Anode dillemna: VP 130S saildrive and Flexofold prop

[David Light]

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

My setup is as follows:

• 2011 Cruiser 32 with a D1-30 engine and 130S Sail Drive.
• 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).
• Saildrive fitted with genuine VP ALUMINIUM anodes as per latest technical bulletin, and tested for good continuity to the saildrive leg.
• Flexofold 2-blade prop with small 32mm ZINC anodes on either side of prop hub.
• As far as I can determine (visually and by continuity and resistance test) the saildrive and prop are not isolated from each other.
• Vessel has a large MG Duff ZINC anode about 1m away from the saildrive which is connected to the vessels grounding.
• Vessel has a cast iron keel which is correctly grounded.
• Hull and keel antifoul is Coppercoat, professionally applied with correct primers on respective surfaces.
• Saildrive is antifouled with Propspeed, reapplied annually.
• Prop is not antifouled at all as recommended by Flexofold.
• 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:

• The vessel is on a swing mooring (not marina) in Singapore.
• The climate here is warm year round (tropics) which results in warm water, probably above 24 degrees average
• The swing mooring is in a channel with relatively high current flow, probably about 3-5kn avg
• There are estuaries and rivers upstream running into the channel resulting in high nutrients and organics in the water on the outgoing tides
• 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).
• 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:

• 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.
• The saildrive anodes are in good condition after 6 months showing relatively little wear.
• There is no discernible wear on the saildrive leg beyond corrosion which was already present from the previous owners
• 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:

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

• 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
• 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!

[symphony2]

First thing. You do NOT need the hull anode. It is not wearing because it is doing nothing. It is not factory fit. Bavaria have never fitted hull anodes on saildrive boats so it has been fitted afterwards by somebody who does not understand the principles of galvanic action. Second the keel is nothing to do with anything  else on the boat. I think the grounding wire you mention is connecting the keel to the mast for lightning protection. Coppercoat has nothing to do with any form of galvanic action except if it is used directly on aluminium.

The wear seems normal on your saildrive anode. So the problem is with the prop. I can't explain that and I suggest you contact FlexoFold. AFAIK the prop should be isolated from the saildrive. I had the same set up as yours except that the prop was an earlier version without the anodes and was trouble free for over 10 years.

I don't think any of the other factors (environmental conditions, the boat's DC systems or stray currents) have anything to do with excess wear. The mixing of anode materials on the same boat is not totally clear - some say it is not good others take the opposite view. The potential of the 2 materials used in anodes is virtually the same. However it might well be worth switching the saildrive anode back to zinc to see if it makes any difference.

[Yngmar]

Since your saildrive and prop are electrically connected, they must have the same anode material on them. You put aluminium on one and zinc on the other, so now you have a nice galvanic cell, with the more anodic (zinc) eroding away. So matching anodes will reduce the problem somewhat, but not eliminate it, because:

Saildrive and propeller should not be electrically connected. There's a rubber bushing in the propeller that avoids this. A common installation mistake is to omit the plastic washer that sits between the propeller cone and the propeller. The cone is unavoidable screwed to the prop shaft, but the washer (a wear part often discarded during improper servicing) is there to avoid it touching the propeller itself. This is at least how it works on the Volvo fixed prop (which doesn't even need an anode), I'm sure Flexofold has something similar or else it'll keep eating anodes for breakfast.

This washer: https://fybmarine.shop/volvo-penta-saildrive-cone-plastic-washer-part-number-850888

The rest of the boat is electrically isolated, because VP are not idiots, and thus doesn't get involved in this at all.

[tiger79]

First thing. You do NOT need the hull anode. It is not wearing because it is doing nothing. It is not factory fit. Bavaria have never fitted hull anodes on saildrive boats so it has been fitted afterwards by somebody who does not understand the principles of galvanic action.

Bavaria factory-fitted a hull anode to my 2014 boat, connected to the grounding circuit.  It's about 120mm diameter and is located just forward of the saildrive.

[symphony2]

That is for the AC circuits and only came in after 2013 to meet the latest RCD requirements. The OPs boat is 2012 so does not have that and as he says the big anode he has is connected to the vessels grounding which suggests the DC negative. However either way it has no effect on the saildrive or prop.

