Aluminium anodes in fresh water

[lumphammer]

We are wintering this year in S Brittany in the Vilaine River, which is supposed to be fresh to brackish water. In theory in these conditions we should be using aluminium or magnesium anodes, but as we are only going to be here till end of March I'm wondering what the consequences of just leaving the nickel (Edit: Of Course I really mean Zinc) ones in place. We will probably get hauled out for a scrub off before we leave, so will use that as opportunity to replace/clean existing anodes.

I could get a hanging aluminium anode. What would the likely consequences be of just leaving things as they are? Most of the time we will be disconnected from any shore supply.

 

[VicS]

We are wintering this year in S Brittany in the Vilaine River, which is supposed to be fresh to brackish water. In theory in these conditions we should be using aluminium or magnesium anodes, but as we are only going to be here till end of March I'm wondering what the consequences of just leaving the nickel ones in place. We will probably get hauled out for a scrub off before we leave, so will use that as opportunity to replace/clean existing anodes.

I could get a hanging aluminium anode. What would the likely consequences be of just leaving things as they are? Most of the time we will be disconnected from any shore supply.

Nickel ones....???? I hope you mean zinc.

Zinc will tend to loose its effectiveness...... a hanging aluminium anode will restore the protection. It might need a clean off itself if the water is more nearly fresh than brackish but magnesium would have a short life if the water becomes more brackish than fresh.

Clean off the zinc anodes before returning to salt water.


Remenber a hanging anode must have a sound electrical connection to what it is being used to protect.

If you connect to shorepower for any length of time fit a galvanic isolator if the shorepower earth is bonded to the anode(s)

 

[viva]

Yes I assume you have zinc not nickel. I keep my boat on the vilaine and I use a magnesium anode as I think the water is closer to fresh than brackish. Don't plan on buying anything other than a zinc anode in France, the French don't seem to see the need for anything other than zinc and I had to buy my anode in the UK. See my post on Scuttlebutt. I connected a stanchion to the anode terminal on the inside of the hull and I hang the mag anode from the stanchion using a crocodile clip

 

[rafiki_]

Best to use magnesium in fresh water. The consequences of getting it wrong are very expensive!

 

[Quandary]

Best to use magnesium in fresh water. The consequences of getting it wrong are very expensive!

With a berth in the Crinan Canal for nine years now, I have some experience of this and it is frequently discussed here, so some of my conclusions.
Aluminium anodes are the best solution if the boat goes to sea from time to time, they remain active in both fresh and salty water and do not disappear as fast as magnesium, however they are not available for saildrives.
Magnesium is the best if the boat is always in fresh water but many boats with bronze props on ss shafts here get away with no anodes at all, but I would not risk it, a magnesium anode can fizzle away in less than a fortnight in salt water.
Zinc anodes on props will usually stay clean in fresh water if the boat is used frequently but a passivated coating builds up on saildrive and other static anodes in about 15-20 days.
I have a three blade folder on a saildrive and use zinc as we go to sea regularly for periods varying from a day to about 25 days but also spend periods of weeks static in the canal. The saildrive anode is abraded at each annual lift out, but in between is supplemented by a hanging anode wired back to the leg (heavy copper with good connections, frequently checked) which is taken out and abraded clean every 15 days or so, I keep a roll of coarse sandpaper for the purpose, a wire brush is not effective

So, if the boat is coming out each time it moves from salt to fresh and vice versa, switch between magnesium and zinc, you will probably get a couple of seasons out of each. I use a set of prop. anodes every six months (they are very small) but a leg anode does about 4 seasons and a hanging anode a lot longer.

 

[Cloven]

Firstly, the Vilaine is very much fresh and not brackish water. We are based in the Vilaine and as others have said, if you are going for anything then it should be a hanging Mg anode. Our regime is a little different to yours as we are based there permanently. We have Aluminium hull and shaft anodes and then an MG hanging anode when in the river and a Zinc hanging anode when berthed in salt water. This regime seems to work in that the MG hanging anode shows the greatest depletion and the Zinc very little. I replace the the Aluminium anodes every 2 seasons but even then only to grind them smooth and use them again on the next cycle.

I your situation, I would suggest a hanging Mg anode if you can get one and make sure that you replace or fully clean up your Zn anodes before relaunch and then not spend too much time before heading back out to sea or the Zn anodes will become ineffective.

Just out of interest, I get my Mg anodes from UK narrow boat suppliers as they are used on canal boats etc. It might be worth seeing if you can get something up near Redon where the Vilaine becomes canalised as I would guess that the local canal boats must use them.

We are wintering this year in S Brittany in the Vilaine River, which is supposed to be fresh to brackish water. In theory in these conditions we should be using aluminium or magnesium anodes, but as we are only going to be here till end of March I'm wondering what the consequences of just leaving the nickel (Edit: Of Course I really mean Zinc) ones in place. We will probably get hauled out for a scrub off before we leave, so will use that as opportunity to replace/clean existing anodes.

I could get a hanging aluminium anode. What would the likely consequences be of just leaving things as they are? Most of the time we will be disconnected from any shore supply.