Miniscule defects in propeller blades that still meet class and regulatory standards can cause cavitation and increased underwater noise, according to a study funded by Transport Canada.
Researchers with Memorial University of Newfoundland, along with partners at Defense Research and Development Canada and prop maker Dominis Engineering, spent three years looking at the effects of different levels of propeller imperfections.
The team studied prop blades with defects below and up to half a millimeter — the maximum accepted level. What they found was that even the smallest change in propeller geometry could cause significant cavitation
“Experimental results show that current widely accepted propeller manufacturing tolerances as stated in the ISO standard need to be thoroughly evaluated and investigated further,” Dominis Engineering President Bodo Gospodnetic, the project lead, said in a statement.
ISO 484-1 standards govern the manufacturing of ship propellers. Most props are made from castings that undergo a machining process. They are then finished using robotic and manual grinding techniques.
“However,” the study said, “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.”
Underwater radiated noise is a low-frequency sound created by oceangoing ships sailing on the ocean. It can be harmful to whales and other sea creatures. Ships running propellers with these small but allowable defects must sail around 50 percent slower than a ship with a defect-free propeller to avoid making noise from cavitation. “The smaller the defect,” the study said, “the less speed reduction is required to remain quiet.”