Propeller Cavitation Damage

[Tree]

Hi All,

Has anyone ever suffered with this from their 28TE? Reason for asking is we hauled the boat out yesterday and after 5 months the propeller is fried!!

First thoughts were electrolysis but on closer inspection I am not convinced as the propeller still has a good ring to it. The Leading edges of the blades are gone but noticeably the forward side of the propeller has damage to it, the stern side although has some slight green to it isn't that bad in comparison.

When we launched last august we noticed that there was some slight vibration above 3400 rpm - checked the engine alignment and everything is within tolerance - the only change we did make was to fit a rope cutter, exactly the same model fitted to my buddies 28 and he has had no issues.

Also, we did find three broken earth wires on the rudder post mount, one of which we didn't find for three months so this will also had contributed to the problems - the anodes have clearly been working as they are due for replacement but as a result nothing else anywhere is showing signs of corrosion.

I've attached these pictures so opinions and thoughts are appreciated - I am intending to see the designer of the rope cutter tomorrow hopefully as he has a huge amount of knowledge on propulsion.

[Tree]

One thing I did forget to mention is when you scratch the surface, where it appears dark in colour its is bright material underneath as you can see in picture three

[jleonard]

To me it looks like electrolosis. It has that "reddish" tint. If it were cavitation/ventilation the wear would be more even.
I would take ot to a prop shop.
What does the shaft look like?

[Tree]

The shaft is fine, as is everything else. Whats concerned me is the pattern of damage on the front side of the prop where as there is little or no damage on the rear side.

There is some galvanic corrosion on it that may be the cause of the prop burn but without a doubt, the prop has a good ring to it despite all the damage - it is salvageable.

Where would i need to be looking for the anode isolation for the prop shaft? Is it earthed through the gearbox or straight off the engine?

[RobS]

...Where would i need to be looking for the anode isolation for the prop shaft? Is it earthed through the gearbox or straight off the engine?

Did you check continuity between the prop shaft, engine and bonding circuit with a VM ? And did you have a collar zinc on the shaft?

[Tree]

Rob, there was never a zinc on the shaft originally however I've stopped off at the yard on the way home tonight to check the continuity and guess what? Yep you've guessed it. Zero!

So I ran a cable to the gearbox from the busbar next to the engine and then retested from the propeller shaft back to various points and had a reading of 566 ohms and it was within a few ohms depending on where I I tested (skin fittings, rudder trim tabs etc)

Propeller specialists view was that the electrolysis has caused some cavitation damage.

I guess that when the engine was refitted (10 hours prior to the purchase and not forgetting the boat was sat for 8 months out if the water) the mechanic never refitted the engine earth to the zincs busbar. I can check to be certain tomorrow as the weather stopped us from pulling the engine today but I bet I find a cable in the bilge.

[RobS]

Every time I change a zinc (engine or exterior ones), I check continuity, overkill I know but it just takes a second to do. When you get the prop reconditioned be sure she gets at least a medium cup, that skeg creates a fair amount of turbulence and you will lose performance and get vibration without a decent cup.

[Tree]

Medium cup being a 5 cup? I do have the prop from Jodi T here which is identical so if need be they can take dimensions from that to see me over until I can have the new prop made that is now on Jodi T and giving significant differences.

[Carl]

Better check those engine zincs too! May want to look into a galvonic isolator as well once you get the system checked out for continuity.

[RobS]

Yes 5 to 7

[Tree]

Better check those engine zincs too! May want to look into a galvonic isolator as well once you get the system checked out for continuity.

I will also be testing the marinas electrics too!

[Yearwood]

It is electrolysis. Electrolysis does not cause cavitation damage. Cavitation damage is generally founds on the non-pressure side of the propeller blades.

[whwells]

Please do have someone check for stray electrical on your docks/slips. Are other boats effected in the same area? Someone may have a hot wired dangling.
Would really want to Dock Master to do some serious checking for the problem.
whwells "Howard"