Vibration Analysis of A 3-Bladed Marine Propeller Shaft for 35000DWT Bulk Carrier

Abstract

DOI: http://dx.doi.org/10.24018/ejers.2019.4.10.914 78 \uf020 Abstract—One of the most dreaded problems on board ships is high level of vibration. The propeller shafts of Bulk Carriers are subjected to dangerous resonance. This dynamic excitation is usually transmitted through the propeller shaft to the rest of the ship. Some ships have been disposed of for years due to objectionable level of vibrations, subjecting them to precarious operation. This thesis aims at analysing the vibration of a 3bladed marine propeller shaft for 35000DWT bulk carrier. The objectives of the work were to mathematically design the 3bladed propeller shaft, carrying out computer aided design of the shaft and numerically performing vibration analysis. The methodology used was the application of Solidworks and Analysis System (ANSYS) softwares to design and calculate the natural frequency of the shaft. The mathematical design gave a Hub (boss) diameter of 0.17m. The hollow shaft had external and internal diameters of 0.10m and 0.09m respectively. Also, torques of 202Nm and 384.72Nm were obtained at the driver and driven shafts respectively. The calculated natural frequency was 249Hz while that of the ANSYS was 280Hz which gave an error of 12%. However, the numerical analysis carried out with ANSYS software also showed that a phase difference of 180 occured at the frequency of 280Hz which is a signal of possible misalignment of shaft. At this frequency, the displacement of the shaft had a maximum value of 7.87× 10m. Reaction forces from the components of the shaft were also observed to have contributed to the vibration of the propeller shaft. These reaction forces, which cause wearing of the stern tube and intermediate bearings due to friction, are represented by phase angles closer to 0. Wear due to friction is a major source of shaft misalignment. In view of the aforementioned results, it is pertinent to recommend that Bulk Carrier should never be operated at the natural frequency of the engine and the critical speed arrangements should always be carried out from preliminary design stage to operational stages.