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Old 18-04-2018, 23:56   #1
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Prop shaft anode is wasted - how urgent is replacement?

I have a bronze propellor on a stainless steel propellor shaft and my prop shaft anode is quite wasted. I am currently in Palma, Majorca and everything here is ridiculously expensive! To save money I would prefer to wait until I am somewhere cheaper in a few weeks when I will get the boat lifted to have the hull spray cleaned and replace the anode at the same time rather than pay a diver to replace it now.

Could anyone please give me some advice on how urgent replacement of the anode is and how rapidly galvanic corrosion will occur to the bronze. Is it safe for me to wait a few weeks or is it a false economy and will it be costing me more in corrosion than the cost of a diver?
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Old 19-04-2018, 00:02   #2
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Prop shaft anode is wasted - how urgent is replacement?

I’m curious about this too...

My uneducated guess is it won’t happen too fast if you’re not plugged in or around other boats that are.

My anodes have lasted a little over a year now because I haven’t been on shore power, but I do wonder about how fast they need to be replaced when they do go.

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Old 19-04-2018, 00:05   #3
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Re: Prop shaft anode is wasted - how urgent is replacement?

No one can answer that question and as my father used to say "how long is a piece of string?"

Do you have a galvanic isolator fitted? they help.

Are you permanently connected to shore power? that is bad news for anodes.

Are you using a generator? again not good news.

Why can't a diver fit a new anode, not a difficult job even for a sports diver rather than a professional in return for a crate of beer.

Now we need a new anode each year because whilst it has some life left in it, the anode won't do two years without completely wasting away. How often are you changing anodes?

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Old 19-04-2018, 00:31   #4
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Re: Prop shaft anode is wasted - how urgent is replacement?

If you have to ask whether replacing a sacrificial anode is urgent, then it's urgent.

You have the sacrificial anode there to protect something. What does the protection is the surface area of clean zinc, aluminium, or magnesium (i.e. whatever metal you are using as the sacrificial anode).

Over the life of a sacrificial anode, its effective surface area changes: 1. by consumption (being sacrificed); and 2. by residues on the surface of the metal developing that prevent the active metal (Zn, Al, Mg, whatever) contacting the water.

You CAN measure the protection granted by your anode by using a half-cell, such as a silver half cell. Anytime that the voltage produced by your anode drops below the level required to protect whatever metal you're protecting, then you needed to replace that anode yesterday.

And just looking at your anode is a good clue.

Let's say you started out with a 400 gram zinc anode on a 25 mm diameter prop shaft. Now you look at it and you guess that it's only half the volume and so probably has a mass of ~200 grams and so is wasted.

Time to replace it?

For sure.
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Old 19-04-2018, 00:32   #5
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Re: Prop shaft anode is wasted - how urgent is replacement?

Unfortunately no-one in Palma does anything for a crate of beer - they are used to superyachts and charge superyacht prices!! We are heading for Sardinia and Sicily and I was hoping to find somewhere cheaper there.....
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Old 19-04-2018, 01:23   #6
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Re: Prop shaft anode is wasted - how urgent is replacement?

Connect a wire from your boat zinc anode to a brush on the ss shaft inside the boat.

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Old 19-04-2018, 02:35   #7
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Re: Prop shaft anode is wasted - how urgent is replacement?

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Originally Posted by Paramotorgreg View Post
Unfortunately no-one in Palma does anything for a crate of beer - they are used to superyachts and charge superyacht prices!! We are heading for Sardinia and Sicily and I was hoping to find somewhere cheaper there.....
Okay, understood. when was the anode last changed and how much is left now?

As Eric says Add another by hanging one over the side with a wire to the negative bus bar or the shaft, either way so long as it will make a circuit.

Minimise the use of of shore power and be ruthless doing this.

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Old 19-04-2018, 02:53   #8
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Re: Prop shaft anode is wasted - how urgent is replacement?

It would suck if the prop was damaged. Penny wise dollar foolish.
So question.... how do you know that it is gone? Did you dive down? I change mine out all the time. Takes a few dives but it gets done
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Old 19-04-2018, 04:21   #9
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Re: Prop shaft anode is wasted - how urgent is replacement?

You say “quite wasted”. If there is some still left it is still doing its job and will likely last a couple of weeks until you haul. If there is significant stray current around you then ymmv between now and when you haul.
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Old 19-04-2018, 08:49   #10
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Re: Prop shaft anode is wasted - how urgent is replacement?

Do you own a mask,snorkel and screwdriver? It ain’t rocket scientry...how’d ya know it was deteriorated to begin with? Probably take 10 minutes...
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Old 19-04-2018, 09:22   #11
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Re: Prop shaft anode is wasted - how urgent is replacement?

I have a similar question about prop shaft anode. Currently in Grenada and been slightly over two years on this anode. We NEVER go into marinas or even use mooring balls; always anchor out and usually well away from other boats. The divers that have been scrapping the bottom for us says zinc is still fine. Various mechanics have told me I don't even need anodes, but I don't buy that. I will have bottom scrapped before I leave for Cartegena in a few weeks and will just replace them regardless. But my question is whether I need them or not? I don't plan on not using them, just want some knowledgeable answers. I do use generator daily, but had a marine electrician completely rewire all 120 and 12 volt on entire boat including generator before leaving Florida. Does anyone have an answer on this?
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Old 19-04-2018, 09:30   #12
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Re: Prop shaft anode is wasted - how urgent is replacement?

Because of the dis-similar metals and electrical currents within and potentially around the boat I would not take a chance not to have the protection. View it as insurance.... although you never go into slip, you never know what may happen in the future. Better safe than sorry..... Just my opinion.
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Old 19-04-2018, 10:51   #13
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Re: Prop shaft anode is wasted - how urgent is replacement?

Quote:
Originally Posted by sher9016 View Post
I have a similar question about prop shaft anode. Currently in Grenada and been slightly over two years on this anode. We NEVER go into marinas or even use mooring balls; always anchor out and usually well away from other boats. The divers that have been scrapping the bottom for us says zinc is still fine. Various mechanics have told me I don't even need anodes, but I don't buy that. I will have bottom scrapped before I leave for Cartegena in a few weeks and will just replace them regardless. But my question is whether I need them or not? I don't plan on not using them, just want some knowledgeable answers. I do use generator daily, but had a marine electrician completely rewire all 120 and 12 volt on entire boat including generator before leaving Florida. Does anyone have an answer on this?
90% of stray current (electrolytic corrosion) issues happen at a dock/marina, but that does not mean it can't be an issue in the form of galvanic corrosion out in some remote area. If your boat is bonded, and it's sitting in salt water, it's essentially a battery. Your risk is going to be a factor of the salinity of the water.

In short using zincs is cheap insurance. They are a lot less expensive than new props and shafts and thru-hulls.
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Old 19-04-2018, 11:01   #14
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Re: Prop shaft anode is wasted - how urgent is replacement?

Hi Pete, I dont know what you mean by "bus bar" and I am not sure exactly what you mean by a circuit if you have a fish dangling in the water off of wire that is attached to your prop shaft inside the boat. Could you either draw me a diagram or add some extra! explanation to this, Please.
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Old 19-04-2018, 11:03   #15
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Re: Prop shaft anode is wasted - how urgent is replacement?

I would worry more about the BRONZE prop than the S.S. prop shaft, and if the anode is half or more gone, it's time to replace,also the anode should be as close as possible to the prop on the shaft.
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