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Old 19-10-2017, 00:08   #1
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Fitting a Zinc Anode question

Hi folks,
It's been a while since my last post on Cruisers Forum but here I am back again after some advice.

I just pulled my boat out of the water for some long overdue maintenance and found the s/steel prop shaft end has corroded away up to the prop nut.When I took the nut off the shaft looks ok with plenty of thread still so I don't have to replace the shaft, thankfully.

Here's a pic of the problem with the nut still on -

https://imgur.com/JTU0xHU




THe corrosion occurred because there was no zinc anode fitted before it went into the water, and the boat was moored at a jetty for nearly 3 years.

So now I want to fit a zinc anode but don't know how to go about it with the prop setup on this boat. There is not enough room behind the prop for a zinc and I don't think I can fit an Anode Nut because of the limited space between prop and rudder. Here is another pic to explain what I mean. (I'm using a different website to host the images so I hope they work ok). -

https://imgur.com/X0XEFjp




Can an efficient anode be fitted to a prop setup like mine, or is the some other way I can protect the shaft from further damage?

Thanks,
bony.
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Old 19-10-2017, 01:12   #2
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Re: Fitting a Zinc Anode question

U need a new shaft and prop.
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Old 19-10-2017, 08:44   #3
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Re: Fitting a Zinc Anode question

You definitely need a new shaft and likely a new prop as well then use a good nut zinc .
Considering that dammage I would also suspect dammages to your engine, transmission, and any other metal items that are thru the hull. ( Thru hull fitting and associated valves) . Basically your entire bonding system is suspect. ( That is if you even have a bonding system installed. If not I would recommend that you install one when you get done repairing everything)
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Old 19-10-2017, 09:09   #4
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Re: Fitting a Zinc Anode question

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Old 19-10-2017, 10:31   #5
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Re: Fitting a Zinc Anode question

If you really can't fit the above pictured zinc, you can use a brush on the shaft inside the hull wired to an external zinc.
I've never seen a prop shaft that looked like that. If it is made from 304 stainless you may have some serious intergranular corrosion.
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Old 19-10-2017, 10:44   #6
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Re: Fitting a Zinc Anode question

Pretty sure he can use a prop nut zinc, and a collar zinc like fstbttms posted. It may be that he has to buy a slightly longer shaft, but I think a new shaft is in his future anyway.
If I were to fit an external anode, I’d bond it directly to the engine block myself.
I’ve been considering an external anode myself
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Old 19-10-2017, 10:52   #7
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Re: Fitting a Zinc Anode question

That prop is toast, completely de-zincified it will break soon.
The shaft is also toast. That corrosion extends way up inside the shaft like the one pictured below and it was in better shape than yours and still broke off.
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Old 19-10-2017, 11:34   #8
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Re: Fitting a Zinc Anode question

[QUOTE=a64pilot;
If I were to fit an external anode, I’d bond it directly to the engine block myself.
I’ve been considering an external anode myself[/QUOTE]

Not sure making contact through the gears, bearings, couplings etc is the best bet. They sell a brush kit for the shaft or you can make your own. Very simple.
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Old 19-10-2017, 12:40   #9
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Fitting a Zinc Anode question

Quote:
Originally Posted by Guy View Post
Not sure making contact through the gears, bearings, couplings etc is the best bet. They sell a brush kit for the shaft or you can make your own. Very simple.


It doesn’t, the bonding connection is from the engine block direct to the anode, same as the ground wire to the negative side of the battery bank.
Now if you have one of those soft mounts that goes between the transmission and the coupling half, then of course your going to need that brush you talking about, cause your driveshaft isn’t electrically connected to the Engine and the rest of the bonding system of the Boat.
You can also run a jumper strap across the non metal disc too.

I have wondered if unbonding all my thru hulls, and electrically isolating the prop and shaft isn’t a better way to go as opposed to all metals bonded together and just a prop nut zinc.

On edit, I am not saying delete a prop zinc, but also adding in another large hull mounted zinc maybe.

My concern if I were the OP would be with my thru hulls, are they also dezincified?
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Old 20-10-2017, 00:56   #10
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Re: Fitting a Zinc Anode question

Thanks for the replies. I figured I might need a new prop shaft but won't really know 'til I get it out and have a googd look. The corrosion appears confined to the very end of the shaft but it is hard to tell without removing it from the boat.

