Application of strain gauge method for investigating influence of ship shaft movement by hydrodynamic propeller forces on shaft alignment

摘要

yy To ensure the stability of marine propulsion shafting system, series of studies regarding shaft alignment have been performed under quasi-static conditions. In the shaft system of a ship, the increase of local load in the stern tube bearing, which supports a propeller shaft, occurs prominently due to the influence of the propeller weight at the shaft end, similar to the case of the cantilever beam. In particular, the aft stern tube bearing is likely to receive a concentrated load in the bottom of aft side. While such magnitude and distribution of local load are determined by the relative inclination angle between the shaft and bearing, the aft bottom of aft stern tube bearing is more severely affected by the local load than other bearings. Such local load can significantly deflect towards the aft end of aft stern tube bearing in case that the shaft sags down, when the eccentric thrust force acts downward due to the propeller force in the hydrodynamic transient status. In this paper, the dynamic behaviour of the propeller shaft is evaluated for a medium-sized oil/chemical product carrier with 50,000 dead weight tonnage acknowledged as a highly efficient eco-friendly ship type. As a result of the analysis, it is confirmed that shaft behaviour would be affected by hydrodynamic propeller forces by the variation of ship draught and main engine load. Further, it is found that the propeller forces during ship acceleration acted as a force lifting the propeller shaft from the aft stern tube bearing and it reduced the possibility of damage to the aft stern tube bearing, thereby, contributing to improving the reliability of the shaft system. This paper deals with the new application of strain gauge method to evaluate the shaft stability. The research results are consistent with those from the previous studies based on the direct measurement in the vicinity of the propeller. The Research findings demonstrate that this new application would be excellent and practical as an alternative to the direct measurement method performed at the propeller position.