Calculation of the Harmonic Structure of Marine Propellers Torque and Thrust 941696
Among the excitation sources of vibration for the marine propeller shafting, the propeller itself plays a significant role. It is the main source of excitation for the axial vibrations and a major contributor among the excitation sources for torsional vibrations. Thus, the possibility to predict the harmonic structure of the torque and thrust, in the design stage of the propeller system has a great importance for the designer. In this direction, the paper presents an analytical method of calculation for the propeller torque and thrust based only on the propeller series diagrams and general empirical formula for the wake and suction coefficients.
A vortex model of the marine propeller, based on Prandtl lifting-line theory, has been developed. Considering the system of bounded and free vortices representing the propeller blade and its interaction with the incipient wake generated by hull motion, the velocity field around the blade profile is determined. Based upon a non-dimensional expression of the circulation, as a function of blade radius and angle of rotation, torque and thrust are computed and expressed as Fourier series.
Parallel to the theoretic study, experimental investigations have been carried-out, both in cavitation tunnel and towing tank, showing an acceptable agreement between calculated and measured fluctuations of torque and thrust.