A case of excessive lateral vibrations of patrol boat shafting: Failures, analysis and solution

Abstract

During the endurance trials of three identical patrol boats with "Vee" type main shafting system (3x900 kW each), three (of nine) shaft lines broke. The excessive lateral vibrations of the main shafting (excited by, cardan joints) were recognized as the main cause of the failures, and some corrective measures were proposed. The authors of this paper were engaged a while later to make a thorough vibration measurement of the repaired boats. As the record showed that the improvement was not satisfactory, some additional more effective measures had to be accomplished. The paper discusses the possible additional measures, and identifies the installation of an additional bearing (at the proper location along the shaft line) as the most promising. Such a bearing shifts the first natural frequency out of the working range, and releases the vibrating system of the existing critical. The paper gives the vibration data chronologically (from the first identification of the excessive vibrations to the last measurements, after the installation of the additional bearings), so that the successive steps of the healing action could be followed. Finally, the paper shows that the applied measure did reduce the horizontal (most dangerous) vibrations sufficiently, but also had some negative (unpredictable) effects on the vertical component of the vibrations.