Invented in 1950 by German engineer Joseph Becker and developed by the ABB company, the all-directional propellant Azimuth Thruster can generate great thrust in addition to generating a rotational torque capable of switching all directions in operation. Given these advantages, this propeller is widely applied in pataches (small ships) used for maritime operations and support purposes such as, for instance, water surface cleaners and tugboats. The propulsion engine varies in design according to ship type, but most choose the propeller for the Z-drive on the engines-2 and shafts-2. The main engine and Z-drive propeller are considerably separated on these ships, and they are located on a sloping surface with different heights, designed to deliver power using long intermediate shaft and cardan shaft. Additionally, high-performance flexible coupling is installed between the main engine and the intermediate shaft to reduce excessive torque fluctuation during the clutch's removal and attachment.
In this paper, we use a water surface cleaner as a model to analyze the causes of the shafting accident that occurred in Z-drive, which is used primarily as a propulsion system in pataches. The study's aim is to verify and improve safety through torsional vibration analysis and measurement. The study's torsional vibration analysis employed a transfer-matrix method by modeling the propulsion shafting system using a concentration mass meter. Furthermore, the major parts' torsional stress or vibration torque was measured and the angular velocity was obtained at the torsional measurement location. In addition, since a ship must be operated safely even in the event of a cylinder misfire, we conducted a theoretical analysis to identify whether there would be a problem with the system in such a situation. The torsional vibration measurement tested the angular velocity variations before and after flexible couplings in their accessible position using a laser torsion-meter. The measurement results were then compared with the analytical results.
Self-excited torsional vibration was identified in the ship's propulsion system with flexible coupling from V company. The rate of vibration increased rapidly, leading to a breakage accident. This was due to 0.5th resonance with the engine's unbalanced load as a vibromotive force. Thus, this resonance is judged to exist at approximately 1,200 r/min when the engine revolutions are dropping. Additionally, it is deemed to exist at approximately 1,180 r/min when the engine's revolutions are increasing. Therefore, given the data, the operation of 1,100 r/min and 1,200 r/min while the engine is running should be avoided whenever possible. The second measurement revealed that the explosion phase angle between A- and B-bank, the primary vibration measurement, was assembled and constructed at 450°. The improvement of the 0.5th torsional vibration by adjusting the explosion phase angle, was not be expected. Therefore, the last measurement was taken after replacing the flexible coupling of V company with the flexible coupling from the Center company. To avoid self-excited torsional vibrations, the intermediate shaft with flexible links was replaced with one with increased flexibility in the three-axis direction. The weight of the output side was reduced as well. Moreover, the system was measured intensively near 600 to 700 r/min. Over two times, the most severe case was the 1.5th of the primary mode of torsional vibration; however, overall torsional vibration was reduced and the system's stability was successfully achieved.