Numéro : 2095 - Year : 1989
Investigation of a new type of contra-rotating propellers and other means of improving the propulsive efficiency.
C.A JOHNSSON, SSPA Maritime Consulting AB, Gôteborg,Sweden
It is well known that, when trying to reduce the vibration and noise excitation for a propeller in a wake, the means mostly used have certain draw-backs, e.g. unloading the blade tips (reduction of efficiency) and introduction of skew back (strength problems). In the current paper these alternatives are compared with more unconventional alternatives, the case of application being a single screw Ro-Ro-ship, characterized by large wake variations. The alternatives investigated are:
propeller in axisymmetric duct
propeller in different non-axisymmetric ducts
propellers in a new contra-rotating arrangement
The contra-rotating arrangement is characterized by a rotatable thruster operating behind the main propeller. Thus the diameter and rpm of the two propellers are different. Most of the transmission problems met, when using a more conventional contrarotating arrangement, are avoided by this concept.
In the paper the design principles for the different propulsors are briefly described and the results of comparative tests in the towing tank and the large cavitation tunnel of SSPA are compared as regards efficiency, pressure fluctuations and noise excitation.