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post #1 of 36 Old 12-11-2016 Thread Starter
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Propeller pitting

Last spring, I replaced my zinc anodes with magnesium ones because I read that magnesium is more effective than zinc in freshwater. When I hauled the boat out recently, I was surprised to find that the magnesium anodes didn't seem to have corroded significantly but the propeller blades had a lot of pitting on them ("before" & "after" photos below). I would expect the anodes to be mostly corroded away and little corrosion on the prop. My first thought was that maybe one (or more) of my neighbors is leaking current into the water, but I have all the same neighbors now as I did the year before (when I had zinc anodes)... or my boat could've been the one leaking current.

A friend suggested that the pitting could've been caused by cavitation, but I'm not sure how likely that is since I have operated the boat under power the same this year as last year to the best of my recollection. I did notice this year though a few instances of a "thump" or other unusual noise when I had to suddenly shift from forward to reverse and apply a lot of power in order to halt forward motion.

Anyone out there have similar experience with propeller pitting?
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post #2 of 36 Old 12-11-2016
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Re: Propeller pitting

cause and effect...

replaced zinc with magnesium

I would clean things up and return to a good quality zinc, what you read may not be best for your situation..

you are chasing things that are not there.
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post #3 of 36 Old 12-11-2016
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Re: Propeller pitting

That's not a little bit of pitting either, If the prop manufacturer saw that he would probably recommend replacing the prop. Im not saying you should do that but a conservative approach would consider the damage to the prop.
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Re: Propeller pitting

Agree with the others .....
First consideration is to return to the usage of a Zn anode.

Such logic can be easily validated by checking with what most other boats in your location are successfully using. As regards the usage of Mg instead of Zn ... the acidity/alkalinity of the water is also a controlling factor in anode selection; seawater is generally slightly alkaline; fresh water is usually slightly acidic ... but not always and especially on the western side of the US depending on location (proximity to deserts, active mining or paper mills, etc. etc).

Another consideration could be: has the nature of the fouling attached to your and other adjacent boats severely changed from the recent past. Reason: attached various microorganisms, bacteria especially, can use certain microscopic (alloy) 'grains' of underwater metals as their nutrient source; thus, such pitting can be due to 'biology'. But, other adjacent boats would also have such a developed condition in their underwater running gear.
If you had an electrical leak from your bilge pump switch, or a high resistance through your engine bonding/ground connection, or a 'shared' electrical leak through your marina's AC 'ground' system, that Mg anode would probably show this, and your Mg anode looks to be relatively 'unused'.

Rx: Id imitate what the other boats in your venue are (successfully) using, as the use of Mg anodes may be entirely inappropriate due to the 'local' water chemistry and its acid/alkaline balance (pH).

Last edited by RichH; 12-11-2016 at 11:30 AM.
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post #5 of 36 Old 12-11-2016
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Re: Propeller pitting

What's the red in the bolt holes? Is it thread lock compound? I've never used it on my prop zinc. Could it have gotten underneath the anode and disrupted the electrical connection? Did you thoroughly clean the mating surface?

While Zn might be better for your specific location, it's odd that it didn't seem to work at all.

I agree with.... "don't fix it, if it ain't broke".... logic when it comes to anodes. I use one shaft zinc, while many use two. I hear pros/cons of having too much zinc and I haven't had a problem. Leaving my program alone.

Wait a min...... do you have an anode on the shaft behind the prop? If so, did you change all your anodes to Mg?


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post #6 of 36 Old 12-11-2016
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Re: Propeller pitting

PYI may be able to clean up the prop. We mailed our MaxProp in a few years ago because the blades were beginning to wobble more than I liked. They built up the gear teeth and re-cut back to spec. Did the same for a few of the blade pits, polished the whole thing and mailed it back within a couple of weeks. Looked brand new again. I think it cost around $600 - not cheap, but way cheaper than a new one.

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Re: Propeller pitting

Ditto on PYI repair experience. I sent mine in, when I found one blade's leading edge must have hit something submerged and had a ding in it. They require you send the entire unit. Came back looking brand new.


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Re: Propeller pitting

Ditto on PYI ... and there's enough corrosion allowance on the Max Prop blades to allow for some serious re-polishing.

'Blade wobble' correction can be as simple as a replacement of the small internal cone gear (and re-honing to fit ... if you know how 'prussian blue' can be used). Quite economical in comparison to an entire 'rebuild'.
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post #9 of 36 Old 12-11-2016
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Re: Propeller pitting

As a Certified Marine Corrosion Analyst, my money would be on a stray current issue. That pitting is significant. With pits of that size the invisible internal corrosion will be much worse than what you see on the surface.

The fact that your anode shows little wasteage may be due to the fact that it is not in good electrical contact with the hub. If there is no electrical contact, there is no protection.

1. Check with a multimeter, the resistance (ohms) between the prop and the anode should be less than 1.0ohm and 0.0ohm would be perfect. The greater the resistance the less protection. If you get O.L. on your meter, you have absolutely no protection. Even with good cathodic protection, it will not likely be enough to overcome stray current corrosion.

2. Have your boat checked for stray current by a qualified (not just one who says he is)marine electrician. damage of this type is almost always caused by ones own boat.
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Last edited by boatpoker; 12-11-2016 at 02:12 PM.
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post #10 of 36 Old 12-11-2016
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Re: Propeller pitting

Excellent post by BoatPoker - gets at 'the meat' of the possible problem.

Observation - If you have the graphics capability to ultra-enlarge both of the attachments to the original posting, you may see the probable effects of the 'over-use' of a 'thread-locker' (Probably Lock-tite™ 'red'). Minnewaska initially alerted us to this, in an earlier posting.
Such cyano-acrylics are known for their dielectric/insulating qualities eg.: Dielectric Breakdown Strength (kV/mm)=~25.
On especially the 'before' pic, one can clearly see the 'oozing' of the suspect substance in the bore holes (with and without cap screws present) of the anode; plus, there seems to be (but at a lesser extent) the same evidence on the 'after' pic.
Although the anode attachment cap screws on a max-prop are not the principal electrical/bonding connection between the anode and the (Max) prop - the circumferential flat surfaces on the aft end of the center hub and the flat base surfaces of the Max-prop M70 zinc 'are' - it is quite possible to 'extrude' the oversupply of Locktight in between these mating / electrical contact surfaces; thus, electrically isolating the anode from the propeller hub (Lock-tite™ covering all mating surfaces).

Rx: Dont slather LockTite™ onto the threads of the cap screws when connecting anodes, as the excess may 'extrude' into those important electrical bonding surfaces.
A mere single drop or two ... INTO the bore holes of the hub at near 'half depth' of where the cap screw will be when full tightened is all thats needed to help hold the cap screws in place. In this way, if there is an excess of the Lock-tite, that excess will be driven deeper into the bore holes instead of extruding to 'outside'.
A drop (or two) of Lock-Tite™ onto the threads of the cap screws will tend to stay near the outer portion of the threaded bore holes as the caps are screwed 'home' and then possibly transfer 'out and away' from the threaded connection and onto those critical mating surfaces when the final 'torquing' is performed. .... worse, virtually nobody 'properly' torques such allen-headed cap screws.

PYI does supply such cap screws with drilled 'castellations' of the 'cap' into which one can place a teeny cotter pin or small dia. seizing wire to help hold them place, just like the cap screws that hold the 'hub' together on a Max-prop ... if you want a 'belt and suspenders' remedy.

Last edited by RichH; 12-11-2016 at 04:02 PM.
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