Study Reveals Big Consequences Resulting From Tiny Propeller Defects
The slightest deviation in the machining, polishing and finishing of a vessel’s 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 was the finding of a Canada Transport-funded study on the impact of manufacturing tolerances on propeller performance released February 21. The study, carried out by Memorial University of Newfoundland, DRDC Atlantic Research Centre and propeller manufacturer Dominis Engineering, found that the slightest change in propeller geometry resulted in “significant” cavitation, and much earlier than previously thought.
The behavior of a section of propeller blade with leading edge defects of 94, 250 and 500 micrometers (µm, or 0.5 millimeters) were studied using Computational Fluid Dynamics at the DRDC-Atlantic Research Centre and 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 Dominis Engineering president. The current tolerance for a defect to the leading edge of a propeller blade is 500µm (0.5 mm.).
Ship’s propellers are manufactured according to ISO 484-1, with the majority of propellers made from castings rough machined on computer numerically controlled mills and then finished using robotic and manual grinding. However, robotic and manual grinding of propeller surfaces can introduce 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,” Gospodnetic said.
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 percent 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 affects cavitation inception significantly and detrimentally. The rules need tightening up,” Gospodnetic said.
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 has remained unchanged.