Corroded SailDrive Prop

[Pieter]

Hello,

For obvious reasons i had to change my SB prop (Volvo Penta MD 2030).
First I thougth the corrosion was caused by electrolyses, but the strange thing is it is only on one spade of the prop...Does anyone recognise this? What can be done to prevent? I read something abt corrosion by cavitation?
See attached pics
Thanks and best rgds
Pieter

[DougR]

Hello Pieter,

At first glance, the nature of the pitting appears to be the result of stray current corrosion, but after studying the photos, I believe the damage is the result of cavitation erosion.

If you examine the photos you will see a pattern of erosion beginning at the V shaped notch on the leading edge of the blade and curving up toward the trailing edge and tip of the blade. This pattern is evident on both the face and back of the blade (high and low pressure sides). What is happening here is that the prop struck something which either dented the leading edge of the blade or even took out a small piece. This damage caused flow disturbance and resulted in a stream of cavitation bubbles streaming aft and toward the tip on both surfaces of the prop. As the bubbles move away from the original disturbance, they begin to collapse, impinging on the surface of the propeller and physically knocking minute pieces of material off the surface of the blade. This will continue until the blade finally fails.

I have seen this same phenomena many times on aluminum sterndrive props. The prop might strike even a small stick at high speed, causing just a small ding of 2 or 3 mm in the leading edge of the prop. However this ding, when running at 35 or 40 knots, causes a narrow string of cavitation bubbles streaming across the blade. This will, in a surprisingly short period of time, cause a prop to throw a blade. For this reason high speed outboards and stern drives almost always run stainless steel props. The stainless props resist the dings to begin with, and if they do get damaged, they are less effected by the cavitation bubbles.

You might be able to get your propeller built up and repaired, but it looks pretty far gone to me.

DougR

[Pieter]

Thanks Doug for your detailed explanation. This prop indeed wont go back into the water.
I feel it is hard to prevent this kind of corrosion (a piece of wood floating around can make a dent in the blade), but keeping a closer eye on things wont hurt.
Thanks again.
Best rgds
Pieter