Issue with propeller zinc

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Issue with propeller zinc

Postby tatoweb on Wed Jul 11, 2018 10:17 pm

Hi all,

I keep my C28 in the water all the time and have the bottom cleaned by a diver once a month.

A couple of times I’ve lost the prop zinc (while all the other zincs had more than half life) and thought the diver didn’t secure it properly and proceeded to put a new one. I’m doing bottom paint now and noticed how the prop zinc is being eaten around the bolt that secures it, so of course once the bolt side separates from the main body, it will fall. Here are some pics.

ImageImage

My questions are:

Does anybody have the same issue?

Is it normal for this zinc to be worn out much faster than the others?

Thank you.




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Re: Issue with propeller zinc

Postby NorthernFocus on Thu Jul 12, 2018 7:46 am

tatoweb wrote:...My questions are:

Does anybody have the same issue?

Yes. I trailer my boat and it stays in the water a week at a time for a total of six or seven weeks each season. I have to tighten the zinc a couple of times over the course of the season. Last year I had to replace it before the end of the season.

Is it normal for this zinc to be worn out much faster than the others?

Yes. When different metals are submerged in an electrolyte(like salt water) or are in contact with one another(like a bronze prop on a stainless steel shaft) you get a small electrical differential between the two which is the cause of galvanic corrosion. This is the principle behind dry cell batteries, e.g. NiCad, Li-Ion, etc.
Dan

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Re: Issue with propeller zinc

Postby tatoweb on Thu Jul 12, 2018 8:17 am

NorthernFocus wrote:
tatoweb wrote:...My questions are:

Does anybody have the same issue?

Yes. I trailer my boat and it stays in the water a week at a time for a total of six or seven weeks each season. I have to tighten the zinc a couple of times over the course of the season. Last year I had to replace it before the end of the season.

Is it normal for this zinc to be worn out much faster than the others?

Yes. When different metals are submerged in an electrolyte(like salt water) or are in contact with one another(like a bronze prop on a stainless steel shaft) you get a small electrical differential between the two which is the cause of galvanic corrosion. This is the principle behind dry cell batteries, e.g. NiCad, Li-Ion, etc.


Thanks for the info. Does your zinc breaks in 2 parts as mine?. I mean, if you don’t change it very often it will just fall?


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Re: Issue with propeller zinc

Postby trailertrawlerkismet on Thu Jul 12, 2018 8:31 am

Looks normal however there are a few things you can do to slow this down. When installing the prop zinc put a stainless washer between the nut head and zinc, this helps keep the zinc from falling off if the hole wears before you get a chance to tighten it. Next you can install a galvanic isolator to help prevent bad current from entering your boats electrical system, thereby helping to prolong all zinc life on your boat. You can also purchase a zinc guppy that gets attached to a boat ground and you then hang it into the water when you are docked. The guppy is just added sacrificial to help prolong your boat zincs.

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Re: Issue with propeller zinc

Postby baz on Thu Jul 12, 2018 8:57 am

I'm having great luck reducing my underwater zinc erosion by having shore power connected for just 24 hrs per week while moored at my Marina in salt water. This periodic 24 hrs is sufficient for keeping my batteries charged up. This process is a lot more effective for me than using and paying for a GFI and having shore power always connected..... I can do this as I live but a few minutes away from my Marina.

When disconnecting from shore power I physically mean this as I remove my dock side power cable from the dock side power pedestal and turn off its break switch.

My periodic underwater diver service has gone from 3 months, to 4 months and now is at 6 months.... and expect this to improve as well.

I do like saving money... :lol:
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Re: Issue with propeller zinc

Postby Tug Travelers on Thu Jul 12, 2018 10:52 am

I lose my prop zinc all the time. I added a shaft brush to tie the shaft and propeller to the main divers dream zinc on the transom. Adding a stainless washer definitely helps but I still lose them. I think that as they erode, they become loose and just spin off. I have noticed that the zinc does not fit tightly against the acorn nut that secures the prop to the shaft. It wobbles in there so as soon as there is a little bit of play it really seems to loosen up and fall off. I have not seen the zinc erode into two pieces like yours has.

