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

But the issue with this, says RNZN Logistics Commander Maritime, Captain Andrew Nuttall, is that the trailing shaft, even though not powered by an engine, still turns because of the windmilling effect of the propeller through the water. This generates heat in the gearbox.

“The pump supplying coolant to remove the heat is driven by the engine and, with it shut down various components began overheating. To avoid that, both engines were run most of the time, even though only one was required.”

This translated into $200,000 of ‘wasted’ fuel used for every 1,000 hours of running. Having both engines operational also required more maintenance.

RNZN and Babcock developed a system 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 the diversion of coolant.

“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.”