Hanging Anode Over Side Re Electrolysis

[demonboy]

Preventing Electrolysis by hanging an anode over the side of the boat. What's the trick? What's the science?

I have my anode attached to a sheathed copper wire. Now what? Where do I attach it to? I was told the backstay is fine but that doesn't make sense. How is that preventing potential electrolysis from working its way to the prop?

Any tips, tricks, clues, explanations gratefully received and will help educate a few boats here who don't really know what they're doing!

 

[pvb]

It has to be securely connected to the underwater metal bits you want to protect. So, for example, it could be connected to the propshaft via brushes (or just connected to the engine if there's electrical continuity between engine and propshaft). But why not just have a hull anode or a shaft anode?

 

[demonboy]

But why not just have a hull anode or a shaft anode?

I have hull and shaft anodes but here in India a haul out is not possible. A boat based here that has since left for Malaysia inspected his anodes and was surprised by how much he had lost. The river water here in India is so muddy going down to replace the anodes is not an option.

So to protect my prop I need to actually attach the copper wire to the engine somewhere, then run it out through a hatch, over the side and into the water?

 

[pvb]

So to protect my prop I need to actually attach the copper wire to the engine somewhere, then run it out through a hatch, over the side and into the water?

Yes, but you must be sure that there's a good electrical connection between propshaft and engine (ie no flexible coupling, etc).

 

[john_morris_uk]

As others have said - there must be a good (ie low electrical resistance) connection between the anode and the item you are trying to protect. If you are sure that the electrical connection is good, the only other 'trick' is to ensure that the anode can 'see' the item it is going to protect. Inother words, it must be close to the item underwater. Hanging the anode over the side alongside the propellor will be fine.

 

[Seajet]

Hanging an anode over the side is pretty standard on alloy and steel yachts, where electrolysis is a big worry, and as long as it's connected sensibly must be a good idea for most boats; I suspect the main reasons it's not done more widely are

A, A lot of people adopt the Ostrich Aproach to nasty things like electrolytic corrosion, stick on an anode then leave it at that,

B, It could be embarassing if one forgets to take it in before setting off, and it goes straight around the prop' or rudder ! A red flag on the line at guardrail level seems wise.

 

[savageseadog]

I'm not convinced by hanging anodes. I suspect they are effective more against stray current corrosion rather than true electrolytic corrosion which really requires the anode to be in contact with the protected metal.

 

[Salty John]

Hanging anodes have to be in electrical contact with the metal they protect. If the rig is electrically bonded to the engine and thence to the prop, the prop will be protected, theoretically, by attaching the anode to the backstay. It should also be immersed in the same pool of water as the prop. i.e the ocean. And it should be close to the thing it is intended to protect.
How effective these things actually are I've never been able to determine. People that use them swear by them, others think they are of dubious benefit.

 

[Quandary]

Hanging anode?

Having looked at my post count, this year I have been trying to reduce my recently excessive input ( I used to be able to keep it at about 20 a year when I had a life) but I could not resist putting my tuppence worth in on this one.
Because my boat is berthed in fresh water for the majority of its time at rest, I use a hanging anode to protect the saildrive leg. The normal external anodes on the leg are vulnerable to rapid surface passivation rendering them ineffective after a week or two. The anodes on the prop. avoid this, probably because the rapid motion wears off the passivation. A heavy insulated copper wire goes from the top of the leg to the cockpit locker where it is connected to the stainless steel wire of the anode, the anode resides in the locker when under way, its surface is cleaned of the white passivated coating with coarse emery paper about every couple of weeks. I also tend to avoid using marina shore power when afloat as much as possible, though the shore power system (Navix) is supposed to be protected
It seems to be effective, but I have no real way of judging how much benefit it has, it does not reduce rapidly but it is a big lump of zinc, I would still rather use it than take the risk of corrosion of the leg with all its different metals. The prop. anodes by contrast seem to reduce very rapidly, the prop is bronze and stainless steel with a s. steel rope cutter.
I used to use a hanging anode on a shaft driven boat before, it originally had had a circuit connected to a big external anode which we removed to improve light weather speed when racing one design, but I suspect it made little difference (to corrosion) in that case. Its a good reason to avoid boats with saildrives if you berth in fresh or brackish water. I am told that the problem would be solved by using aluminium anodes but only zinc or magnesium are available and the latter would only last a couple of weeks in the sea.
However it is not a lot of trouble or cost put against the consequences of having to replace the leg.

