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Saildrive Ring Anode - VP MS25S saildrive

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symphony2:
This anodeoutlet.co.uk/blogs/anode-help-centre/aluminium-versus-zinc-anodes/ gives a reasonable explanation.

Aluminium based anodes have been used for some time on Mercruiser and Volvo outdrives which also have aluminium housings for some time, partly because in the US which is the major market brackish water is common and aluminium is more tolerant of this.

diverphil:
as symphony2 posted, alloy is what vp are now using, i went to the trouble of sourcing and turning it down as I already had a spare and a spare zinc one for keel, when I've used these then I will change over to alloy. as they are around £80 for the machined ones it made sense to use what I had already.

symphony2:
not sure why you would want an anode on the keel. It is cast iron so no chance of galvanic corrosion. It may waste a bit because the iron is not totally pure, but any corrosion of the keel itself will be simple rust and the anode won't stop that. Suggest you remove it and seal the hole well with eopxy. I have just done this on the sheet steel bilge plates on my current boat as again the anofes do nothing.

Older Bavarias don't need any anodes apart from the saildrive and folding/feathering prop and bow thruster if fitted. Post 2012 boats do however have a small additional hull anode, but this is a ground for the 240v system which became compulsory under the RCD that year. Not retrospective though. 

Bubble32:
Thanks for all the responses. I get the point that VP wouldn’t change from zinc to anything if the anything didn’t work. However, on my 2003 B32:
1. There are zinc anodes on the Bruntons prop & I have a few years’ worth of spares. Bearing in mind some of the responses above, I am not sure it would be a good idea to have zinc on the prop & (something else) on the saildrive.
2. Also, there is a secondary anode for the saildrive. I have no idea whether or not this does very much but the original paperwork with the boat suggests that it was an option when fitting out the new boat. I replaced this earlier on the year (https://yachtbubble.com/2022/03/15/first-coat-now-on-and-a-long-term-concern-resolved-tuesday-15-march-2022/). It is connected to the aft-facing gearbox cover on the saildrive. I can’t undo the bolt (one of the 4 on the cover) that grips the ring connector from the external anode.

symphony2:
There is no connection between the anode on the prop and the one on the saildrive housing. The 2 are electrically isolated. It would make absolutely no difference if one was zinc and the other aluminium. The other will likely not do anything until the main one on the housing is completely gone. The nature of galvanic action is between the cathode and the anode with seawater as the electrolyte and the anode closest will waste first.

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