Aluminium or zinc anodes?

[rbmatthews]

I have a dilemma. We are keeping our boat 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. Which is right, and what should I do?

The boat is a Jeanneau SO39i and the hull is GRP, not aluminium, if that is relevant. 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.

I have already read Vyv Cox’s article, and a few other posts on here. It seems like it might be OK to mix the metals, but confirmation or otherwise would be good.

 

[SimonD]

I have a dilemma. We are keeping our boat 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. Which is right, and what should I do?

The boat is a Jeanneau SO39i and the hull is GRP, not aluminium, if that is relevant. 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.

I have already read Vyv Cox’s article, and a few other posts on here. It seems like it might be OK to mix the metals, but confirmation or otherwise would be good.

Have you tried Boatanode24? Aluminium-indium-anodes - Opferanode24.de. They're a German company and have a very large range of aluminium anodes including ones not available from UK suppliers.

 

[Stemar]

I've no pretensions to expertise - someone with some will be along in a minute to explain why I'm wrong, but ISTM that anodes have an effect over a limited distance, so an isolated zinc anode away in the bow shouldn't affect ally anodes around the stern gear.

 

[Tranona]

I have a dilemma. We are keeping our boat 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. Which is right, and what should I do?

The boat is a Jeanneau SO39i and the hull is GRP, not aluminium, if that is relevant. 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.

I have already read Vyv Cox’s article, and a few other posts on here. It seems like it might be OK to mix the metals, but confirmation or otherwise would be good.

The alloy used for aluminium anodes has virtually the same potential as the zinc alloy. Galvanic action is nothing to do with your electrical system. The thruster anode is to protect the mixed metals in the construction of the gearbox and shaft and the anode relies on direct connection through the locking screw into the shaft. I assume you have shaft drive so will have an anode for the propeller, usually a shaft anode just forward of the P bracket and maybe anodes on the propeller itself if folding/feathering.

As already mentioned anodes have to be either directly connected as yours are or if hull mounted in line of sight to the things they are protecting and electrically bonded to them. Neither of these conditions apply to the thruster anode in relation to the stern gear. So no issues with them being different.

 

[vyv_cox]

See this page on my website Anodes

 

[TSB240]

Our boat was based in a very low salinity dock and fitted with aluminum anodes except for the bow thruster. (None available)
No problems encounterred.

 

[vyv_cox]

Our boat was based in a very low salinity dock and fitted with aluminum anodes except for the bow thruster. (None available)
No problems encounterred.

Was? Have you moved? I read that it went bust again, is it closed?

 

[TSB240]

Was? Have you moved? I read that it went bust again, is it closed?

Boat now in Dickies shed after breaking loose from Rowen Bay mooring in storm Noa last April. Should be good as new by next year!
Marina in administration but still in full operation. Some good news of new ownership is expected by end of month. I understand that they are local, sympathetic to the heritage and community views but have a greater synergy than any of the big Marina operators with the boating community.

 

[rbmatthews]

Thanks for all your helpful advice and comments, everyone. I think I'll try 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.

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. We'll see.