 

[Cloven]

Having looked at my post count, this year I have been trying to reduce my recently excessive input ( I used to be able to keep it at about 20 a year when I had a life) but I could not resist putting my tuppence worth in on this one.
Because my boat is berthed in fresh water for the majority of its time at rest, I use a hanging anode to protect the saildrive leg. The normal external anodes on the leg are vulnerable to rapid surface passivation rendering them ineffective after a week or two. The anodes on the prop. avoid this, probably because the rapid motion wears off the passivation. A heavy insulated copper wire goes from the top of the leg to the cockpit locker where it is connected to the stainless steel wire of the anode, the anode resides in the locker when under way, its surface is cleaned of the white passivated coating with coarse emery paper about every couple of weeks. I also tend to avoid using marina shore power when afloat as much as possible, though the shore power system (Navix) is supposed to be protected
It seems to be effective, but I have no real way of judging how much benefit it has, it does not reduce rapidly but it is a big lump of zinc, I would still rather use it than take the risk of corrosion of the leg with all its different metals. The prop. anodes by contrast seem to reduce very rapidly, the prop is bronze and stainless steel with a s. steel rope cutter.
I used to use a hanging anode on a shaft driven boat before, it originally had had a circuit connected to a big external anode which we removed to improve light weather speed when racing one design, but I suspect it made little difference (to corrosion) in that case. Its a good reason to avoid boats with saildrives if you berth in fresh or brackish water. I am told that the problem would be solved by using aluminium anodes but only zinc or magnesium are available and the latter would only last a couple of weeks in the sea.
However it is not a lot of trouble or cost put against the consequences of having to replace the leg.

Further to Quandry's response, we also berth in freshwater but sail on the sea.

Could I correct him slightly by saying that Aluminium anodes are available. I have both an Al shaft anode and an Al hull anode - the latter electrically bonded internally. In addition to these, when in the freshwater berth, I have a hanging Magnesium anode which is connected to a bolt which passes through the transom and then is bonded internally. This is removable and when I go out to sea, but when stationary in a salt-water berth/mooring etc, I use a Zinc hanging anode connected to the same bolt. The connections on both hanging anodes are a ring fitting over the bolt and a SS wing nut to secure. Had this set up now for 4 seasons with absolutely no problems.

The Mg anode is showing some slight depletion as it is often in place for a month or more or even over the whole winter period. The Al anodes similarly show a little depletion but both are good for at least another 4 seasons or more. The Zn anode is almost as good as new.

Hope this helps.

 

[Quandary]

I can not find aluminium anodes for Volvo saildrives legs and I have been looking for three years now, the problem is compounded because of the Ambassador rope cutter which requires modified leg anodes at £62 a pair.
I am interested in your interchangeable idea as long as I could be sure of maintaining good electrical contact but since the hanging anode wire is stainless steel with less conductivity anyway, this may make little difference. I have not used magnesium so far because of the rapid depletion and also because I suspect that the leg is much more vulnerable in salt water than in fresh, I presume the zinc is as effective as long as it is exposed, rubbing the surface is not too big an issue and is usually done once each time we take the boat to sea.
I am usually much happier to see anodes depeting noticeably, it must indicate that they are working? Some years ago I used to depend on a single shaft anode, at haul out one year it was long gone and the folding prop. blades were noticeably pink, I always duplicated anodes after that and watched to ensure they were depleting. My last shaft drive boat in the canal lost very little weight from its anodes but I put this down to the relative purity of the water.

 

[Cloven]

I can not find aluminium anodes for Volvo saildrives legs and I have been looking for three years now, the problem is compounded because of the Ambassador rope cutter which requires modified leg anodes at £62 a pair.
I am interested in your interchangeable idea as long as I could be sure of maintaining good electrical contact but since the hanging anode wire is stainless steel with less conductivity anyway, this may make little difference. I have not used magnesium so far because of the rapid depletion and also because I suspect that the leg is much more vulnerable in salt water than in fresh, I presume the zinc is as effective as long as it is exposed, rubbing the surface is not too big an issue and is usually done once each time we take the boat to sea.

Apologies - I missed your point about the fact that you have a saildrive - I have a conventional shaft drive.

The Mg anode I use came from a narrowboat supplier and was not only large but very cheap with a fixing lug at each end. The fact that there is depletion of the MG anode and no damage to the prop etc indicates that it is working so the electrical contact must be OK. The big advantage is that it is very easy to replace as it does not require access to the hull.

