Anode dillemna: VP 130S saildrive and Flexofold prop

[davidlight]

I sincerely hope that someone here has the knowledge or experience to guide me through this situation.

My setup is as follows:

1. 2011 Bavaria Cruiser 32 with a Volvo Penta D1-30 engine and 130S Sail Drive.
2. Engine and sail drive correctly isolated and tested for resistance according to VP technical guidelines, and saildrive isolated from rest of vessel including gear and throttle shifter (tested).
3. Saildrive fitted with genuine VP ALUMINIUM anodes as per latest technical bulletin, and tested for good continuity to the saildrive leg.
4. Flexofold 2-blade prop with small 32mm ZINC anodes on either side of prop hub.
5. As far as I can determine (visually and by continuity and resistance test) the saildrive and prop are not isolated from each other.
6. Vessel has a large MG Duff ZINC anode about 1m away from the saildrive which is connected to the vessels grounding.
7. Vessel has a cast iron keel which is correctly grounded.
8. Hull and keel antifoul is Coppercoat, professionally applied with correct primers on respective surfaces.
9. Saildrive is antifouled with Propspeed, reapplied annually.
10. Prop is not antifouled at all as recommended by Flexofold.
11. Vessel has a solar panel and Victron MPPT keeping the batteries topped up as well as a Victron inverter charger which remains off when I'm not on the boat.

The conditions are as follows:

1. The vessel is on a swing mooring (not marina) in Singapore.
2. The climate here is warm year round (tropics) which results in warm water, probably above 24 degrees average.
3. The swing mooring is in a channel with relatively high current flow, probably about 3-5kn avg.
4. There are estuaries and rivers upstream running into the channel resulting in high nutrients and organics in the water on the outgoing tides.
5. From what I can tell, by taste at least, the water is still very salty even on the outgoing tides (although not nearly as salty as the Med for example).
6. The vessel is about 200m from the shore, and 20m from the nearest boat (2 boat lengths or thereabout).

[TL;DR] So this is my dilemma:

1. The Flexofold prop anodes are wearing away rapidly. They last all of a month, maybe 6 weeks, and I change them out as soon as they start flaking away. Where I've left the prop anode change for anything more than 2 months, the resultant corrosion on the prop has been severe, especially considering it is not even 2 years old.
2. The saildrive anodes are in good condition after 6 months showing relatively little wear.
3. There is no discernible wear on the saildrive leg beyond corrosion which was already present from the previous owners.
4. The hull anode is still in very good condition even after two years, but it is a 2.2kg anode so fairly chunky.

As is common in our community (and especially around the bar at our local sailing club), everyone has an opinion on what might be causing the rapid wear on the prop anodes. I've been told the following:

1. Anode wear is accelerated in warm waters, in waters with high organic concentrations, and in waters with high current flow.
2. VP are idiots - the engine and saildrive MUST be connected to each other so that a hull anode can further protect the saildrive.
3. Coppercoat is from the devil and will cause your boat to sink overnight. It is the single cause of ALL corrosion on boats.
4. There is an electrical source and/or another boat in my immediate vicinity introducing stray current into the sea.
5. Mixing anode types will cause the less noble anodes to wear even faster, in this case the small zinc anodes on the prop.
6. The problem is that I'm using aluminium instead of aluminum.

I'm sure there are many more opinions, but I'm really trying to stick to the science here.

The Flexofold prop is made from a combination of Nickel, Aluminium, and Bronze. If I understand the nobility of metals correctly, this alloy should be more noble (less active) than aluminium and zinc on its own, with zinc being least noble of the three.

As I see it the zinc anodes on the prop are wearing first because the zinc is the least noble (most active) metal on the saildrive and prop, and being small, they are wearing rapidly as they are sacrificial for the saildrive as well (which is relatively large). The wear may also be accelerated by the water conditions. Theoretically, once the zinc anodes are worn, the prop anodes should wear down next before the prop itself does, but for whatever reason this is not the case as we are seeing corrosion on the prop hub and blades too.

As a solution, I am contemplating either:

1. changing the saildrive anodes back to Zinc as VP use to supply for many years before they moved to Aluminium anodes as standard for seawater, so that both the prop and saildrive use zinc anodes. Incidentally then all three anodes on the boat (prop, saildrive, and hull) will be zinc anodes; or
2. changing the prop anodes to aluminium so that both the saildrive and prop use aluminium anodes.

