More than $250,000 in annual fuel savings for two of the Royal New Zealand Navy’s (RNZN) fleet will be possible, thanks to a unique engineering solution developed by the RNZN and its fleet maintenance partner Babcock (NZ).

Offshore patrol vessels HMNZS Otago and HMNZS Wellington were designed to run mostly using just one of their two main engines and propeller shafts. The speed generated by one engine is generally enough for their operational requirements.

But the issue with this, RNZN Logistics Commander Maritime Captain Andrew Nuttall says, was that the trailing shaft, even though not powered by an engine, still turned because of the wind-milling effect of the propeller through the water, which subsequently generated heat in the gearbox.

“The pump to supply coolant to remove the heat was driven by the engine,” Captain Nuttall says. “But because this was shut down in the case of the trailing shaft, the result was that various components began overheating.

“To avoid that, both engines were required to be running most of the time, even though only one was technically required to power the vessel.”

HMNZS Wellington’s engineering officer Lieutenant Tim Johns and Babcock’s senior mechanical engineer Patrick Clissold view the coolant transfer system.

This meant that $200,000 of essentially wasted fuel was required for every 1,000 hours of running. Having both engines operational also required increased maintenance and It also meant that marine technicians had to manually adjust the cooling system at all hours of the day and night to ensure the ship’s safety.

The RNZN and Babcock combined engineering design team developed a unique solution that cross-connected the cooling system, so that when an engine was shut down the propeller could still rotate, while the heat generated was cooled through diversion of coolant.

The solution developed by the team would result in significant financial benefits, Captain Nuttall says.

“The saving in fuel alone is impressive. Combine that with the reduced maintenance costs and this solution represents an excellent example of the value our engineering team can add.”