Zinc or aluminium anodes?

[RubyTuesday39]

I have a dilemma. We are keeping our Jeanneau SO39i in the Baltic with its low salinity, and I have been advised that it is better to switch from zinc to aluminium anodes for everything, as they work well both in sea water and brackish water, whereas zinc only works well in seawater. My understanding though is that you shouldn’t mix aluminium and zinc anodes due to the difference in anodic activity, the aluminium being the more active of the two, meaning that it will protect any zinc anode from doing its job. Therefore anodes should be either all aluminium or all zinc.

So far, so good. However, I have discovered that the anode on the Vetus bow thruster only comes in a zinc version. That would suggest that I must have everything in zinc. However, Vetus themselves have said that it is quite OK to mix zinc and aluminium anodes, which contrasts with most other advice I have had.

I am not sure if Vetus’s advice is based on the possibility that the bow thruster is electrically isolated from the rest of the boat’s electrical system (it has its own dedicated battery and relay system to the controls), but I have not been able to check this as the boat is in Sweden at the moment. In any case, I am not sure if the thruster system is actually isolated anyway as I presume it is connected to the main battery charger to keep its battery topped up.

So, which is right, and what should I do?

Also, does anyone know exactly how many anodes there are on a Jeanneau SO39i? So far I know that there is one on the prop, another one on the hull, and the one in the bow thruster. Does the engine (Yanmar 3JH4E) have an internal one, and presumably there is one in the hot water cylinder? Any others I should know about?

Any advice gratefully received.

Robin Matthews.

[Chris Meyer]

Dec 7, 2023 12:32:53 GMT RubyTuesday39 said:

I have a dilemma. We are keeping our Jeanneau SO39i in the Baltic with its low salinity, and I have been advised that it is better to switch from zinc to aluminium anodes for everything, as they work well both in sea water and brackish water, whereas zinc only works well in seawater. My understanding though is that you shouldn’t mix aluminium and zinc anodes due to the difference in anodic activity, the aluminium being the more active of the two, meaning that it will protect any zinc anode from doing its job. Therefore anodes should be either all aluminium or all zinc.

So far, so good. However, I have discovered that the anode on the Vetus bow thruster only comes in a zinc version. That would suggest that I must have everything in zinc. However, Vetus themselves have said that it is quite OK to mix zinc and aluminium anodes, which contrasts with most other advice I have had.

I am not sure if Vetus’s advice is based on the possibility that the bow thruster is electrically isolated from the rest of the boat’s electrical system (it has its own dedicated battery and relay system to the controls), but I have not been able to check this as the boat is in Sweden at the moment. In any case, I am not sure if the thruster system is actually isolated anyway as I presume it is connected to the main battery charger to keep its battery topped up.

So, which is right, and what should I do?

Also, does anyone know exactly how many anodes there are on a Jeanneau SO39i? So far I know that there is one on the prop, another one on the hull, and the one in the bow thruster. Does the engine (Yanmar 3JH4E) have an internal one, and presumably there is one in the hot water cylinder? Any others I should know about?

Any advice gratefully received.

Robin Matthews.

Hey Robin,

Great questions.
We are in fresh waters in Michigan with our SO389.   We actually use magnesium for the prop end and saildrive collar.
Our bow thruster uses zinc since we can't buy in any other form.  So far, it works fine.
I think there is one on the engine, but we have never changed it.

Chris

[Charlie-Bravo]

Distance between the anodes will have an effect, your thruster being at the pointy end will probably be some distance from your hull anode.

I have been reading up on these things recently as I have just purchased an aluminium motor catamaran boat , and was wondering what is best anode wise. My outboard motor has zincs, my hull should have an aluminium alloy with iridium, as I think all aluminium anodes have, ….. makes the anode get attacked before the hull as it doesn’t get an oxide layer like your average aluminium.

I would go with Navalloy anode on the hull , leave the rest in zinc, and look at them at the next haul out to see what gets eaten the most.

CB

[MartyB]

I'm in salt, but there is a major river that flows into the bay/marina I am at.
Be it good or bad, the aluminum anodes get eaten up faster than the zinc versions. From my standpoint, that is better than the non-eaten zinc versions.
From a ecological standpoint, the aluminum is a better anode. Zinc in heavy salt is better at protecting boat parts.
With this said, some brands have a better quality, and a bit more material than other brands. Martyr and Sea Sheild seem to be the most popular version in Puget Sound region from an availability standpoint.

Marty

[Don Reaves]

There should be nothing damaging about having both zinc and aluminum anodes. The issue is simply that the aluminum anodes will protect the zinc anodes as well as anything that the zinc anodes protect. Until the aluminum anodes are used up, of course, at which point the zinc anodes will take over as well as they can, given the salinity of the water.

As Charlie-Bravo pointed out, the bow thruster will probably be protected by its zinc anode due to its proximity.

[RubyTuesday39]

Thanks for all your advice and comments, everyone. I think I'll put the new aluminium ones on the hull and the prop, and put a new zinc one on the bowthruster and see how it goes next time we lift her out.

