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.