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

One year my MaxProp zinc had disappeared, along with the screws and the blades were badly pitted--though not as bad as the OP's. PYI recommended "plating" the blades with the original bronze formulation and remachining the blades to the original design. They also tightened up the play and returned the prop looking like new. That was a number of years ago. They charged about $900, if memory serves, but that was less than the $2000+ cost of a new prop.

Since then, I have used Pettit Barnacle Buster zinc spray on the prop and shaft, with excellent results. The attached thumbnail shows the prop after haul-out this fall. The bottom and prop were power-washed by the boat yard, with no other treatment of the prop.

BTW, my boat is not at a marina and shore power is rarely connected, so the original problem--only once in 20 yrs, is a mystery.
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post #12 of 36 Old 12-11-2016
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Re: Propeller pitting

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Originally Posted by fallard View Post
One year my MaxProp zinc had disappeared, along with the screws and the blades were badly pitted--though not as bad as the OP's. PYI recommended "plating" the blades with the original bronze formulation and remachining the blades to the original design. They also tightened up the play and returned the prop looking like new. That was a number of years ago. They charged about $900, if memory serves, but that was less than the $2000+ cost of a new prop.

Since then, I have used Pettit Barnacle Buster zinc spray on the prop and shaft, with excellent results. The attached thumbnail shows the prop after haul-out this fall. The bottom and prop were power-washed by the boat yard, with no other treatment of the prop.

BTW, my boat is not at a marina and shore power is rarely connected, so the original problem--only once in 20 yrs, is a mystery.
Likely stray DC current from your own boat ...... bilge pumps are the most common cause although any DC conductor exposed to bilge water could have been the culprit.

The hysterical laughter you hear as you drive a way in your"new" boat ..... is the seller.

Last edited by boatpoker; 12-11-2016 at 09:24 PM.
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post #13 of 36 Old 12-11-2016
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Re: Propeller pitting

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Originally Posted by kbbarton View Post
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?
1.- The corrosion you experienced is not cavitation corrosion.

2.- There is no need to use Loctite on the anode mounting screws.

3.- The fact that you do not have any new dock neighbors doesn't not eliminate the possibility that a neighboring boat is causing the problem. Their electrical systems are not static and undergo changes and problems, just as yours does. That said, when stray current is a problem, it is typically an issue aboard the boat in question, not a neighbor's.

4.- Consider installing a limited clearance collar anode on the shaft as additional protection, if you do not already have one there.
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post #14 of 36 Old 12-11-2016
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Re: Propeller pitting

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Originally Posted by boatpoker View Post
Likely stray DC current from your own boat ...... bilge pumps are the most common cause.
You'd think that with bilge pumps and bilge pump switches being the most probable and/or most serious cause .... that someone would invent a low to nil amperage *alarm* (ignoring the typical amps draw when that equipment is either operating or at dead 'zero' amps draw ... the alarm activated at a set point amperage range 'between' these two) for bilge pumps and their 'switches'.
Otherwise on a routine maintenance/inspection basis, one has to physically measure for 'any' amps/current with a VOM .... and 'most' will forget to do so, like myself.
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post #15 of 36 Old 12-11-2016
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Re: Propeller pitting

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Likely stray DC current from your own boat ...... bilge pumps are the most common cause although any DC conductor exposed to bilge water could have been the culprit.
My bilge pump is dry 99.9% of the time. Exceptions, like when the raw water pump seal failed, are rare and short-lived events. The disappearance of the tail cone zinc and all 3 screws hasn't happened before or since over a period of 20 years and there do not appear to be any DC paths to seawater.
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Re: Propeller pitting

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My bilge pump is dry 99.9% of the time. Exceptions, like when the raw water pump seal failed, are rare and short-lived events. The disappearance of the tail cone zinc and all 3 screws hasn't happened before or since over a period of 20 years and there do not appear to be any DC paths to seawater.
You say "rarely" connected to shore power, not never.
You say your bilge is dry 99% of the time. not always dry.
While galvanic corrosion is a very slow process, stray current corrosion can be extremely fast. I know one vessel that lost two shafts, two propellers and two rudders within two weeks improperly wiring a new bilge pump. One demonstration in one of my corrosion classes dissolved an aluminum bar in a morning by leaking DC from a battery booster pack into a tub of salt water.

The hysterical laughter you hear as you drive a way in your"new" boat ..... is the seller.
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post #17 of 36 Old 12-12-2016
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Re: Propeller pitting

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Originally Posted by fallard View Post
My bilge pump is dry 99.9% of the time. Exceptions, like when the raw water pump seal failed, are rare and short-lived events. The disappearance of the tail cone zinc and all 3 screws hasn't happened before or since over a period of 20 years and there do not appear to be any DC paths to seawater.

This corrosion occurred over a matter of a few weeks. The cause? Broken insulation on a bilge pump wire. The owner was convinced it stemmed from his neighbor's powerboat and so refused to do anything about it. The prop ultimately fell off the shaft.

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Re: Propeller pitting

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Originally Posted by RichH View Post

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.
Only the machine screws used to secure the end cap and spinner halves will accept cotter pins or seizing wire. The screws used to attach the anode do not.
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post #19 of 36 Old 12-12-2016
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Re: Propeller pitting

The replacement anode screws, from PYI, do come with holes that I presume one could thread safety wire through. I've never attempted it. Seems it would introduce one more dissimilar metal to the mix and perhaps impact balance.

https://store.pyiinc.com/products/sc...ant=4653659457


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Re: Propeller pitting

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Originally Posted by boatpoker View Post
Likely stray DC current from your own boat ...... bilge pumps are the most common cause although any DC conductor exposed to bilge water could have been the culprit.
Would this be a submerged pump, more than a diaphragm with a hole pickup, or both?

I supposed a remote float switch, for either, is wet virtually 100% of the time.


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