Which would be better, and if neither, what else should I consider?

Thank you in advance!

[boatpoker]

Your aluminum anode is not aluminum. It is an aluminum alloy. An aluminum alloy on the same circuit as a zinc anode will actually protect the zinc. Mixing anode types is unwise.

Aluminum anode - alloy with 95% aluminum, 5% zinc, 0.02% indium and less than 0.3% of other trace elements.

Zinc anode - 99% zinc and trace elements of cadmium, copper, lead and aluminum.

The chart below shows voltage potentials of the different anode materials

If your propeller corrosion is as dramatic as you appear to suggest, it is unlikely that galvanic corrosion is the issue as it is a very slow process. Stray current is more likely.

In every corrosion survey I have ever done, the victim vessel was the master of its own disaster. I have yet to see a marina or another boat be the cause of stray current corrosion.

The proper way to trace the source of stray current corrosion is to buy one of these ... https://boatzincs.com/categories/too...electrode.html This comes with a pretty well written booklet on how to use it.

[Leadfree]

Because the prop is in motion, its zincs wear away more quickly. F.ex: I have sail drives and Gori props. Drive zincs last for years, collar zincs on props for months. Prop zincs pit and wear from the friction.

DEFINITELY isolate as per VP instructions. You say the prop isn't isolated, I'm not sure what that means in your case; Gori have a rubber bushing which isolates their props. But the isolation of the SD should cover the prop, I should think?

If as suggested above you do have a stray current issue, it is likely your own boat, so start there

[davidlight]

Thank you for these insights and inputs. I did have a marine electrician go through the vessel in the water and again on the hard, and they were not able to find any potential source of stray current. I took their word, but perhaps I should get a second opinion.

So this is the first item to check again. Then on to the second: To clarify, from what I can tell, the prop and the sail drive are not isolated form each other, but indeed the saildrive is isolated from the engine and therefore the prop is also isolated from the engine.

I've read and understood that the Flexofold prop hub should have an isolation bushing where it joins onto the drive shaft, but as far as I could tell from the installation (which I've done three times removing and refitting the prop for cleaning) it makes contact with the hub where nut fastens the prop onto the shaft. Looking closely again at photos of the hub, I see the bushing between the 'inner' hub where it slides onto the shaft spline, and the outer hub that hold the blades.

Assuming that is meant to provide isolation between the drive shaft and the prop, would this reduce the anode wear, beyond the friction wear described above?

[davidlight]

I've ordered the CORROSION REFERENCE ELECTRODE - thank you for recommending this.

[GordMay]

Greetings, and belated welcome aboard the CF, David.

I don’t believe I’ve ever seen a more detailed technical description of a problem, and it’s proposed solution.
I only wish I could help.

[davidlight]

Thanks GordMay! The devil's in the details!

[Leadfree]

Originally Posted by davidlight

I've read and understood that the Flexofold prop hub should have an isolation bushing where it joins onto the drive shaft, but as far as I could tell from the installation (which I've done three times removing and refitting the prop for cleaning) it makes contact with the hub where nut fastens the prop onto the shaft. Looking closely again at photos of the hub, I see the bushing between the 'inner' hub where it slides onto the shaft spline, and the outer hub that hold the blades.

Assuming that is meant to provide isolation between the drive shaft and the prop, would this reduce the anode wear, beyond the friction wear described above?

If I read you right, you are saying that you are missing the isolation bushing? By all means, install it...

[davidlight]

Unfortunately this is integral to the prop hub and not a separate component installed by the user. I'm attaching the photo (not mine) to try and illustrate where this bushing is located. As I understand they hydraulically press the inner hub into the plastic/rubber bushing and then press this assembly into the outer hub. The inner hub is then theoretically isolated from the outer hub.

But there are reported instances where this bushing could be damaged, pressing the inner hub deeper into the outer hub, compromising the bushing, which I then understand would cause conductivity between these two parts. It hasn't happened to me, but there is a user with a cat who had rope wrapped around his prop, causing the inner hub to push deeper into the outer hub that the entire assembly was scraping the saildrive anode. He reported a 4mm difference between his port and stbd hubs, so it clearly doesn't take much to compromise that bushing.

