New techniques allow Wärtsilä to design propellers with higher efficiency than in the past without increase of vibration and noise levels. Assuming that geometry and other boundary conditions do not change, there are four options to increase efficiency through a new propeller design: (i) Reduce the blade area, (ii) change the blade contour, (iii) modify the radial pitch distribution, (vi) apply the Lips tip-rake concept. Reducing the blade area can increase the efficiency of a cargo vessel with a power density of 735 kW/m2 by about 3% without any unacceptable increase in pressure pulses or the risk of erosion. Changing the blade contour, i.e. reducing the chord lengths at outer radii, where the friction losses are highest, can also increase the efficiency. But a more important argument for this option is the significant reduction of pressure pulses, because the generated lift, and therefore the efficiency, is most significant at the outer radii. For the same reason high pitch values at the outer radii have a positive impact on efficiency, but they impose a negative effect on cavitation behaviour and consequently lead to higher pressure pulse levels. Moreover, high tip loadings cause intensive tip-vortex cavitation, and high pitch values at inner radii cause hub-vortices can lead to cavitation erosion on the rudder, so that the loading of the propeller blade should be shifted away from the tip and the root. A Lips tip-rake propeller differs from a 'normal' propeller only in the radial rake distribution. At approximately 90% of the propeller radius towards the tip the blade is bent towards its pressure side. This modification has a similar effect to increasing the propeller diameter. The Lips propeller has been in operation successfully since June 2001 on the bulk carrier 'Atlas Sea', owned by Elmira Shipping, and according to Elmira speed has improved considerably. Calculations show that propeller efficiency would increase by three percent without increasing the pressure pulses. When the mission profile of a vessel changes, replacing an existing propeller can be necessary. A new service curve for the propeller must be defined, which should fit within the new engine power diagram. For example, a fixed pitch propeller originally designed to operate on two engines cannot be used on one of the two engines only. This propeller has to run 'lighter', i.e. a pitch reduction is needed, which can be achieved in cutting the trailing edges of the blades.


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    Title :

    Replacement of fixed pitch propellers. Practical experiences


    Additional title:

    Praktische Erfahrungen: Ersetzen von Festpropellern


    Contributors:

    Published in:

    Publication date :

    2002


    Size :

    5 Seiten, 7 Bilder, 2 Tabellen



    Type of media :

    Article (Journal)


    Type of material :

    Print


    Language :

    English