Cast iron keel anode

[sailoppopotamus]

I'm new to boat ownership and have recently acquired a 1988 Jeanneau Sun Light 30 with a cast iron keel which is currently on the hard. As it stands, the boat has anodes on the keel, prop shaft and at the end of the shaft on the propeller. I'm replacing the propeller with a Flexofold -- they shipped their prop without an end anode, and when I asked them about this they said that a prop anode is not needed and that the prop shaft mounted anode is sufficient (I have a shaft, not sail drive). So I'm down to two anodes. Furthermore, my keel is quite rusty so I'm about to have it sand blasted and re-coated. In preparation I have removed the anodes on either side of it. Upon removing the anodes, I discovered that there's anti-fouling beneath them. I have two questions:

1. Is a keel anode really needed? A cursory search on the internet suggests that no, boats with a cast iron keel do not need an anode on the keel. I'm not sure I understand why exactly. Searching for pictures of my boat hasn't helped much; some have a keel anode, some don't, so I'm not sure if the boat left the factory with one. My question thus is, do I need to replace the anode, or was it unnecessarily fitted by some PBO? In which case, should I just patch the hole through the keel with some epoxy filler?

2. In the event that the keel anode is actually necessary, it seems perhaps wrong for me to have an epoxy barrier and antifouling between the anode and the keel. I would guess that what I would have to do is to sand blast the keel, and then do the whole epoxy barrier - primer -antifouling dance while leaving two bare-metal circles on either side of the keel so that I can mount the anode in direct electrical contact with the keel. But even this seems wrong, since the anode-keel interface obviously won't be completely sealed, so the keel will quickly corrode away and undermine the electrical contact.

Any input on my dilemma would be greatly appreciated. For what it's worth the current anodes are circular with a 9cm diameter. I'm pretty certain that the prop shaft anode is necessary, so I'm definitely replacing that, I'm just not sure about what to do with the ones on the keel.

 

[Concerto]

You do not need an anode on a cast iron keel. The antifouling beneath it is a big clue it was fitted by a previous owner, who did not have a clue.

A rusty keel can be treated as a DIY job. Mechanically remove all paint and any rust. Treat the whole keel with Fertan rust converter, preferably twice. Fill any holes with a filler, it does not have to be epoxy based as this is harder to sand smooth than a polyester based filler. Now coat the keel with at least 6 coats of International Primocon, followed by 2 or more coats of antifoul.

This will not stop the rust completely, nothing will as cast iron is porous and there will be some residual moisture trapped in the casting. The next time you are out of the water, grind back any rust spots and spot treat with Fertan and Primocon. After a few seasons doing this you will become almost rust free.

 

[dankilb]

Our ‘89 Jenneau never had a keel anode. We blasted and re-epoxied a year or so ago and a briefly considered fitting one, but research suggested it wasn’t worth it.

Btw Jen keels came from the factory with a decent epoxy fairing/coating. This lasted us 25+ years. We found the casting itself to be of decent quality. So, a full blast and re-epoxy should reward with long service if done right.

I’m a known dissenter from the Primocon method on these forums and do believe that these keels, in particular, can be re-coated to prevent rust. Only time will tell, with ours!

 

[vyv_cox]

I agree with the other contributors that an anode is not needed. This is a situation in which a paint film is the most effective protection.

Additionally, if you wished to provide only galvanic protection to a keel, dependent on precise composition it would need anything up to a dozen normal yacht-sized anodes to be effective.

 

[greeny]

This discussion has raised a question for me and I'm interested to understand the reason anodes shouldn't be fitted. I've just helped a friend get his boat back in the water and we replaced the 2 disc anodes, one either side of the keel which had all but gone in 1 year. He does them every year he says, along with his shaft anode. His keel is not connected to anything inside so why do the anodes disappear if he doesn't really need them? Or are you saying that without the anodes, the paint will 100% insulate the keel from the water. I'm thinking this may be true but only until the rust breaks through and then you have conductivity. I'm not questioning the knowledge of you guys who have more experience than me but trying to understand why they're not needed when his are being eaten away so quickly.
As an aside to this, my last boat did not have any anodes on the keel but the paint job was in good nick with no rust break through and so no conductivity.

 

[Tranona]

An anode will not prevent rust - that is simply the result of mixing iron with water. The anode goes because of the small amount of impurities in the casting. As Vyv says the bulk of the casting is such that small amounts of galvanic action will eat the anode - but won't prevent rust on its own. The only way that stands a chance of stopping rust on iron keels is blasting and applying a good coating system, typically 5 or 6 coats of epoxy. Conventional primers such as Primocon and antifoul can also provide good protection.

My "new" boat has sheet steel keels which of course rust if exposed to water. They have anodes on them which by the look of previous photos have been there for 15 years and have barely wasted. They wont do anything because they are just attached to a single metal. Likewise the bottom fitting on the rudder is 316 stainless and a previous owner fitted 2 disc anodes which have not wasted at all. Totally unnecessary as again only one metal (bolts are also 316). The only danger here is possible crevice corrosion where the bolts go through the wood, but they are all firmly set in sealer so won't get damp.

