An investigation into the loss of a propeller from the Interislander ferry Aratere reports the most likely cause was an irregularity in the propeller itself.
The KiwiRail funded investigation, led by the international engineering consultancy firm Aurecon, established, recorded and analysed the physical evidence.
After consideration of extensive expert technical reports and advice, KiwiRail’s conclusion was that the most likely cause of this incident was a misshapen propeller blade.
However, the manufacturer of the propeller does not agree with this conclusion.
The starboard propeller tail-shaft fractured when Aratere was sailing from Picton to Wellington on November 5, 2013. The ship continued safely on to Wellington, and later the propeller recovered from the sea floor near Tory Channel.
The Transport Accident Investigation Commission investigated the incident and were yet to report on its findings.
Independently, KiwiRail launched its investigation.
“Our investigation led by the international engineering consultancy firm Aurecon was lengthy and exhaustive,” says KiwiRail Group General Manager Network Services Todd Moyle.
“It involves experts producing reports that considered some possible scenarios and causal factors,” says Moyle.
“The thoroughness of this investigation has enabled us to be confident in the conclusion of Aurecon’s Project Director Stephen Jenkins that all potential factors, other than an irregularity in the propeller itself, are unlikely to have caused this event,” he says.
“For example, we have ruled out an external event or a power surge as causes. The investigation also found that the fitting of a new mid-section to Aratere in 2011 did not compromise the propulsion system, which is in the rear section of the ship,” says Moyle.
“While our investigation points to an irregularity in the propeller as the most likely cause of the failure, we note that the manufacturer has defended the propeller and did not agree with our conclusion,” he says.
“KiwiRail uses only recognised suppliers for the critical engineering products that we purchase for our ferries and this experience means we will be even more vigilant in future,” says Moyle.
“In this case we have reached a confidential agreement with the manufacturer which means that although they accept no liability for our loss, we nevertheless have confidence about our ongoing commercial relationship with them,” says Moyle.
On Tuesday 5 November 2013, one of the two shafts that drive the Aratere’s propellers broke when the ship was near Tory Channel. She was able to return to Wellington where divers confirmed that the tail section of the ship’s starboard propeller shaft had broken, and the propeller lost.
Monitoring systems on board Aratere provided no warning of imminent failure. The crew were not aware of anything unusual before the failure.
The repair work on Aratere was carried out at Singapore’s Keppel shipyard in conjunction with a routine two-year, dry-dock.
The most significant work included replacing both port and starboard propeller tail shafts and bearings and realigning the shafting system.
The original smaller propellers that had been on Aratere before her lengthening were re-fitted because new propellers would have taken up to nine months to procure and fit.
Aratere re-entered service in early July last year. Mechanically and electrically Aratere ran well but her smaller propellers were less effective than the larger propellers they were replacing.