FWIW: This is the official word from Flexofold on this:

"Yes, you are right, there is a plastic ring at each end of a large rubber clutch hub.
Which is a built-in safety so that if you should get rope in the propeller blades, it must be able to slide so as not to damage your sail drive.
We call it a safety clutch.

It should be completely electrically isolated from the outer part with the blades."

All things read and considered now, I'm wondering if this is a significant part of my problem (granted, I'm exploring the stray current issue as I accept this may also be at play).

[Rae Simpson]

Hi David Light:

I'm working through similar issues except doubled.

I have a catamaran, dual engines with a C120 Sail drive and a C130 drive. I installed the Flexfold props in spring of 2024. They provided a nice boost in performance over the previous VP folding props from last century, but the propeller zincs were gone in under two months. The VP zincs would usually last about a year.

The biggest part of the problem - IMHO- is that the zinc on the Flexfolds are too small compared to the VP.

After the factory zinc's disappeared so quickly, I switched to aluminium zincs on the props and legs. After four months, the propellor zincs looked questionable so decided to change them out. They were stuck in place - apparently aluminium swells a bit as they corrode. A lot of prying and scraping got them out.

I found about 20-25% of the zinc remained. So I should get about 6 months life as is.
Marginally satisfactory.

I used the Boat Zinc CRE to chase ground leaks. That was helpful and I found some room for improvements - but - the zincs are doing their job - corroding away and protecting my underwater metals, just lasting for a shorter duration than I'm used to.

I took Flexfold's advice and did not antifoul the props. Bad idea. Reducing the exposed area of the metal will reduce stray currents potential and anode corrosion. Propspeed worked well on my old VP props.

Bob Olsen at BoatZincs was helpful in getting some of this sorted.

Sail drive electrical isolation tidbits: I used an ohm meters and tested conductivity from the engine side:

C120 is not isolated.
C130 has some isolation, the case was not conductive to the engine, but the shifter arm was conductive as were the mounting bolts and the two rubber covered bolts on the top of the transmission.

Hope this helps...

Cheers....

Rae

[davidlight]

Hello Again! I have received the Corrosion Reference Electrode as well as a set of Aluminium Anodes for the Flexofold prop hub both from BoatZincs.com and proceeded with the anode replacement and tests. I tested the hull potential using the Corrosion Reference Electrode carefully following the instructions provided in the manual, and the findings were interesting to say the least given the discussion so far.

At the outset, it was interesting to note that the aluminium anodes supplied by BoatZincs.com are significantly lighter compared to the zinc anodes supplied by Flexofold. I appreciate that aluminium is lighter than zinc, but was surprised by just how much. I will weight these with a digital kitchen scale in future for reference. Sadly the screws supplied by BoatZincs.com are not supplied with the thread locker already applied as those supplied by Flexofold. I've tried to find this type of threadlocker in vain. Any suggestions are most welcome!

Photo of zinc and alu anodes included for comparison.

With the electrode in the water, at slack tide, and the other end connected to the engine ground (alternator negative), and all breakers and isolators turned off, the readings were as follows:
- With the ZINC prop hub anodes almost entirely worn out, the reading was approx -1055mV. Photo included.
- After replacing with the ALUMINIUM prop anodes, the reading was -1060mV. Photo included.

Turning on the battery and subsequent DC loads had a -3-5mV effect at most, so negligible as I understand.

Following further inspection, the keel is definitely bonded to the mast, but possibly not to the DC negative (ground) as I initially suggested in my previous post. For the sake of completeness, I did a test to the keel bolts and the reading was -525mV. Photo also included for interest and reference.

I also measured on the sail drive, which being electrically isolated from the engine, would have produced a different result, but I didn't take a photo of this, and also can't remember the value, so will check this when I'm on the boat again this weekend.

Armed with this information, I understand then that I don't have a stray current issue coming off of my boat, but appreciate that this doesn't exclude stray current from nearby sources, and that I will need to move the boat to another location to test this. I am planning to do so this weekend.

What do you make of this? Could it really just be that these anodes wear down quickly because of where they are placed on the prop hub and their small size, coupled with the continuous high current flow over the prop on the swing mooring?