 

[vyv_cox]

This discussion has raised a question for me and I'm interested to understand the reason anodes shouldn't be fitted. I've just helped a friend get his boat back in the water and we replaced the 2 disc anodes, one either side of the keel which had all but gone in 1 year. He does them every year he says, along with his shaft anode. His keel is not connected to anything inside so why do the anodes disappear if he doesn't really need them? Or are you saying that without the anodes, the paint will 100% insulate the keel from the water. I'm thinking this may be true but only until the rust breaks through and then you have conductivity. I'm not questioning the knowledge of you guys who have more experience than me but trying to understand why they're not needed when his are being eaten away so quickly.
As an aside to this, my last boat did not have any anodes on the keel but the paint job was in good nick with no rust break through and so no conductivity.

Don't be fooled by the fact that anodes are wasting. When dissimilar metals are connected and immersed the more anodic one will be consumed trying to protect the more cathodic. In the case of a keel the cathode is huge by comparison with the anode, which therefore wastes quickly, but provides extremely limited protection from corrosion.

In my talks on the subject I use the theoretical situation of a rivet in a sheet of metal, immersed in water. A steel rivet in an aluminium sheet will last well as it is the cathode. The aluminium anode is far larger than the cathode, so will also last well, although there can be local wastage around the rivet.

In the reverse case an aluminium rivet in a steel sheet will have a very short life as it tries to protect the far larger area.

 

[sailoppopotamus]

Thanks to everyone for your replies. Indeed the consensus seems to be that the anodes are not necessary so I'll just fill the holes and proceed with the job.

 

[greeny]

Don't be fooled by the fact that anodes are wasting. When dissimilar metals are connected and immersed the more anodic one will be consumed trying to protect the more cathodic. In the case of a keel the cathode is huge by comparison with the anode, which therefore wastes quickly, but provides extremely limited protection from corrosion.

In my talks on the subject I use the theoretical situation of a rivet in a sheet of metal, immersed in water. A steel rivet in an aluminium sheet will last well as it is the cathode. The aluminium anode is far larger than the cathode, so will also last well, although there can be local wastage around the rivet.

In the reverse case an aluminium rivet in a steel sheet will have a very short life as it tries to protect the far larger area.

Thanks Vyv and Tranona,
So in theory he can remove the anodes, fill, fair and paint the holes along with the rest of the keel and he should be ok. Just got to convince him now.
I do have another question and hope it's not considered thread drift because its on a similar subject. The boat has a stainless steel exhaust box. Rubber hose from engine to box, rubber hose from box to skin fitting. The box is continually being eaten away in the bottom area causing leakage. Is this just corrosion from the hot seawater in the exhaust or can the seawater in the hose also be allowing electrical continuity from the engine to cause current flow and hence the problem. The box is bonded to the hull anode to try to stop this I think but the anode doesn't seem to decay at all so is not doing much.

 

[Tranona]

Much more likely to be crevice corrosion, probably starting at the welds. Very common in Volvo style rubber mufflers where the end plates are stainless with tubes welded in for the inlet and outlet. The outlet for obvious reasons is under water held in the box and the welds corrode partly because it is difficult to get clean welds there but mainly because they spend all their life in stagnant seawater - perfect conditions for crevice corrosion. Exactly the same with a stainless box, one of the reasons they are rarely used these days. Solution is to replace it with a plastic or GRP waterlock. Bonding to a hull anode will not do anything as to work anodes have to be in the same electrolyte as the metal(s) and in line of sight. Neither of those conditions exist in this case. Saw a similar misunderstanding recently in a boat I viewed to buy. The heat exchanger had just been replaced (at huge cost) because it was corroded. Normally it is protected by a pencil anode but the owner was not aware of the need to replace it and suspected it had not been done for several years. He then bonded it to the hull anode in the mistaken belief that would prevent corrosion. Actually the boat had no need of any anodes as it had a proper bronze propeller, just like the boat I actually bought.

 

[vyv_cox]

Warm seawater is a very corrosive medium so unless the stainless trap is very well made using the right material and filler rods there is a high likelihood of corrosion. I favour the plastic ones myself

 

[greeny]

Thanks for that information guys. I was afraid the answer would be along these lines. The SS box is fitted in a place where nothing else will fit from the usual suppliers so plastic or grp is not possible unless we make our own. We even went as far as to make cardboard "mock ups" of the available boxes that looked close but nothing was any where near fitting.
The "original" box looked very "self made" and was completely shot. He had a new one made to the same dimensions by the marine welder down here who seemed to know his stuff. He also said that we may be fighting a battle we can't win with SS. Didn't fill me with confidence that the new box is going to last very long.
The closest fit is one of the Vetus boxes but it's too deep and clashes with the prop shaft which runs under and sightly to one side of it. Just had a thought. If we can find a plastic welder maybe we can cut one of those down and reduce the height by 1". I'll have to look at the inside configuration to see if that is even possible. That would give us the clearance we need. Something to work on for next year maybe but messy and again relying on a good welder. Thanks again guys.