The full story about this problem is that I spent a number of years completely renovating the boat and during that time I was given a second hand prop shaft which was in much better condition than the original shaft which had serious wear at that point where it passes through the stuffing box/packing gland.

As the donated shaft was too long I got a local engineering bloke to cut it to length and turn a new taper on the end to suit my prop and to cut a thread for the prop nut. The guy was slightly out cutting the taper which resulted in the prop sliding further up the shaft about half and inch. This put the prop closer to the hull than I wanted because I had planned on fitting a zinc anode to the shaft in that space behind the prop. It also meant I had to put a large spacer in between the prop and the nut, as can be seen in the photos.

Time went on, renovations continued and the need to do something about finding a method to fit a shaft anode was put on the backburner. When the renovations were finally completed, the boat was relaunched but in the meantime I somehow forgot about fitting the zinc anode. The boat was moored in seawater at a jetty which had no shore power but there were other boats moored nearby. Due to health problems and family matters the boat got little use for almost three years, but I did go down there fairly regularly to clean it and run the engine etc.

Fortunately, my corrosion problem appears to be confined to the prop and the end of the shaft. My through hulls are as good as the day I fitted them and the engine, gearbox, and shaft coupling all appear to have not suffered any damage. That part of the prop shaft which is inside the boat shows no sign of corrosion either.

When the weather improves I will pull the shaft out of the boat and have a closer inspection of it. I don't know what the grade of the stainles steel is and I understand what you are all saying about pitting and intergranular corrosion. I will take that into account when I inspect it more closely.

I doubt if I could fit a collar zinc with the setup I have at the moment but if I have to get a new prop and shaft I'm thinking I might be able to go a couple of inches longer to give some room behind the prop for a collar zinc. It would mean moving the rudder out a bit further but I think it could be done, hopefully without too much hassle.

I will also definitely look at electrical bonding for the boat. It has none at the moment. I will need to read up more on the subject to get my head around it but it sounds like it's something I should have done three years ago.

I will add some more photos later when I have the shaft out.

Thanks again to everyone for the replies.

bony.
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Old 20-10-2017, 05:09   #11
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Re: Fitting a Zinc Anode question

I don't know where you are located but I would contact the people at Aquamet about some shaft material. As stated that prop looks dead. Hit one of the blades lightly after you take it off. If it doesn't ring and aol you get is a dull thud you should think about replacing it. That prop may make a suitable spare. With a single engine boat you really don't want to be taking chances with the running gear. You could make the shaft an inch longer to give you room for the collar zinc. Replacing zincs should be an annual check up.
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Old 20-10-2017, 05:14   #12
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Fitting a Zinc Anode question

May be that you do not need a longer shaft, just a proper taper?
I would not do anything to cause me to move the rudder, I’d fit a hull zinc before I did that.
You don’t want too much shaft overhang, you will get prop vibration. If you do.
Also you can fit a prop nut zinc if you don’t have room for a collar zinc
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Old 20-10-2017, 06:02   #13
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Re: Fitting a Zinc Anode question

FYI:
Here ➥ Propeller Clearance Diagram - Cruisers & Sailing Photo Gallery
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Old 20-10-2017, 07:49   #14
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Re: Fitting a Zinc Anode question

A shaft brush as mentioned earlier connected to the boat's bonding system along with an external anode that is also connected to the boat's bonding system should end this problem. And consider using a diver's plate style for your anode because they have large surface areas.

Also recommend that you measure the system's performance with a half bridge to verify your anode is doing its job.
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Old 20-10-2017, 20:03   #15
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Re: Fitting a Zinc Anode question

Quote:
Originally Posted by dsharp View Post
I don't know where you are located but I would contact the people at Aquamet about some shaft material. As stated that prop looks dead. Hit one of the blades lightly after you take it off. If it doesn't ring and aol you get is a dull thud you should think about replacing it. That prop may make a suitable spare. With a single engine boat you really don't want to be taking chances with the running gear. You could make the shaft an inch longer to give you room for the collar zinc. Replacing zincs should be an annual check up.
I'm located in Australia. I will be looking at fitting a new prop anyway because I wasn't happy with the one I had. I think it's the wrong size or pitch for the boat.

Thanks.
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