

The Rolls-Royce Promas consist of a twisted full-spade rudder with bulb that is smoothly connected to the propeller hub by a hubcap, and is adapted and optimised to the propeller design. A well-designed twist adapts the rudder to the rotation of the propeller slipstream and reduces the local angle of attack on the rudders leading edge. This gives a more efficient rudder with lower drag and better recovery of rotational energy from the propeller slipstream.

The best results are achieved on blunt single screw vessels with a block coefficient of 0.75-0.85 and a design speed in the 14 to 16kt range. Here the efficiency gain can be as much as 6-9% compared with conventional solutions. For faster and slenderer single or twin screw vessels such as car carriers, efficiency improvements of 2-5% can be expected.

Wärtsilä’s Energopac is a very similar design and is claimed to create less drag than conventional rudder systems. Most notably, when using small – corrective – steering forces to maintain course, the difference in rudder resistance is significant. The high-lift performance of Energopac requires smaller steering angles, which consequently reduces rudder resistance. The reduction in fuel consumption depends very much on the type of vessel, its operational profile, and on the reference propeller and rudder. Proven savings in required power for a vessel’s trial speed vary between 2–9%. The potential savings are large for vessels with highly loaded controllable pitch propeller systems, such as RoRo-vessels, ferries, container / multipurpose vessels, and vessels with an ice class notation.

The two systems described above are designed from the outset with matched rudder and propeller but modifications to existing vessels are also possible. Promas Lite, a simplified version of Promas is intended for retrofit upgrading. The vessel’s existing rudder is retained, but is fitted with a prefabricated bulb, while the propeller is equipped with a special hubcap and new blades. shipinsight.com | June 2013 | 23