"If we designed a conventional propeller which produced no cavitation at all, it wouldn't propel the ship very efficiently," Thomas explains. "Of course for a cruise ferry like Honfleur, passenger comfort is a priority. But we also want to maximise the benefits of switching to gas power, and make her as efficient and as green as we possibly can. This new 'tip-rake' propeller design allows us to reconcile those two priorities."
The new design sees the tips of the propellers ‘rake’ forward in the same way that modern wing designs rake upwards. This shape, and the effective lengthening of the prop’s blade, allows the areas of high and low pressure to equalize smoothly, making for more refined progress with no loss of efficiency.
Thomas, his colleagues and the propeller supplier arrived at the new design using sophisticated computational fluid dynamics software - or CFD - which can model the flow of water through the propellers, calculating where those areas of low pressure will occur and how to resolve them.
But the simulations don't stop there. The hull has been designed to ensure that the seawater arrives at the propellers at the optimum speed and angle. And Honfleur's naval architects have calculated how any remaining noise or vibration will travel through the ship's structure, and designed her steelwork to minimize its transmission.
"When you put all these factors together, I'm confident that there will be almost no noise contribution from the propeller," Thomas says. Unlike the huge, 2.5m tall wingtips on your Airbus A320, you won't be able to see Honfleur's elegant tip-rake propellers. And that clever design means you'll barely be able to feel them either.