 

[Seajet]

Quandry,

I'd have thought the s/s wire, which you are aware is less conductive, may be a snag, I'd go for a mild steel wire and put up with replacing it now & again ?

I had a boat with a Volvo saildrive, it would have been a lot simpler for everyone if I'd just mailed my bank card to the Volvo dealers and told them my pin no. !

Big problems with corrosion - inc' core plugs - I bought the boat too late to catch it.

I also had a nasty surprise when I had her hoisted to sell, the boss of the folding prop' had a crack in one of the blade mounts.

Despite enlisting the aid of British Aerospace Kingston ( RIP ) with every type of welding etc going and a very skilled 'coppersmiths' dept, the answer was that the high-zinc casting was such low quality it couldn't be worked on.

Such low quality that this simple lump set me back £480.00 in 1989...

 

[Quandary]

Apologies - I missed your point about the fact that you have a saildrive - I have a conventional shaft drive.

The Mg anode I use came from a narrowboat supplier and was not only large but very cheap with a fixing lug at each end. The fact that there is depletion of the MG anode and no damage to the prop etc indicates that it is working so the electrical contact must be OK. The big advantage is that it is very easy to replace as it does not require access to the hull.

Thanks for that. I am tempted to buy a magnesium anode like yours but rather than interchange them simply connect both to the terminal in the locker, deploying the appropriate one depending on where I am. I presume a heavy insulated copper wire on the mag. one would be fine in fresh water? I tend to drop over the side as far forward as I can reach to place it close to the leg but perhaps this is unnecessary if electrically connected. Do anodes of different metals used simultaneously cause a problem, I presume not if they are lower in the table than any of the metals they are protecting?

 

[Swagman]

Hanging Anodes

Jamie,
We got familiar with hanging anodes a few decades back when we owned an aluminium flyer in Australia. It had no internal engine but with a need to keep batteries charged via shore power, we had 2 hull / keel anodes plus four slim hangers!
It chewed through all the anodes quick enough for me to see a value in continuing to use them and replace each year. We never saw any corrossion issues.
When we decided to leave a later plastic boat with a saildrive in Lagos for a winter (brackish tidal waters) and heard it was famed for electrolysis problems, we decided to also hang an anode. With a mild steel wire which ran over the side and clipped to the saildrive head bolts it merrily eroded through the winter months and we never had some of the corrossion problems others complained of. I'm guessing that meant it worked for us.

 

[chinaman]

Further to Quandry's response, we also berth in freshwater but sail on the sea.

Could I correct him slightly by saying that Aluminium anodes are available. I have both an Al shaft anode and an Al hull anode - the latter electrically bonded internally. In addition to these, when in the freshwater berth, I have a hanging Magnesium anode which is connected to a bolt which passes through the transom and then is bonded internally. This is removable and when I go out to sea, but when stationary in a salt-water berth/mooring etc, I use a Zinc hanging anode connected to the same bolt. The connections on both hanging anodes are a ring fitting over the bolt and a SS wing nut to secure. Had this set up now for 4 seasons with absolutely no problems.

The Mg anode is showing some slight depletion as it is often in place for a month or more or even over the whole winter period. The Al anodes similarly show a little depletion but both are good for at least another 4 seasons or more. The Zn anode is almost as good as new.

Hope this helps.

Thankyou Davewarburton and Quandry for these comprehensive summaries. All I now need to know is where to get hanging Al and Mg anodes. Z-Guard does "hanging sharks" at £35, but other than that I'm finding them few and far between on the net. Any suggestions welcome.

 

[demonboy]

BTW, thank you to everyone who commented on this thread. Apologies for the delay but I've been away from the computer and all that.

I will be putting into practice one of the solutions mentioned here but have an even better one for checking my prop anode: sail to the Maldives for a couple of weeks! Apparently they are relaxing the cruising rules slightly for short-term visitors. Since it's only two days away this might be a very exotic way of diving down below and checking/replacing my anodes

 

[atyas]

Answers will be find in the "White Paper on Cathodic Protection" here www.galvatest.com

 

[PCUK]

Don't forget you can also melt down any old anodes you find and make up your own hanging version. Cheap recycling!!!

 

[atyas]

Be aware anodes must be absolutely free of any impurities otherwise they don't work. You can do many ways, but searched results depends on the cost equipments to be protected...