Three props better than two – Rolls-Royce’s Levander

A naval architect by background, Levander explained that by adding a shaft housed in the centre skeg and a third propeller, fuel savings of 10-12% can be achieved. The findings have been confirmed both by computational fluid dynamics and model tests. He also points out that the idea is not entirely new – the Titanic, for example, had three propellers.

A larger propeller blade area means lower loads which raises open-water efficiency, he said, as well as improving manoeuvring capability. These are important design criteria for certain ship types, particularly ferries and cruise vessels.

Any increase in first cost, Levander believes, would be more than offset by improved operating flexibility, reduced power demand, smaller engines with fewer cylinders and even a reduction in the number of engines – from four to three, for example. Three separate engine compartments would also provide additional safety and improve a vessel’s compliance with Safe Return to Port requirements, he said.

Various options are available for different ship types. Small expedition cruise vessels, for example, could gain fuel savings of up to 12% with one centre shaft and two Azipull thrusters. This would again reduce propeller loading whilst also providing a clean wake for the wing thrusters and improved flow through the main propeller. Such an arrangement already exists on board the Finnish Border Guard patrol vessel Turva, built in 2014 at STX Rauma Shipyard.

The vessel is powered by three Wärtsilä 34DF engines capable of burning LNG or diesel with the engines located in two separate engine rooms. In the after engine-room space, a 12-cylinder dual-fuel engine is coupled mechanically to a controllable pitch propeller while, in the forward engine room, two generating sets provide power for two electrically driven Azipull thrusters.