CP Propeller Equipment - Martin's Marine Engineering Page

For each condition and all angular positionsbehind the actual hull, the flowaround the blade is calculated. The extentof cavitation is evaluated with respectto noise and vibration, fig 30.Angle of attack (degrees)4Suction2ActualHigh skewTo suppress cavitation–induced pressureimpulses even further, a high skewdesign can be supplied, fig 31. By skewingthe blade it is possible to reduce thevibration level to less than 30% of anunskewed design. Because skew doesnot affect the propeller efficiency, it is almoststandard design on vessels wherelow vibration levels are required.Today, the skew distribution is of the“balanced” type, which means that theblade chords at the inner radii areskewed (moved) forward, while at theouter radii the cords are skewed aft. Bydesigning blades with this kind of skewdistribution, it is possible to control thespindle torque and thereby minimizethe force on the actuating mechanisminside the propeller hub, fig 32.0–2–4Pressure0.4 0.6 0.8 1.0Dimensionless ratio of radii r/RSpindle torque (kNm)420–2–40 90 180 270 360Angle (degrees)0.40 0.60 0.801.00r/R2 03 24 21–0.1Fig 31: High skew designSkew angleCentre lineshaft2 03 24 11–4.1Single bladeAll bladesFig 32: Spindle torqueFor high skew designs, the normalsimple beam theory does not apply anda more detailed finite element analysismust be carried out, fig 33.2 04 05 37–8.0Fig 30: Cavitation chart and extensionof sheet cavitation – suction side10 20 30 4050602 04 01 12–4.0ISO stress levels in N/mmFig 33: Finite element calculation ofpropeller bladeThis document, and more, is available for download at Martin's Marine Engineering Page - www.dieselduck.net21