I have spoken with Ranger Tug about it. It is a problem keeping these zincs on the propellor shaft. I have not been able to find a zinc that fits tightly on the acorn nut.
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Re: Issue with propeller zinc

Postby baz on Thu Jul 12, 2018 10:57 am

I added a shaft brush 'professionally' on my 2010 R-25 Classic to see if that helped. It did not. The issue persisted.
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Re: Issue with propeller zinc

Postby Red Raven on Thu Jul 12, 2018 12:06 pm

Lots of history on the troubles with that propeller zinc. A search on this site for for prop zinc and you get lots of examples and info.

I struggled the first few seasons to get the zinc to last through a 6 month season. Here is what we have learned and done to get 6 months or more. We now have plenty of zinc left when we pull it out in the fall.

1) as delivered on our boat the prop zinc was only zinc tied to the bonding system. I suspect this is common to all the classic R25’s and R27’s. The trim tab zincs are not tied to the bonding system because the trim tabs are screwed into the hull and don’t bolt through. The thruster zincs are also independent. The rudder and prop present a lot of metal to protect for such a small zinc. With the factory configuration in the salt water in the NW continuously it only lasted about three months. This is probably a suitable arrangement for occasional trailering use though.

2) as noted above, things are complicated by the poor fit of the prop zinc to the castle nut, it spins at high speed, and is held by only one bolt which gets loose as the zinc is consumed. Given all this it is surprising it stays on at all! Many have been able to improve this situation by using lots of thread locker (make sure it dries before submersion), adding a shim (stainless washer or copper penny) between the flange and the castle nut, and reducing the zinc consumption at the mounting surfaces (paint, tape, or washer). This will help the prop zinc stay attached longer but eventually it will still be gone without added zinc.

3) What else to do? Increase the amount of zinc tied to the bonding system. Easiest is to add a zinc fish you drop in the water when not under way. Somewhat of a pain though and you might forget to pull it when heading out! Next easiest is to add shaft zincs to the swim grid supports under the water line. The supports are already tied to the bonding system so you can just add the zincs and you are done. More difficult, you can add a transom zinc to the starboard side of the transom. Requires drilling holes through the boat and tying to the bonding system but allows for a large zinc to be installed that will last more than 6 months. This is what I did.

We also added a galvanic isolator which I recommend to protect your boat from other unprotected boats in the marina. It will not however protect your boat from its own galvanic potentials.

I hope this helps!

Curt
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Re: Issue with propeller zinc

Postby baz on Thu Jul 12, 2018 12:46 pm

Curt:

I hate to be negative about many of the solutions stated on this forum for dealing with excessive zinc erosion, but...I've previously done all the things you've mentioned on my 2010 R-25 Classic and all of it had minimal effect on the underwater zinc erosion. I spent a ton of money that was ill/badly invested IMO. I even opted to install a large plate anode on transom to help with the zinc erosion and for a new Smartplug shore power cord and boat's Smartplug power receptacle thinking that maybe my stock cable was not up to the task. The only way I've found to to reduce corrosion on the underwater zincs is to continuiously (once per week) disconnect my boat from shore power for a week at a time. I'm doing this now with my R-27/OB and the effect is significant per my diver service reports. My boat is in PNW salt water 100% of the time.

Having an outboard today means I have no prop shaft zinc to be concerned about, but on my R-25 it was a problem for sure.
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Re: Issue with propeller zinc

Postby Red Raven on Thu Jul 12, 2018 2:29 pm

Hi Baz,

Of course every situation is different. Not sure what is different in your case. All I can say is I have had 50% of my prop zinc still attached after 6 months using the above techniques the last two years. Before, it was gone in less than three months.

The improvement by disconnecting from shore power implies your issue is primarily the other boats in your marina. This “should” have been stopped by the addition of a galvanic isolator. Perhaps yours was installed incorrectly or perhaps there is something else going on at your marina. I don’t know.