If anyone is interested, Vetus said they were thinking of making an aluminium anode for the bowthruster (a BOW5512D) at some stage, but that it wouldn't be for some time yet. In the meantime, they assured me the Zn one would work fine due to its distance from the other anodes, which is basically what Charlie-Bravo said.

R.

[ohana]

Hi - recommend you take a look at Vyv Cox website - see   coxeng.co.uk/metallurgy/anodes/

Vyv is very knowledgable and obliging and I would take his advice every time. If you cannot find a specific answer to your question on his site, he posts on the YBW forums and if you put your question there, or PM him on that site, Im sure he will respond

[RubyTuesday39]

Dec 8, 2023 23:26:28 GMT ohana said:

Hi - recommend you take a look at Vyv Cox website - see   coxeng.co.uk/metallurgy/anodes/

Vyv is very knowledgable and obliging and I would take his advice every time. If you cannot find a specific answer to your question on his site, he posts on the YBW forums and if you put your question there, or PM him on that site, Im sure he will respond

Thanks Ohana,

Yes, I have read the Vyv Cox article you mention, and also put my queries on the PBO site. Answers were similar to here. 

Thanks.

[Trevor]

Hello Robin,

Your configuration may be different to this but I thought the SO39i has two anodes as standard on the running gear. One is on the fixed prop and the other is on the prop shaft just forward of the cutlass bearing. If the boat has a stainless folding prop the prop anode may not be required.

The anode on the shaft protects the shaft, the prop and the P bracket and the P bracket is bonded to the engine ground ( and hence the shaft) by a copper braid glassed into the hull and attached to the p bracket.

The seacocks are isolated and not bonded to any cathodic protection system, so the SO39i left the factory without anodes attached to the hull.

Yours may have been modified but the factory setup was very simple and for the most part it seems quite effective.

Regards

Trevor

[RubyTuesday39]

Dec 13, 2023 10:33:08 GMT Trevor said:

Hello Robin,

Your configuration may be different to this but I thought the SO39i has two anodes as standard on the running gear. One is on the fixed prop and the other is on the prop shaft just forward of the cutlass bearing. If the boat has a stainless folding prop the prop anode may not be required.

The anode on the shaft protects the shaft, the prop and the P bracket and the P bracket is bonded to the engine ground ( and hence the shaft) by a copper braid glassed into the hull and attached to the p bracket.

The seacocks are isolated and not bonded to any cathodic protection system, so the SO39i left the factory without anodes attached to the hull.

Yours may have been modified but the factory setup was very simple and for the most part it seems quite effective.

Regards

Trevor

Trevor

Thanks for your thoughts on this. I am not sure what would have been the configuration when our boat left the factory, but when we bought it, it had just the three anodes - the one on the bow-thruster (which was a later addition anyway), a bolt-on pear-shaped one on the hull, bonded to the engine, and one on the FeatherStream propeller (specially made for the prop by Darglow Engineering, the manufacturers). The propeller is also a later addition, so I have no idea what the original configuration was - it may well have been what you describe. We have continued to use it as we found it, and it seems to work OK. I am assuming the shaft is protected by the prop anode.

The boat is in Sweden at the moment, but I will check all this out when we get back to her next year.

Robin.

[Trevor]

Thats an interesting difference to what I am used to. The only reason I know the configuration in some detail is I purchased a SO36i in 2007 and the SO39i is prettywell the same boat but 3ft longer. The SO36i was configured back then as I described which is the same as the yacht currently owned whch is a 2008, SO42DS.

[Charlie-Bravo]

Trevor doesn’t mention a hull anode, although you do, and perhaps they were not fitted as standard, and some don’t feel they are required …… each to their own.

Your prop anode primarily protects the prop, the shaft anode protects the shaft and helps longevity of the prop anode perhaps , which being the smallest generally get eaten first, the P bracket usually has a non conductive bearing in it , isolating it from the shaft and prop anode, and the P bracket would normally be connected to the hull anode along with an engine block connection via a dubious braided cable often hidden under gel coat internally.

Well worth checking continuity of P bracket to hull anode …. or shaft, with a cheap multi meter , as the braided cable is known to die from ‘green disease ‘, disconnecting it from any benefit of an anode.

Lastly, place the shaft anode 4 to 5 inches away from the P bracket, the bearing within the P bracket is water fed, placing the anode too close can seriously reduce its life.

CB

[cpetku]

Based on low salinity I would probably try aluminum, but one must check the condition of the anode occasionally when switching materials to make sure it isn't consumed too quickly. In my case (fresh water in Michigan) the Zinc's never showed any wear so I went to magnesium. If the anodes are'nt showing sacrificial wear, then they aren't really protecting anything. Magnesium gets pitted in a year and I change them every 2-3 years as the anode is consumed more and more. BoatZincs dot Com has a good selection and you can get a reference electrode there as well to determine if you're protected well enough. I used my electrode once when installing the anodes so in my case it was over-kill to buy it.