[davidlight]

Thanks for this reply Rae. Following similar advice I think I will PropSpeed the Flexofold on the next haul out. I was advised against this before as I was told that any damage to the antifoul then exposes one small part of the metal which then concentrates any corrosion to that spot. Not good considering, but the prop and hub is already damaged so I guess I can't do much further harm...

[housedad]

Hi,

Exploring the same kind of issue. My old prop an Eliche Radiche in some kind of Nickel Bronze Aluminum alloy did not corrode at all as long as I had it, but unfortunately is fell off after having stuff tangled in it (drifting fishing nets).

The replacement is a Flexofold 2 blade exactly as the one we have seen in this thread.

My engine is a Volvo Penta MD2030 and a VP saildrive leg. As described here the drive is insulated from the engine. In my case the isolation from the prop shaft to the housing is maintained. I have measured for any kind of continuity between the engine and battery minus to the leg and to the prop shaft as well as the prop hub, always come up with zero.

I do suspect my shore power setup but need to be able to understand what's wrong so I can fix it. I have also measured for continuity between the shore power earth lead and the engine/battery minus/leg and prop shaft, and found nothing.

First two years with the new prop we noticed that the anodes where totally gone but no serious corrosion on the blades or the hub.

Third year I was living on the boat sailing around for two months, this is substantially more that the previous seasons. Of course whenever moored in a marina I used shore power to get e.g. hot water, etc.

After last season the prop had some pretty ugly corrosion, pictures attached.

After a lengthy discussion in a Swedish forum I decided to paint the prop with the antifouling as the rest of the bottom. This fall is looked much better but there is still something going on.

I read this thread and think we are sharing the same kind of problem...

My boat is a Dufour 35 Classic, GRP hull, no anodes except on the drive leg and the prop. I can see no wiring to the keelbolts or anything else that suggest battery negative to water contact. Mast is deckstepped.

First pictures are the state after last season, some after cleaning.
I then sanded the prop and applied many coats of antifouling.

I do think the corrosion is related to my shore power but I do not understand how, and I do not understand why the prop is "attacked" NiBrAl is a pretty noble alloy and in the case of e.g. a stray current then if a understand correctly the prop must be the negative and I cannot see how that can happen since it's very well isolated from the rest of the boat.

Also the anode on the drive leg has little or no wear. Boat is located on the west coast of Sweden --> salty sea water.

Any ideas on what to check/look for is appreciated.

And I will of course follow continuation of David's dilemma

[Rae Simpson]

Live aboard and sailing activities likely increased zinc usage and corrosion for us both.

Testing voltage on individual circuits on my boat, most were well under the recommended readings. However when most circuits are turned on, the combined voltage is over recommendations - leading to increased corrosion potential and zinc consumption.

Also when props are turning - the rotation may increase the wear and reduce the life and the protection provided by the zincs - aluminum or other wise. Flexfolds have the zincs protrude from the prop - resulting in increased wear compared to VPs props where the zincs are largely imbedded.

I am checking my zincs more frequently - monthly or semi monthly - depending on how they corrode over the low use winter season. Next haul out (in the spring) I will apply prop speed to reduce the exposed propeller surface area, hopefully reducing zinc consumption and corrosion.

Ironic - if you don't use the boat much you might be OK with these props and zincs. But use the boat allot and zinc trouble are likely to raise their ugly head.

Cheers...

Rae

[boatpoker]

Originally Posted by Rae Simpson

Live aboard and sailing activities likely increased zinc usage and corrosion for us both.

Testing voltage on individual circuits on my boat, most were well under the recommended readings. However when most circuits are turned on, the combined voltage is over recommendations - leading to increased corrosion potential and zinc consumption.

Also when props are turning - the rotation may increase the wear and reduce the life and the protection provided by the zincs - aluminum or other wise. Flexfolds have the zincs protrude from the prop - resulting in increased wear compared to VPs props where the zincs are largely imbedded.

I am checking my zincs more frequently - monthly or semi monthly - depending on how they corrode over the low use winter season. Next haul out (in the spring) I will apply prop speed to reduce the exposed propeller surface area, hopefully reducing zinc consumption and corrosion.

Ironic - if you don't use the boat much you might be OK with these props and zincs. But use the boat allot and zinc trouble are likely to raise their ugly head.

Cheers...

Rae

If you are not using one of these, you cannot measure corrosion potential.
https://boatzincs.com/cre/?srsltid=A..._27kdTS0_byJt2