The impact of cavitation on performance

Cavitation and the associated noise can have a significant impact on the performance of a propulsion system. A complex and fascinating process, cavitation is a serious issue for propellor-driven vessels.

Cavitation forms when a propeller spins rapidly, it creates a low-pressure area behind the blades. If the pressure in this area drops too much, small pockets of vapour can form in the water, which then collapses as they move into higher-pressure areas, creating noise and damaging the leading edge of the propellor. 

Along with the well-documented increased maintenance costs and decreased efficiency, the collapsing bubbles can generate a lot of noise, which is increasingly a concern for superyachts operating in environmentally sensitive marine areas. To avoid cavitation, propeller design and manufacturing must meet strict tolerances and standards, however, a new study suggests that the problem may be more acute, and tolerances narrower than relations suggest. 

A recent Canada Transport-funded study conducted by Memorial University of Newfoundland, DRDC Atlantic Research Centre, and propeller manufacturer Dominis Engineering has found that even the slightest deviation in the machining, polishing and finishing of propeller blades could result in underwater radiated noise and cavitation, even if defects are within the maximum tolerance allowed by classification societies and the ISO 484-1 standard.

The three-year project involved studying the behaviour of a section of propeller blade with leading edge defects of 94µm, 250µm, and 500µm using Computational Fluid Dynamics (CFD) at the DRDC-Atlantic Research Centre and Memorial University of Newfoundland.

The research revealed that a ship with a "defective" propeller must travel at a given percentage slower than a vessel with a "correct" propeller to operate below the cavitation inception speed and remain quiet. The smaller the defect, the less speed reduction is required to remain quiet. For example, according to the study, a ship with a propeller defect of 0.5mm would have to sail at 45 per cent of the speed of a defect-free propeller to avoid cavitation noise. The smaller the defect, the less speed reduction is required to remain quiet.

According to project lead, Dominis Engineering President Bodo Gospodnetic, "The current widely accepted propeller manufacturing tolerances, as stated in the ISO standard, need to be thoroughly evaluated and investigated further. The current tolerance for a defect to the leading edge of a propeller blade is 500µm (0.5mm)."

The study also found that robotic and manual grinding of propeller surfaces introduces inaccuracies and deviations from the approved design, which can lead to cavitation, erosion, noise, vibration, and loss of propeller efficiency. "The leading edge is a very challenging area to manufacture accurately yet it has a strong influence on sheet, streak, and vortex cavitation," said Gospodnetic.

Although ISO 484-1 has been a standard for propellers since 1982, the allowable tolerance and geometry remain unchanged, even after being reviewed in 2015 and 2022. Gospodnetic believes that the rules need tightening up.

"The findings of the study have important implications for ship designers, builders, and operators as 80% of underwater radiated noise comes from the propeller. If vessels are legislated to be quiet in sensitive habitats, such as the Juan de Fuca Strait, then they will have to limit their speed to below the cavitation inception speed," concludes Gospodnetic.

The researchers are looking for funding to continue their investigation in a second-phase model test in a cavitation tunnel. The data from the study will form a basis for making decisions about the quality of propeller manufacturing precision required to meet ship noise objectives.

The full report is available via seabornecomms

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