How do you distinguish between cavitation and corrosion of the prop? I have some deterioration on the leading edge of my propeller, but the rest of the propeller looks normal.
Mark Matthews responds:
As I understand this issue, cavitation and corrosion are separate concepts that can become related. With most propulsion systems, a number of dissimilar metals come into contact with one another near propeller. Stainless-steel shafts, bronze propellers, and salt water make the area an enticing one for corrosion. Sacrificial zincs on the propeller are designed to dissolve before the rest of the components do, so it's important to replace these on a regular basis. Stray electrical currents in marinas from faulty wiring and/or shore-power cords draped in the water, all contribute to the equation.
The leading edge of props operates much in the same way the leading edge of a sail does. That is to say, it is the most important part of the prop. When the leading edge gets eaten away by corrosion, cavitation—that is the introduction of air bubbles into the thrust generated by the propeller—results. Cavitation occurs when negative pressure on the forward or "suction" side of the prop is so low that air bubbles reach the high-pressure area on the blade and collapse. The result is that water vaporizes or "boils" due to the extreme reduction of pressure on the back of the propeller blade. This usually occurs at high speed and/or under load.
Many propellers partially cavitate during normal operation, but excessive cavitation can result in physical damage to the propeller's blade surface due to the collapse of microscopic bubbles on the blade. There may be many causes of cavitation, such as incorrect matching of propeller style to application, incorrect pitch, physical damage to the blade edges, and others.
As the bubble of water vapor moves along the blade surface, it collapses back into the blade causing propeller damage and noise. This type of vibration can usually be solved by a close inspection of the leading edge of the propeller and making sure that it is smooth, consistent, and free of any damage.
If the propeller is in good condition and the problem still persists, the cause may be a hull appendage (water intake, strut, etc.) up stream of the propeller.
The results of cavitation can be very severe. Deep pitting of the propeller blade can occur in a very short period of time and may even result in a thrown blade. Cavitation can also show up as a discoloration of the propeller blade, which can be an early indication that cavitation is occurring. Damage caused by cavition usually occurs in a well-defined pattern on one or all the blades, depending on the source. If the pitting or discoloration appears to be random, with no identifiable pattern, the cause is probably not cavitation but rather galvanic corrosion.
Here's hoping that this information is useful to you. Best of luck.