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Old 07-07-2020, 23:08   #1
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Damage to propeller

I found some pitting on my new propeller and need advice.

I did all the tests with the special electrode over the side. The potential of the propeller shaft is -850 millivolts. This is good. I then checked for stray current by turning on the various DC circuits with no changes.


Then I measured DC milliamps from shaft to ground with a 40 milliamp reading. When I disconnect the boat ground from the submerged grounding plate this number falls to .3 milliamps. Less than 1 milliamp.

What is going on?
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Old 07-07-2020, 23:14   #2
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Re: Damage to propeller

Pitting on prop can be caused by cavitation, this is more likely in your case than electrical issues.

Could this new prop be "under" propped; i.e. have you run it at high RPMs?

Attaching a picture to the thread would probably go a long way in diagnosing the issue, but failing that you could google images for "propeller cavitation damage" and see if any of them look like what is happening to yours.
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Old 08-07-2020, 09:49   #3
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Re: Damage to propeller

Here are the pictures before prop was cleaned
The manufacturer of the propeller attributes this to stray current corrosion since zincs were present
The propeller is no undersized or oversized . The blades were replaced with smaller ones when engine would not reach maximum rpm. Now it does.Propeller does not run at high rpm.

Btw this is a 3 month old propeller.
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Old 08-07-2020, 11:14   #4
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Re: Damage to propeller

I once helped a friend of a friend change zincs in his then new to him boat on the hard. There was some shaft pitting but not too much. Turned out the PO installed the zinc with the protective paper(plastic?) still on it where it hugs over the shaft.

I was taught by a boat builder friend that the shaft prior to zinc installation must be nicely cleaned, buffed, etc. - should look as good as new, so as to make as close fit as possible. And more then one zinc is always better if there is room.
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Old 08-07-2020, 11:34   #5
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Re: Damage to propeller

2 shaft zincs and 1 one zinc on prop

I did check and no paper

Does anyone have any comments on the 40 milliamps reading? This is a test I read about to find stray currents.

The very helpfully representative at propzincs says that the 40 milliamps is created by the zincs and all is well. I bought the reference electrode from them and they are kind enough to answer questions. Note that this particular reading in milliamps was not taken using the reference electrode. I used this reference electrode to determine the potential of the propeller shaft.
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Old 08-07-2020, 12:41   #6
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Re: Damage to propeller

Certainly looks like stray current to me. 99.5% of the time stray current comes from the boat that is suffering the damage.

You seem convinced that your boat is ok. How about your neighbour ? The improvement when you disconnected the ground suggests the current may be coming from another boat. Do you have a galvanic isolator ? .... cheaper than a new prop.
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Old 08-07-2020, 13:13   #7
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Re: Damage to propeller

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You seem convinced that your boat is ok.
It would be a rare boater indeed who admitted that source of prop corrosion might be his own boat.
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Old 08-07-2020, 13:17   #8
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Re: Damage to propeller

I am no expert when it comes to this type of problem and it seems that you definitely have a problem.

I think I can see two different issues from your photos. The pink areas are dezincification caused by electrolysis and I believe I see some pitting which could be caused by cavitation.

I can't tell if you have a maxprop or similar featherer but I remember reading something by Steve Dashew years back that with flat blade props cavitation is correlated to tip speed. Something along the lines of when your top speed exceeds 900 FPM cavitation can become an issue. Usually an issue with higher revving diesels with not very tall reduction gears.

If you are in a Marina with dockmates I'd listen to what Boat poker said. Go out on anchor and redo your tests.
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Old 08-07-2020, 13:17   #9
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Re: Damage to propeller

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Originally Posted by fstbttms View Post
It would be a rare boater indeed who admitted that source of prop corrosion might be his own boat.
Heck, it would be a rare boater indeed that would admit most of his boats problems are probably due to his faults..
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Old 08-07-2020, 13:21   #10
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Re: Damage to propeller

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Originally Posted by fstbttms View Post
It would be a rare boater indeed who admitted that source of prop corrosion might be his own boat.
Ground faults are hard to find but easy to test for, disconnect all your batteries and see what happens to the readings. But leave shore power and ground plate connected.
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Old 08-07-2020, 13:21   #11
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Re: Damage to propeller

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I can't tell if you have a maxprop or similar featherer...
It's a Flex-O-Fold.
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Old 08-07-2020, 13:25   #12
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Re: Damage to propeller

Thanks for that! I can see it now.
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Old 08-07-2020, 13:26   #13
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Re: Damage to propeller

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It's a Flex-O-Fold.
Do you see the pink spots as well as the pitting or are my eyes failing me again?
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Old 08-07-2020, 13:27   #14
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Re: Damage to propeller

I'm gonna throw this out there.....could be a bad prop....a bad casting ??
I look at those photo's and its not indicative of "pitting" to my eye...

Bronze props could be made from nickel aluminum bronze or manganese bronze....so it's possible that materials in the casting are being " eaten"... maybe it's not bronze at all.

Prop could have been made in China....always suspect..... ??

Prop cavitation can also make for little " explosion" on the prop surface, causing those holes.

Is the pitting noted only on the back of the blade ?? Any pitting noticed on the front of the blade ? See above for prop cavitation. If the front of the blade is " clean" likely a "cavitation" problem rather than an electrolysis problem.

Is your boat " plugged in" while you are working on it.

What does the zinc look like after it's been on the shaft ? Is it still on the shaft. I note a zinc on the end of the prop, but are there others ?? Does the boat have a hull zinc ?? How long does a zinc last on your boat when it's in the water ??

If you see electrolysis or pitting on the prop, you will likely also see it on other bronze thru-hulls. Do you ??

Many questions. Trying to resolve this on a cruising forum without being there is difficult.

I note that the prop appears to some kind of " folding" prop...I'm not familiar with 4 bladed folding props, so can't tell materials are used here.


But to your question, any stray current on your boat could find it's way to the prop.

Have you switched off all electrical power prior to your test. Even switching off all power does not mean some or other wire inside the boat has not abraded and is touching something it shouldn't.

Bilge pumps are usually hard wired, check those.

Do your testing with the main electrical better switch " on" and " off"....

What about 110v wiring...??

Your shore power cable could be in the water at the dock, causing problems.

Neighboring boats can cause a problem.

The fact that you are getting different readings as you disconnect grounding plates could also be a clue.

Yes, many questions.....trying to get you to think thru' some of these....
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Old 08-07-2020, 13:34   #15
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Re: Damage to propeller

google "bronze prop pitting photo's"....many pics of "pitting available for your review....you will note that "pitting" has a defined "pitting" surface, unlike your photo's.
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