A sub-optimal fit, vibration, corrosion and the uneven thrust produced by differently-pitched blades all contributed to the “fretting” which saw the Interislander ferry Aratere ultimately lose its starboard propeller shaft in Cook Strait in November 2013.
Such were the key findings of an investigation by the Transport Accident Investigation Commission (TAIC), which recommended parent company KiwiRail improve its process documentation for both the fitting of new propellers as well as on the final fit achieved.
The TAIC also urged Standards New Zealand to forward the report to the International Organization for Standardization, so as to establish if current standards for manufacturing large-diameter marine propellers are appropriate for modern, high-efficiency propellers operating closer to cavitation margins.
In response, KiwiRail group general manager Todd Moyle acknowledged the “valuable” recommendations made about the standard for advanced propeller manufacture and confirmed the Interislander had already improved document management processes.
However, having undertaken its own “extensive investigation”, KiwiRail did not accept an additional TAIC viewpoint that it had not followed manufacturer advice on the best means of fitting the propellers and therefore had potential contributed to the failure, he says.
“The proposed method of fitting the propellers was impractical because of the very long shaft fitted,” says Mr Moyle.
“The method we did use to fit the propellers was discussed with the manufacturer, who accepted it as an alternative.”
Mr Moyle adds that control of the Aratere was never lost during the incident — “and at no point were passengers or the vessel at any risk”.