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Anodes- Zinc or Magnesium?

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Hi all again...

Thanks for your help with oil change!

I want to replace the anodes on my stern drives. I know that I should use Zinc for saltwater and Magnesium for fresh but just exactly where do we sit?

The boat will spend most of its time on the southern broads, so the area from Goodchilds where we are moored, Burgh Castle, with frequent trips to Brundall, Reedham, Oulton broad and Beccles. I would like to do the odd sea trip, but suspect that this is not enough in its own right to warrant zinc.

So can anyone tell me whether the water in the area is more fresh than salt???

What do you guys recommend?

kind regards

adam and suzanne

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For your proposed mooring (heavy brackish) and proposed cruising area neither, Aluminum on the drives for those conditions is the latest thinking and what I have used for some time now. If you want the science behind it then the MG Duff site is the place to go, what they don't know about anodes would fit on the back of a postage stamp, it's all they do, they are "anodes r us" :grin:

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Whole heartedly agree with David, Used Aluminium on Mercruiser legs on White Lady when I was in Burgh Castle next door to Goodchilds and they worked perfectly, they were about 1/3 used up after 12 months, have stayed with aluminium since moving to more or less fresh at Reedham but had the same problem you are about to have, due to the new boat being on Volvo engines and Legs.

Performance Metals do a range of high quality Aluminium anodes for Volvo drives and they came recommended from the big boys site. They were all the right shape and size, came with all the fixings and looked to be of very high quality, they also have a coloured indicator to show when they need changing but for what they cost I tend to do mine every 12 months when the boat is lifted. Cant comment on how they are wearing as they have only been on for 4 months.

Got mine from Aquafax Lutton. think they are also available from AR Peachment over at Brundal.


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Aluminium, as David says.

We changed our drives anodes fairly early on as boat owners and therefore before we'd fully got to grips with ownership. Our boat had been unused for four years when we bought her and the magnesium anodes were almost wasted. As we planned to take her to sea we were (incorrectly) informed the magnesium would disappear in the blink of an eye. Enquiring about aluminium I was told it wasn't available for Volvo 280 drives (at least not from Volvo). So we opted for zinc.

In the months that followed further research made it clear that was unwise on several levels and about 6 months later I changed the anodes for magnesium ones. The zinc ones had glazed over and were doing no good whatsoever. Fortunately there was no evidence of galvanic corrosion on our drives though.

My last look at those drives was almost exactly a year later when she was hauled out for a survey. The magnesium ones were fairly corroded by then but we had been to sea several times and spent a total of about 10 days moored at the RNSYC though our normal mooring at Brundall was in proper fresh water so somewhat different from Burgh Castle. So in my experience, the zinc is useless in fresh water and the magnesium does go pretty quickly in salt - all the evidence pointing to aluminium.

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well thankyou all for that...

and the general consensus is aluminium! And even speaking to volvo today. they would recommend the same.....the chap at Lowestoft Northgate marine was brilliant and spent over an hour on how I should service the engines... so i thoroughly recommend!

kind regards adam and suzanne x

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