Forensic Issues from the Investigation of a Marine Shaft Failure

Abstract

The starboard propeller shaft of a twin-screw diesel electric rail ferry in New Zealand failed just after the ferry left port. Weather was not a factor. The ship was on a regular schedule of three sailings a day. The starboard propeller was found in 120 meters of water approximately two nautical miles from the channel — some distance from the point where power was observed to reduce to zero on the shaft. The fracture surface of the shaft showed a classic fatigue failure pattern. However, there were questions to be answered, including what initiated the failure, and why a tension failure occurred in a shaft that was primarily under compression from the reaction forces of the propeller. This paper will look at some interesting factors in the investigation, the techniques used to limit the investigation (and its cost) to relevant areas, a few of the false trails that were followed, and the processes eventually used that were the most convincing.