KiwiRail believes propeller at fault in loss

Published on : Thursday, December 3, 2015

Kiwi RailKiwiRail has completed its investigation in to the loss two years ago of a propeller from the Interislander ferry Aratere  and believes the most likely cause was an irregularity in the propeller itself.


In an extremely rare event, the starboard propeller tail-shaft fractured whenAratere was sailing from Picton to Wellington on November 5, 2013. The ship continued safely on to Wellington and the propeller was subsequently recovered from the sea floor near Tory Channel.


The Transport Accident Investigation Commission investigated the incident and is yet to report on its findings.  Independently, KiwiRail launched its own investigation.


“Our investigation was led by the international engineering consultancy firm Aurecon and was lengthy and exhaustive,” KiwiRail Group General Manager Network Services Todd Moyle says.


“It involved nationally and internationally-recognised experts producing reports which considered a number of possible scenarios and causal factors.


“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.


“For example, we have ruled out an external event or a power surge as causes. Importantly, the investigation also found that the fitting of a new mid-section to Aratere in 2011 did not affect or compromise the propulsion system, which is in the rear section of the ship.”


That left possible potential causes being:

  • Fretting at the interface between the propeller shaft and propeller hub
  • Cavitation arising from one or more misshapen or out-of-balance propeller blades
  • Vibration due to misalignment of the various individual shafts which together make up the complete propeller shaft
  • Stress on the shaft from the weight of the propeller cone, which was designed to be larger than the equivalent cone on the original propeller

“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 does not agree with our conclusion.


“KiwiRail uses only reputable 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.


“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,” Mr Moyle says.


 “Since returning to service Aratere, which is Interislander’s only specialised rail freight ferry, has been a workhorse of the fleet. She has a full schedule of sailings, regularly making six journeys a day across Cross Strait and she significantly contributed to the Interislander having its best-ever season last summer.


“Together, our three ferries currently make more than 4000 crossings of Cook Strait each year, carry more than 800,000 passengers, around 220,000 passenger vehicles, and more than 2 million lane metres of freight, including rail. The reliability performance against scheduled sailings for the past year was 99 per cent.”


 “Kaiarahi recently joined the Interislander fleet and forward bookings indicate another successful summer season on the way,” Mr Moyle says.


Source:- Kiwi Rail


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