Root cause failure analysis of propeller shafts in fast gun boats in Sri Lanka Navy

Abstract

The Fast Gun Boats are playing a vital role for Sri Lankan naval fleet to prevent anti-smuggling and poaching activities in the northern seas at present. The aim of this research is focused on identify the failure of propeller shafts in fast gun boats and to find out the root causes for such failures. These crafts operated last four decades and failure of propeller shafts has been identified. Failure of propeller shafts is a critical situation concerning not only the operational requirements of the Sri Lanka Navy but also the repair cost & time. The objective of this research is to develop a mechanism to identify and analyze the root causes for failure of propeller shafts. This objective can be accomplished by examining the failed propeller shafts and investigating the fracture surfaces of propeller shafts to identify the root cause and propose recommendations to investigate the design of the propeller shafts and find solutions to overcome those failures. The initial findings which were mainly based on past records of failure, the operating running hours of propeller shafts and the pattern failure of propeller shafts was analyzed by Scanning Electron Microscope (SEM). According to the findings, the corrosion on shaft surface (pitting) causes the failure of propeller shafts. The experimental investigation highlighted that corrosion penetrates for further depth at the bottom of the corroded layer. Further, stress raisers were initiated from the bottom of the corroded layer and corrosion penetrates through stress raises. After further investigation, it was confirmed that oxygen is available inside the stress raisers. Therefore, oxygen actively participated in increasing the corrosion. The results of the experiment provide a better understanding of the corrosion in propeller shafts. Further, it was highlighted that the corrosion adjacent to the edge of the shaft is severe as galvanic corrosion. Thereby, the design itself promotes the failure at propeller shafts and this study was able to prove the same for future studies.