Increasing levels of underwater noise generated by shipping can threaten marine mammals and fish, which rely on sound to navigate, communicate and find food.
Now a new technology capable of reducing the underwater radiated noise generated by ships’ propeller cavitation has been developed by researchers at Strathclyde University and West Sussex-based Oscar Propulsion.
Cavitation occurs when tiny bubbles form in the water as a result of the propeller moving through it. When these bubbles collapse, they create an audible shock wave.
Propeller cavitation can generate as much as 180 decibels of underwater radiated noise, and can be heard by marine life 100 miles away. It can also damage the propellers themselves.
The new system, known as PressurePores, reduces propeller tip vortex cavitation by applying a small number of strategically bored holes in the propeller blades. This reduces the sound produced by the propellers, without significantly reducing their efficiency, according to David Taylor, CEO of Oscar Propulsion.
The team first tested the idea in a cavitation tunnel at Newcastle University, said Taylor. “We made some relatively randomly-placed holes in different propeller models, and compared these with propellers without holes, and with that relatively crude test we managed to get about a 14dB reduction in noise,” he said.
After this success they carried out computational fluid dynamics modelling (CFD) at Strathclyde University, which confirmed the result of their physical tests. They then used the CFD model to analyse the effect of more specifically targeting the location of the propeller holes.
They found the PressurePores system could reduce cavitation volume by up to 14 per cent, and underwater radiated noise by up to 21dB.
Finally, they further verified their results in tests on the sub-cavitating propellers used by the Princess Royal, a 19m research catamaran operated by Newcastle University.
They found that as few as 17 strategically-placed holes per blade tip were enough to reduce noise levels.
Although the system does have a small impact on efficiency, the company is now working with propeller blade designers to investigate whether this can be eliminated, said Taylor.
Propeller blades are currently designed to minimise cavitation, by curving them inwards, for example.
“You won’t have to do all of that if you have PressurePores, because you’ve already reduced the cavitation,” said Taylor. “Therefore you can improve the efficiency of the blades by not having to do things to reduce cavitation,” he said.
PressurePores can be incorporated into new propellers or retrofitted into existing devices. The system is particularly suitable for naval vessels, fishing fleets, offshore vessels and cruise ships operating in sensitive environments, the company said.
The technology can also be applied to all types of propellers, including pods and thrusters.