[Yngmar]

I'm sure Flexofold has something similar or else it'll keep eating anodes for breakfast.

Out of interest I did a bit of googling, and what I found is:

• Lots and lots of complaints where Flexofold props do eat anodes for breakfast.
• Flexofold installation manual has no sign of the propeller being insulated from the shaft, but it does have a big disclaimer for excluding corrosion damage.
• Flexofold has now released propellers with composite hubs (which presumably insulates the prop from the shaft).

Cynics may spot a typical corporate pattern here: Design flaw, denial, eventual solution released as "new improved version" 

[marioxp]

It seems that the propeller alloy is less noble than the aluminum anode (although according to the declared composition it shouldn't be). When the amount of zinc in the anode on the propeller drops, it begins to degrade.

I think you won't go wrong if you put a zinc anode on the saildrive instead of an aluminum one. More zinc might help.

But all this is happening to me very quickly.

[David Light]

Thank you for these replies! 

Foremost allow me to clarify that the detail provided above serves to provide context and avoid you having to ask questions to some of the more obvious or common things relating to an issue of this nature. I appreciate that to the trained eye some of these aspects may not be relevant. Sadly my eye is untrained and there are many possible causal links... 

Regarding the contentious hull anode, I'm the second owner of this boat. I confirmed with the first owner, who is based in Thailand, that Bavaria indeed did fit the hull anode ex-factory as an option for deliveries in tropical climates. They confirm that the other Bavaria's in their fleet, even before this time, also had this fitted ex-factory. This seems to check out since the cable connected to the hull anode is inside a sleeve with other factory fitted cables, including the fuel sender, and some engine wiring. Like tiger79, mine is also fitted slightly forward and off to starboard side of the saildrive. On the inside its under the aft-berth close to the diesel tank. So seems this was something that was maybe not common for the European and/or American market, but certainly for the tropics. Given that my hull anode has shown no wear in the last two years, I agree with symphony2 that it's not doing anything.

I should also add that I've changed all my thru hulls to Trudesign two years ago, so the only metal under the water is the keel, saildrive, prop and the bit of the rudder shaft that comes into contact with water as the boat rides the swell.

For context, I've been in touch with Flexofold for the past 18 months regarding this issue and their response has been fairly unhelpful. They attributed this rapid wear to the current/flow on the swing mooring and warm water temperature, and have happily sold me 13 sets of anodes which I've replaced at great effort every other month. I've more recently been in touch with them again and have had swift replies from them, although not entirely helpful either in resolving the matter. I'd like to think there isn't anything sinister about the fact that they are aware of this and have subsequently released a new composite model, since there are equally many happy owners taht are not facing the same issues as I am, and will instead try to understand what is different or unique in my application which could be causing this issue.

I agree with you that the rate of anode depletion on the prop is unusually high, hence the reason I reached out here for opinion and advice. 

Regarding the stray current, this has been foremost on my mind, but I did have a marine electrician test for stray current with the boat in the water, and then do some further testing when she was on the hard in December, and he reported no issues. All values were around -1,000mV which he said is normal and correct in salt water. But I think I need get a second opinion on this and have today ordered a Corrosion Reference Electrode from BoatZincs.com to test for myself. I will report back once I receive it and have done the tests.

So the issue of the prop isolation seems to be something to look into. As Yngmar points out, the prop should be isolated from the saildrive, which is achieved by a bushing inside the prop hub. Flexofold today confirmed that there are two plastic inserts that separate the bronze bushing on the drive shaft spline from the bronze prop hub. It seems that this bushing is a potential failure point, since the spline insert can push into the hub causing it to make contact, which may be exacerbating my issue as implied above. On this model of the Flexofold there isn't a plastic washer on the end-nut as described by Yngmar so I can confirm that there isn't any omission there. I've removed and refitted this prop three times (myself) already, and followed the instructions in the manual to the T every time. But as I recall, there WAS conductivity between the prop and the saildrive, and I suspect herein lies the crux of some of the issue. Unfortunately the manual doesn't specify an electrical test to check isolation (like the VP manual does) nor states that the prop must be isolated electrically from the drive shaft. Maybe it's common knowledge for some but it certainly wasn't for me! 

