Zinc, magnesium or aluminium?

[Sanderling_]

For 2013 we will be dry sailing out of the Vilaine.

Our sailing patterns are such that we will be afloat for at least one week a month with two x four to six week blocks early and late in the season. Many of our week long stays will be in the river (fresh water) whilst some will be at sea (salt water) as will the longer trips.

We have a hull mounted pear anode and a second anode on the saildrive. To maximise our precious time afloat, I would expect Sanderling to be launched prior to our arrival each trip so swapping anodes dependent on our planned itinery will not be practical!

I would like advice on the choice of anodes, do I:

1. Use a Mg or Zn hanging anode dependent on where we are.
   
   If so, do we need it to drag it whilst sailing?
   
   What are the risks of just using it when moored?
   
   Whilst this might replace the pear, what would I do with the saildrive anode?
   
   
2. Use a Mg anode and ensure that we check it mid season
   
   Am I right to understand that Mg works in salt water but just erodes faster?
   
   
3. Is an Aluminium anode suitable for both fresh and salt water?

Thanks, Tony

 

[VicS]

For 2013 we will be dry sailing out of the Vilaine.

Our sailing patterns are such that we will be afloat for at least one week a month with two x four to six week blocks early and late in the season. Many of our week long stays will be in the river (fresh water) whilst some will be at sea (salt water) as will the longer trips.

We have a hull mounted pear anode and a second anode on the saildrive. To maximise our precious time afloat, I would expect Sanderling to be launched prior to our arrival each trip so swapping anodes dependent on our planned itinery will not be practical!

I would like advice on the choice of anodes, do I:

1. Use a Mg or Zn hanging anode dependent on where we are.
   
   If so, do we need it to drag it whilst sailing?
   
   What are the risks of just using it when moored?
   
   Whilst this might replace the pear, what would I do with the saildrive anode?
   
   
2. Use a Mg anode and ensure that we check it mid season
   
   Am I right to understand that Mg works in salt water but just erodes faster?
   
   
3. Is an Aluminium anode suitable for both fresh and salt water?

Thanks, Tony

The troubles are that Mg is too reactive for use in salt water and will soon be consumed, while zinc and aluminium become passivated in fresh water and require physical cleaning before becoming effective again in salt water.

Aluminium is suitable for salt or brackish water, but not fresh


there is a little advice, but not a lot, on MGDuffs website http://www.mgduff.co.uk/leisure-craft/cathodic-protection.html

 

[theoldsalt]

I keep my boat so far up a river that the water is fresh and enter salt water when I take the boat out - most weekends. Since it spends most of the time on it's mooring I was advised to change the anode from Zinc to Magnesium despite no previous problems with the Zinc. What a mistake; at the end of the season the prop was in a mess. I have now changed back to Zinc and when lifted out at the end of last season there was no problem with the underwater gear.

So I can only conclude that Zinc works better in fresh than Magnesium does in salt water for a boat used in both types of water.

 

[Sanderling_]

There seems to be much conflicting info online about the use of aluminium anodes.

It would appear from Performance Metals that an aluminium/indium alloy, Navalloy, spec MIL-A-24779(SH) may be the answer as they say that this works in fresh, brackish and salt.

Next issue is to find a supplier that does it in pear and Volvo 120saildrive in the Uk!

 

[Tranona]

Would not worry about it. Stick with zinc. It takes a long time for zinc to become passive in fresh water. A week would not make any difference. When I was planning a transit with my saildrive motor I asked the question widely and the consensus was not worth changing. You might use a hanging mag anode bonded to the saildrive casing if you are static in fresh water for any length of time. What is you hull anode bonded to. Should be no need for it on a saildrive boat.

 

[Rossynant]

+ 1. Boats on freshwater lakes here have zinc anodes or no anode at all.

 

[Cloven]

We are based on the Vilaine although afloat in a marina.

I use the following:

Permanent Aluminium shaft and hull anode.

+ Hanging Mg anode when in fresh water.

+ Hanging Zn anode when at anchor/in marina when at sea.

No problems after 6 seasons.

The largest depletion is of the hanging Mg anode which is only used in fresh water. The Zn anode shows almost no depletion and and Al anodes very little. I bought a large Mg anode from a narrowboat chandlers which has a lug at each end. I then cut this in half and made up a wire to hang it and it is connected through the hull to the internal electrical system. The first half did about 3 seasons and then I replaced it with the second half. Although the surface of the used Mg anode was very pitted, there was still plenty left so I used an angle grinder to clean it up back to the smooth metal. That went back in at the end of last season and I am about to do the same cleaning up job with the second half. I can send you pics of before & after if you think its worth it.

Possibly see you next season on the Vilaine.

 

[Sanderling_]

What is you hull anode bonded to. Should be no need for it on a saildrive boat.

It has about six wires to it; keel, fuel tank, engine block (I think), mains earth (galvanic isolator) +? +?. I don't think that the through-hulls are bonded to it but I might be wrong.....

I hung a Mg anode over the side as we were afloat up river all last summer (connected to the hull anode), it didn't erode at all.

 

[Sanderling_]

I use the following:

Permanent Aluminium shaft and hull anode.

+ Hanging Mg anode when in fresh water.

+ Hanging Zn anode when at anchor/in marina when at sea.

No problems after 6 seasons.


Possibly see you next season on the Vilaine.

Dave, thanks for that.

Aluminium plus Zn or Mg hangers for salt and fresh seems like the best compromise.

Yes, I am sure that we shall meet. We arrived at LRB last spring with a dry sail contract out of the new yard up the top of the hill but it wasn't until November that they could lift out more than 7T!

Tony

 

[Tranona]

Can't see the point of the hull anode. Nothing you describe needs any protection. The only reason for having a hull anode on a saildrive boat is if it is bonded to the casing as with a hanging anode. No good bonded to the engine as this is isolated from the drive. Is there any significant of activity on it?

 

[Sanderling_]

The hull anode has needed replacing each year due to erosion so it must be doing something. The wiring is all within the main loom so I suspect the bonding of these items was done by Moody's when new.

 

[Tranona]

Difficult to know what it is doing as there is nothing inside the boat where there are dissimilar metals in contact with seawater. Suspect it may be connected to the saildrive. Normally a hull anode would be used to protect the prop on a shaft drive boat and possibly the rudder depending on construction.

 

[Cloven]

Dave, thanks for that.

Aluminium plus Zn or Mg hangers for salt and fresh seems like the best compromise.

Yes, I am sure that we shall meet. We arrived at LRB last spring with a dry sail contract out of the new yard up the top of the hill but it wasn't until November that they could lift out more than 7T!

Tony

Hi Tony

Yes we are at LRB, Pontoon C4 - Hunter Channel 323 "Seachange"

If the ensign is on the stern, we are in!!

We'll be out late April until end June so hopefully see you at some point.

Dave

 

[vyv_cox]

We berthed in fresh water in Holland for seven years but sailed on the sea most weekends, summer and winter. I retained my zinc prop anode throughout. By haul out time it would have some zinc hydroxide deposits on it, easily wire brushed off. No corrosion problems.