Cathodic Protection for an A25

[dkirsop]

After my first season of ownership I hauled my boat and discovered I had a corrosion problem. The prop nut anode was completely missing and there was a pink surface blush on the prop. Mid-way through the next season I hauled the boat and found the anode pretty much gone, I replaced it with a new one and re-launched. At the end of the season all that remained was a slim washer sized ring under the head of the bolt. Something had to be done or serious damage to the propeller would occur. Note that the skeg anode is in reasonable condition.

Surprise.jpg

My prop shaft is isolated from the main engine, there is no electrical connection. This was confirmed with a multimeter connected between the shaft and the engine, so the corrosion had to be related to electrolysis between the shaft and the propeller. A larger anode was needed. I decided to use a "Guppy Anode" to provide the extra protection when moored.

Guppy.jpg

This is a commercial product but it is simple and less costly to make your own. An alligator clamp, #8 wire, a large anode and some fasteners is all that is required.

The first thing I did was test for electrical continuity between the alligator clamp and the anode. Good thing I did as there was an open circuit. The Guppy had been crimped to the wire insulation. An easy fix but annoying for having paid for a working product.

The alligator clamp was also too small to grip the shaft. I solved this with two hose clamps fastened to the shaft. The clamp screws were placed 180 deg. apart to achieve balance.

Guppy Clamp.jpg

Whenever I am moored I hang the guppy over the side of the boat. A quick check with the multi-meter recorded a current of several milliamps at my permanent moorage. If memory serves right this was in the 20-30mA range which is significant when considering electrolysis.

This season when I hauled my boat about 50% of the prop nut anode remained. Success!

Prop Anode.jpg

Below is a comparison with a new anode, 120g Old vs 220g New.

Old vs New.jpg

The guppy shows signs of corrosion but there is years of life left in it yet. I would estimate at least another five years.

I only use the guppy at my permanent moorage as this seems to be a local problem. When cruising I don't bother with it as this represents only a short period of time in comparison to my time at my home port. Hope others find this helpful.

[kerrye]

Is there a cotter pin which hold your prop nut on? If so, how does that anode fit over the cotter pin? If there isn't a cotter pin, what secures the nut to the prop shaft?

[dkirsop]

There is a cotter pin that fits through the prop nut and folds flat into a narrow slot on the nut. If you blow up the first picture you can just make out the slots in the nut. It's hard to see due to the zinc residue. The zinc then slips over the nut and is held in by a screw at the end.

[kerrye]

Thanks. I'll look for one of those. Does the zinc screw into the end of the prop shaft or threads on the end of the prop nut?

[WillieC]

How are you measuring current? I have a low end Fluke, if there is such a thing. Might have to upgrade.
You describe the anode loss we experience as well, only we are out in the middle of nowhere for much of our moorage. I made one as you describe, only had #10 wire at hand, close enough, and was able to measure millivolts at all the marinas we visited last summer. Quite a variance out there. I seriesed the meter between the clamp and the shaft to take my readings. I even kept a record, but at some point my brain switched to recording ohms. Doh! That's why I retired. Couldn't trust me around real voltages.

[dkirsop]

Kerrye, the screw threads into the end of the nut.

WillieC, I just use an el cheapo Radio Shack digital meter in series between my shaft and the clamp. No point in losing high quality tools overboard. I think the water in my marina warms during the summer month which contributes to the loss. I also think there may be a power imbalance in the nearby power lines giving rise to possible stray currents.

[dkirsop]

Kerrye, your question prompted me to go back into the shop and take a closer look at the prop nut. I noticed the PO had lengthened two of the slots to align with the hole in the shaft for the cotter pin. Easy to do with a hacksaw blade. Here is a picture. The screw at the top holds the anode on.

Prop Nut.jpg

[kerrye]

That makes sense. I had looked at some a month or so ago and didn't see any which had slots close enough to the front of the nut to fit the hole on mine. So lengthening the slots will work.

[DesertAlbin736]

Seems like stray current coming from somewhere more likely to be the culprit, ie electrolytic vs galvanic corrosion. How likely is it that zincs are being eaten up prematurely just from galvanic action of bronze prop in contact with s/s shaft alone without some source of stray current?

Good reading on the subject here:

https://bbsc.org.au/resources/Documents ... sy-SYC.pdf

Where do you dock/moor your boat? If I recall correctly unless you have access to a private dock the only commercial marina on S. Pender is Bedwell Harbour/Poets Cove. Other than that there's Port Browning and Otter Bay on N. Pender. If you're hanging on a mooring with access by dinghy only, then stray currents from other boats or faulty dock power shouldn't be a problem.

