Electrolysis/Corrosion on my Prop - SailNet Community

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  #1  
Old 03-06-2006
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Unhappy Electrolysis/Corrosion on my Prop

I have a 1995 Hunter. When I pulled her this winter I noticed extreme corrosion of my prop. When I purchased the boat in Late 2004 I saw no indication of this on the prop. I am having prop replaced and obviously want to prevent this from happening again. All of my seacock show no signs of electrolysis.

What can I do/should I do to protect my prop and boat. From what I understand this could be coming from my boat or one near by me in the marina?

Are there things that I need to check and are there steps I can take to protect this?

Thanks!

Matt
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Old 03-06-2006
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Matt,

The stray a/c can be difficult to find.

Did the propeller/shaft have anodes when you purchased the boat?

Your propeller installation probably has one (or more) sacrificial zinc anodes. Make sure that you keep an eye on their decay once you have installed the new prop. This may require a dive once a month or so. If the a/c near your dock is suspected to be high - move the boat to anchorage before diving on the prop/shaft. If the decay rate is such that the anode(s) will be less than 40-50 percent before your next scheduled haul, you will have to move the boat, or find the stray current. Talk to the marina operator - they may be helpful.

Propeller replacement is expensive enough that if the decay continues at a rapid pace, you should hire a marine electrical technician to help you find the source, or at least verify that it is not coming from your own equipment.

Wayne

Last edited by wwilson; 03-06-2006 at 11:37 AM.
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Old 03-06-2006
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Other causes

We noticed pitting on our prop and think it may have been helped by anti-fouling paint that was applied without the proper undercoatings. We've since been more careful about applying priming coats. We still renew the zincs too, though.
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Old 03-06-2006
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painted prop

paulk,

Have you found a paint that protects the propeller for a season? Most paint doesn't adhere to the prop blades long enough to provide much protection if the boat is driven under power for any length of time. I've pretty much given up and just clean and polish out of the water once a season. I've tried one or two paints that claim to protect a propeller, but ended up being more advertising hype than benefit.

I'd be interested to hear if you have found better results.

Wayne
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Old 03-07-2006
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Offline,

Your post didn't comment on the status of your shaft zincs. Those are intended to protect other components like props. If you didn't have any in place, there's your problem. If you did and they eroded away, start using an extra zinc hung into the water off the stern, connected to th engine. There a model shaped like a fish at most stores. If you have this problem with the zincs in place, there probably some poor quality material in the prop or shaft.
To prep the prop, use a zinc prop paint such as: http://www.pyiinc.com/index.php?sect...u=mppaint&sn=2 This stuff has worked Ok for us, keeps growth off the prop, but not much to do about the problem of corrosion.
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Old 03-08-2006
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Prop Paint

sailingfool,

Thanks for that PYI, Petit paint URL. I have a MaxProp - its good to see that the paint comes recommended by PYI. I'll give it a try this season.

Wayne
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Old 03-08-2006
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Quote:
Originally Posted by offline2004
What can I do/should I do to protect my prop and boat. From what I understand this could be coming from my boat or one near by me in the marina?

Are there things that I need to check and are there steps I can take to protect this?
As previously mentioned, keep an eye on your zincs. Everything working as it should, in a saltwater environment you should get 6-9 months out of them. Significantly more or less is indicative of a problem. There are many factors that can affect zinc life. The most common ones are electrical problems aboard your own boat. If you notice rapid zinc depletion, have a marine electrician go through your boat's system to see if he can pin down a cause. Installing an isolation system can help protect you from external causes of electrolytic corrosion. Again, a marine electrician can advise you on this.

If your boat lives in a freshwater environment, it is important to note that anodic protection should be provided by magnesium anodes, not zinc. The zinc anodes do not have enough electrical potential to overcome the greater resistance inherent in freshwater.

Last edited by Fstbttms; 03-08-2006 at 10:05 PM.
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Old 03-10-2006
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I unfortunately do not know what the status of the Zinc's were. Unfortunately this is a case where ignorance was not bliss.

With the new prop they machine shop put on a new on set of zincs the prop shaft.

Walking the yard I have noticed that several people have 2 sets on the shaft. Is this a case of more is better or and I simply going to have 2 sets of zincs deteriorate at the same pace as 1 set?

Thanks!
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Matt

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1995 Hunter 336 Hull #100
Herrington Harbour South
Friendship, MD
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Old 03-10-2006
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Matt...more is better...as one of the other posters said, you can also hang a zinc "fish" from a backstay to allow you to monitor the "sacrifice". The electrical problems may NOT be comming from your boat as we have noticed vastly different rates of zinc attrition in different marinas where other boat's and the marina's electrical system may contribute to th problem. Check with other owner's nearby to see if they have the same problem.
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Old 03-13-2006
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Quote:
Originally Posted by offline2004
Walking the yard I have noticed that several people have 2 sets on the shaft. Is this a case of more is better or and I simply going to have 2 sets of zincs deteriorate at the same pace as 1 set
In theory, if you have one zinc on the shaft and it lasts X months, two zincs will last 2X months. It is possible to over-zinc your boat but I think you would have to really load up for this to take place. Two zincs on the shaft is a common configuration. If you decide to hang a "fish" over the side, the inboard end must be in direct electrical contact with the item for which protection is intended. This usually means termination on the transmission or shaft itself. Only clip it to the backstay if your rig is bonded to the engine. Remember also that the zinc's physical distance from the shaft/prop will affect it's ability to provide protection. The proper solution is to have adequate zinc on the shaft.
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