Design for Industry Small propeller defects can make a big noise

The slightest deviation in the machining, polishing, and finishing of ships’ 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.

A Canada Transport-funded study on the impact of manufacturing tolerances on propeller performance (by the Memorial University of Newfoundland, DRDC Atlantic Research Centre, and propeller manufacturer Dominis Engineering) found the slightest change in propeller geometry resulted in “significant” cavitation much earlier than previously thought.

The behavior of a propeller blade section with leading edge defects of 94 µm, 250 µm, and 500 µm was studied using Computational Fluid Dynamics (CFD) at the DRDC Atlantic Research Centre and the Memorial University of Newfoundland in a three-year project that concluded last year.

“Experimental results show that current widely accepted propeller manufacturing tolerances as stated in the ISO standard need to be thoroughly evaluated and investigated further,” said Bodo Gospodnetic, project lead and president of Dominis Engineering.

The current tolerance for a defect to the leading edge of a propeller blade is 500 µm (0.5 mm).

Ship propellers are manufactured according to ISO 484-1, with most propellers made from castings rough machined on CNC (computer numerically controlled) mills and then finished using robotic and manual grinding. However, 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 e ciency.

“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.

Researchers found 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. For example, a ship with a propeller defect of 0.5 mm would have to sail at 45% 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.

“The 0.5-mm defect tested is one of the tightest ISO 484-1 propeller manufacturing tolerances, yet it has been demonstrated that it a ects cavitation inception significantly and detrimentally. The rules need tightening up,” said Gospodnetic.

ISO 484-1:2015 has been a standard for propellers since 1982, and although the standard was reviewed in 2015 and 2022, the allowable tolerance and geometry remain unchanged.

“We know that 80% of underwater radiated noise comes from the propeller, but if ships 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,” said Gospodnetic.

While initial CFD studies show how very small defects can influence cavitation inception, research partners seek funding to continue their investigation in second-phase model tests in a cavitation tunnel. DW