Propeller erosion? corrosion?

Here is a propeller from a steel hull trawler, single screw. The prop is pretty lightly loaded but is exhibiting severe erosion of the blade surface (see below). This is happening on both sides of the propeller nearly equally. I suspect that we have a galvanic corrosion issue.


The propeller is manganese-bronze, would Nickel Alum Bronze be less susceptible?

 

What's the shaft made of and do you have a shaft ground?

 

It is a steel shaft and it is grounded.

 

Just the blade tips or the whole blade generally? Do you have another ground plate and how is the engine grounded? Are the zincs in good shape or are they all oxidized?

 

Most of the blade appears to be affected, though it more pronounced at the outer radii. I assume that is due to higher flow speed, though there is probably some cavitation there as well.
I am trying to find out the condition of the zincs when the boat was pulled. If they were completely gone, I think that is probably the issue, if they are perfect, then they aren't doing anything.

 

Just from the pattern I'd say galvanic corrosion. Manganese-bronze is funny in that went it corrodes, only the exposed area wastes. Looks like some tried to clean the blade and that is what left the pattern. Cavitation generally leaves a very irregular, but clean surface. Are the very tips truely bright, or is that just a photo artifact? That may be a cavitation effect.

 

Is the pic showing the pressure side of the blade? You said the prop is lightly loaded, what's your definition? Has the prop been repitched? What's in front of the prop, particularly in the outer radii? I'm asking because you may actually have a pressure-side cavitation due to negative angle of attack on the outer sections of the blade.

 

Doubtful that it is pressure side cavitation. The shaft angle is less than 1 degree and the hull is smooth and clean in front of the prop. It is a typical single screw installation with the hull creating an axial deficit at the top and bottom of the disk. I certainly expect some suction side cavitation at the top, and there is somevevidence of cavitation at the tip on the SF.

The shiny look at the outer edge is most likely due the sun shining on the prop and the camera washing out.

 

I see two possible causes.

1. The zinc on the prop shaft doesn't make contact with it.

2. There is electrical activity in the water surrounding the berth. That could be an offset voltage of the shore power ground if that is connected to the ship's ground or current carrying metal surfaces nearby.

Less obvious, but not entirely impossible is water pollution with strong acids. Check the PH with a test strip.

 

Manganese Bronze is really what is termed a high tensile brass. That is Brass with around 5% Manganese. There are very few new props made from this material now because it corrodes so easily.

One solution is to paint it. Epoxy > 150 microns then either a hard specialised prop antifouling or one of the newer silicone reisn (Epoxy silicone ).

 

Prop colour...a reddish pink ...is the typical indicator of galvanic or electrolysis damage . If you observed this colour change , Double check for electric leaks. Wear on your anode system is also an indicator. This leak could be coming from your dock. Does the boat have an isolation tranformer on its AC shorepower link ?

As Mike Johns stated, nothing wrong with a barrier coat of epoxy primer to reduce the exposed metal surface. Correctly prepare the surface and Use the epoxy specified for Props.

Normally I notice cavitation erosion first on the edges of the prop...your prop appears clean on the edges.

 

Thanks for all the feedback.

I don't think that a coating is a good long term solution, since it will likely break down anywhere there is any cavitation. We will have to investigate where the issue in the system is.

BTW
Many propellers are still made from Mag-bronze due to its lower cost (less copper and no nickel)

 

I've seen a lot of earlier Mag Bronze/Brass props painted to stop de-zincification. The paint can stay in surprisingly good condition.

A lot of props were cast in Mag Bronze a few decades ago, but they went out of favor around the world and the real bronzes predominate now ( even stainless steel came and went as a material of preference).

 

Worse case, you add an impressed current protection system.

 

I'll bet the shaft ground sucks.