I also learned today that it is not uncommon for anodes on a hub to show greater/accelerated wear due to the increased friction caused by the flow across the anode as the prop spins. Apparently this is also an issue facing Gori props with protruding anodes on the hub. I'm ruling this out for the time being since I haven't used the boat in 6 weeks as I was abroad, and the rate of wear has remained alarmingly high.

Then there is the issue of the dissimilar anode material, which Flexofold has implied could have an impact, but as they do not sell aluminium anodes for this prop, suggested I change the saildrive anodes to zinc instead of changing the prop anodes to aluminium, and continue to monitor the situation. I was able to order aluminium anodes for this prop from BoatZincs.com which will come together with the Corrosion Reference Electrode. I'm going to fit both the zinc and the aluminium anodes on the prop alternately to test the corrosion potential of each respectively.

Thank you again for your thoughts and replies above!

[symphony2]

I expect that anode on the hull is the ground for the 240v circuits as fitted to post 2013 boats. There was similar on my 2015 33 - essentially the same boat as yours although it was a small rounjd button type rather than the larger one as shown in tiger79s photo. so it is not a sacrificial anode like the ones on the sailsrive.

[David Light]

Thanks symphony2! Would you mind explaining the function of this 'A/C' anode? Is it a 'ground' path for the earth of the AC circuit so that the RCD can trip?

Without hijacking this thread, I'm keen to learn about your 33. You're correct, your 33 is the upgraded version of my 32 so the same hull (as for the 34 btw). As I understand the upgrades from the 32 to 33 were mostly cosmetic, slightly larger berths (by moving the bulkheads a few millimeters), bigger saloon windows (would love those), larger mirrors (very important for SWAMBO), improved fridge insulation (mine is not too bad except for the lid), changes in the interior hand rails, and the addition of a second helm (option). Apart from the latter twin helm (option), moving the keel slightly forward, and the cosmetics aside, do you know of any major mechanical or electrical changes between these two models or the later 34 that address fundamental issues that were introduced in the 32? I'm asking on the odd chance that you know... 

[symphony2]

It is unclear why the 240v circuit has to be grounded as there does not seem to be a problem without it as there are thousands of boats built before the rule changed that are not grounded.

The main changes from the 32 to 33 were the completely new deck moulding to replace the BMW designed one on the original. This is most obvious in the windows and the cockpit which is wider and better shaped coamings. The interior was also completely new, designed in the UK with overall improvements in fit and finish. As you note the keel was moved a bit as was the mast location. Electronics  were Garmin whereas I think earlier boats were Raymarine. I have not sailed a 32, but mine with the shallow keel sailed well particularly when I changed the furling mainsail to one made from Vectran. I also added a top down furler cruising chute. It was built to order so had extras to make single handed sailing by an OAP easier such as the 30hp engine a bow thruster and extra winches. I had it for 5 seasons sailing up and down the S Coast from my home base in Poole. I sold it to take on a rebuild project on an old boat.

Not aware of any significant changes to the main mechanical and electrical systems except the introduction of twin wheels which came in just after mine was built as an option and epoxy resin infused hulls in 2017. I have not seen a 34 for 2 years since there was one on display at Southampton boat Show. it had lots of "new" extras like performance sails but no other real changes compared with mine. The shock was the price which to a similar spec to mine was 70% more than I paid in 2015.

[tiger79]

It is unclear why the 240v circuit has to be grounded as there does not seem to be a problem without it as there are thousands of boats built before the rule changed that are not grounded.

It's a safety issue, to reduce the risk of electrocution to a person in the water.

[David Light]

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?

[marioxp]

If the saildrive is isolated from the engine it is also isolated from the common DC negative and then saildrive should be viewed separately.

I'm interested in what values ​​you would get if there were no protective anodes on the saildrive and propeller at all.

I assume that as long as there is a anode body, even if it is worn out, it still provides protection, and you read this protection on a multimeter. But the question remains as to why they wear out so quickly. I think that now that both anodes are aluminum, they will wear out more evenly, although the anodes on the propeller are smaller, so it is to be expected that they will wear out bit faster.

If we look at the theory, zinc should be wear out before aluminum.

If I had put a zinc anode on the sail drive, and an aluminum anode on the propeller, if I had been you.

 if I were you, I would put a zinc anode on the sail drive and an aluminum anode on the propeller, and watched how anodes behave.