Still looking forward to getting back up your way next summer.

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[dkirsop]

I keep my boat at Thieves Bay, a private marina, with no dockside power. I suspect there is a stray current coming from the nearby power lines. There is a large transient population on the island and I suspect the power company is unable to maintain phase balance in its lines on a consistent basis. Any phase imbalance ultimately finds its way to ground, or water if the line is near the shore.

[DesertAlbin736]

I suspect the power company is unable to maintain phase balance in its lines on a consistent basis. Any phase imbalance ultimately finds its way to ground, or water if the line is near the shore.

Yeah, could be. Especially if there's no common neutral tying back to the substation.

So if you're on S. Pender then you must have to drive over the Pender Canal bridge to get to Thieves Bay?

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[WillieC]

" How likely is it that zincs are being eaten up prematurely just from galvanic action of bronze prop in contact with s/s shaft alone without some source of stray current"

My thoughts exactly. I have no 120VAC, the DC is bare bones original (larger alternator with isolated ground terminal) but massively cleaned up, the prop shaft is electrically isolated from the engine, the rudder skeg a few inches away has a zinc anode that never seems to erode, I moor hundreds of feet away from the nearest shore and at least that far from other boats on mooring balls. Bronze prop, SS shaft. None of the through hulls are bonded and they look like new. The Sub base is way around the Great Bend and there have been no reported sightings for many years in these parts. Then again, they are sneaky. And a couple months will eat the propshaft zinc at least 50%, maybe more as it looks like the bolts are ready to fall off.

One more question. My propshaft zinc, a plain old-fashioned donut style, is about half an inch away from the new super-duper resin fiber cutlass, direct from AlbinmotorSweden, between the cutlass and the prop. None of this fancy dan bullet zinc incorporated with the propnut stuff. (Tacoma Prop said that the bullet zinc is the preferred method, I just don't have room for it as the rudder is right there.) I also have a PSS shaft seal that blows water into the carbon portion of the PSS. It discharges under some pressure at the cutlass. Could that also contribute to the rapid wear on the zinc? And what does the carbon of the PSS add to the galvanic mix, even though it is on the inboard end of the shaft? (The PSS came with the boat and I had it serviced last spring in Mukilteo. Great folks there. I do not know if the anode erosion is different with the original stuffing box.)

Since we have the trailer, we regularly pull the boat out of the Canal in the summer, so keeping an eye on the anode is no big deal, just seems to wear more than what others are experiencing. The boat was previously moored at Squalicum in Bellingham, no trailer then, and the zinc was changed maybe annually by divers. I will be changing my own zincs, thank you very much.

[dkirsop]

The ABYC listing for Galvanic Series of Metals places zinc near the top of the list with a corrosion potential of approx. -1.0V. Stainless steel shafting is near the bottom at -0.15V. When connected and placed in a conductive fluid (e.g.. salt water) a current flow driven by -0.85V will occur and the zinc anode will sacrifice to the stainless steel shaft. It is the flow of electrical current that results in loss of metal. Silicon Bronze is listed in the middle of the table at -.27V so the zinc will also sacrifice to protect the prop. This is important as silicon bronze contains 2% Zinc.

Remember that pink blush I mentioned at the start of this thread? That was the zinc in the prop sacrificing to the stainless shaft. If left unchecked then the zinc will eventually leave the bronze and the propeller will become very brittle - not good. So it is important to make sure there is an anode at or near the prop.

Conductivity of the water will affect the rate of loss. In simpler terms the following will affect the life of the anode: water temperature, salt concentration and potential stray currents. Boats moored in fresh water will go largely unaffected. The anode must be large enough that it will last between regular periods of inspection.

The flow of water through your shaft log will have negligible effect on the zinc.

The bottom line is: so long as you maintain your zinc you have no worries, and the frequency of maintenance will be governed by the size of the zinc. The zinc must be connected either to the shaft or the propeller to keep the circuit intact.

[WillieC]

Great summary, Dave.
I might add to " The zinc must be connected either to the shaft or the propeller to keep the circuit intact." Obviously in contact with the water. A zinc added at the north end of the shaft in the bilge to serve as a backup for one's PSS shaft seal does not serve as a sacrificial anode as it's not in the water. If it is, replace your bilge pump.

Your point about changing the zinc as needed really tells the story. I can buy a lot of zincs for what I paid for my nice new prop.