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Now you see it, soon you won't - Honfleur's super-efficient propellors

Honfleur’s propellors – a closer look

The graceful, upward-curving wingtips of some modern passenger aircraft serve an important purpose. By smoothing out the airflow at the end of the wings, they make your aircraft more efficient and more environmentally friendly. Honfleur's twin propellers use the same theory, and have a dramatic new shape as a result.

 

"When they were first delivered, the guys in the yard thought they looked so odd that they asked us if they'd been made correctly," says Thomas Stoye, the propeller design specialist at FSG, the northern German shipyard where Honfleur is being built.

 

But their complex, graceful form is exactly as intended, and the result of some sophisticated computer modelling. It resolves one of the perennial problems of propeller-driven ships, and passengers will be able to feel the difference when they’re aboard.

 

A wing generates lift by creating areas of high and low pressure, and a propeller propels a ship by doing the same in water. A propeller design which generates higher pressures will pull a ship forward more efficiently, making it travel further for the same power and fuel use.

 

But this can also leave areas of very low pressure in which the seawater effectively 'boils', creating pockets of gas which quickly collapse again. This turbulence around the propellers - called cavitation - is one of the main causes of noise and vibration in the passenger areas of a ship.

 

CFD rendering showing reduced tip vortex propeller blade cavitation

 

Area of low pressure cavitation affecting hull

 

"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.

 

Aftbody underwater view of the tank test model showing shaft lines rudders and propellers

 

 

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