Identification of ship propeller torsional vibrations

The article presents identification methods of ship propeller torsional vibrations in dimensional space and ways for their dumping. Propeller torsional vibrations are caused by static, dynamic and hydrodynamic unbalance of propeller itself and its operation in non-uniform velocity field of ship hull. The propeller working in uneven velocity field of the hull creates cyclic thrust forces, peripheral vibrations of shaft as well as flexural and torsional load of propeller shaft. Vibrations cause additional thrust load on shaft bearings and shaft bending moment. Vibrations of ship hull, particularly its stern section cause variation of pressure field created by propeller. Amplitude of particular parts of the hull comes from the action of cyclic forces created by working ship propeller. They can be very large. Therefore, the ways to reduce vibrations caused by running propeller are very important. Given in the article methods of vibration dumping calculations conduct to determination of vibration dumping coefficients. Calculation results are comparable to measuring results in spite of fact they were determined under assumption of different velocity fields of water flowing through propeller circle. Amplitudes of vibrations depend on number of propeller blades and pulsate with the basic frequency equal to product of the rotational speed and number of blades. Causes of such vibrations and additional load of vessel stern are described in the article. An attention was paid to assure the greatest possible uniformity of velocity field by adequate design of stern shape and accessories. The ways to reduce torsional vibrations of propeller by the appropriate selection of blades number and their accurately machining are also given in the article.