All,

You can test the galvanic potential of your boat to determine if you have enough zinc/protection, with and without the connection to shore power by buying one of these.

http://www.boatzincs.com/corrosion-reference-electrode-specs.html

Curt
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Re: Issue with propeller zinc

Postby baz on Thu Jul 12, 2018 2:39 pm

Curt: I agree each situation can be different...

It appears your boat is not in salt water 100% of the time. Obviously this will/would mean less zinc erosion for you.

I've had my boat(s) moored at the Port of Edmonds for close to 10 years, and at 4 different slips. In all cases the underwater zinc erosion was severe. I did have the Marine's operations folk test the shore power pedestals each time I moved to a different slip and they indicated all was AOK. No doubt the Marina is 'hot' and quite likely some boats are not electrically grounded/bonded correctly.

So, my action was to take control myself and simply have my boat disconnected from the Marina's shore power for as long as I can between battery charging top-ups. My diver service told me this was the surest way to counter the zinc erosion and they perform service all around the Seattle area marinas so their advice was credible for me.... and so far the evidence is overwhelming.
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Re: Issue with propeller zinc

Postby Tug Travelers on Thu Jul 12, 2018 3:10 pm

Baz, I completely agree with disconnecting shorepower. I used to have a 28 ft Skipjack with twin duoprop outdrives. The zincs would only last two months. On that boat you had to haul it to replace the zincs as they were behind the propellors. What a pain. I tried all sorts of different things.

The Mercruiser Mercathode system that imparts a current in the water to protect the outdrives. Mounting extra zincs on the boat. The ONLY thing that significantly affected zinc erosion was disconnecting shorepower and adding solar to the boat to keep the batteries charges.

The real test for this is the Boat Zincs reference electrode. The difference in the voltage was painfully obvious when you plugged the shorepower in.

On our R31 I am trying the aluminum anodes from Boat Zincs. They say they have had good luck with them lasting longer and still providing the protection. I just changed the thruster anodes and the trim tab anodes after four months. I think it is a little better then before we added the galvanic isolator but I think the real answer is unplug from shore power.
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Re: Issue with propeller zinc

Postby baz on Thu Jul 12, 2018 3:33 pm

Tug Travelers:

Nice to get an opinion on this shore power disconnection process. Thanks.

I do have solar panel and in my covered slip and I do get full sun in afternoons on rear part of the boat. With this in mind I'm apt tho try going two weeks with shore power disconnected to further improve the zinc erosion. :)

The evidence is simply what the diver service tells me when they visit per a schedule. So far I've gone from 3 months, to 4 months and now am at 6 month diver inspections/visits. I'm expecting this to improve further.

This process is saving me money; less diver service visits, less zinc replacements, no need for spending money on the likes of a GFI, extra bonding installs for trim tabs, hanging a sacrificial gupy overboard, extra charges for electricity use by Marina, and so on. Of course, this process of having shore power connected for brief periods really is only practical if you live but a few minutes from your Marina.
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Re: Issue with propeller zinc

Postby captstu on Thu Jul 12, 2018 8:12 pm

The photograph of yours and corrosion shows a particular problem.

What each of the general solutions above appear to have merit and appear to be correct the problem you have is somewhat different.

When is it corrodes near the zinc’s attachment rather than uniformly across the whole zinc it is because the castle nut was bottom painted prior to installing the zinc.

There must be no paint on the castle nut, no corrosion on the castle nut and nothing that would stop a good electrical connection between the zinc and the castle nut. If any of these defects exist electrical circuit will be completed through the hex machine screw rather than thru the shaft and nut.

Next time you change your zinc, have the installer clean the castle nut until it is shiny clean. Then install the sink and tap it with a hammer until it is tight against the castle nut. Then put in the hex machine screw. You’ll still have a problem with zinc loss but at least it will be normal loss.


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Re: Issue with propeller zinc

Postby bill46 on Thu Jul 12, 2018 8:28 pm

If you keep your boat in salt water add a galvanic isolator. My since went from months to years.
Also prop zinc from factory came with new bolt and large SS washer. Diver said what is this, then recalled missing zincs and decided it was a good idea.
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