Keel electrolysis/ corrosion

[Irmascherkid]

Good afternoon to one and all,
Looking for some input as to the problem I have just found out since I lifted the boat out for some maintenance.
At first glance the pitting and holes could have been merely the normal stuff that you see, however once the boat was all choked up I had a good poke around only to discover that there was a fair bit of damage to the lower aft part of the keel now this is something that can easily be remedied so my burning question is this am I better to fit a anode straight onto the keel or should I drill the holes in the hull so it would sit just aft of the keel and then bond it to the keel bolts?

What have you done and what was the deciding factor?

Thanks in advance
Dave

[Symphony]

There is no value in fitting an anode to a cast iron keel. What you see is almost certainly straightforward rust nor caused by Galvanic action. The keel is all one metal and galvanic action occurs when there are two dissimilar metals in contact.

A slight caveat to this is that some castings do contain small pockets of slag or other impurities that may cause localised galvanic cells which corrode, but an anode won't stop this.

Clean away the corrosion, treat with a rust convertor such as Fertan and prime with something like Primocon. May be worth filling the pits bfors priming with an epoxy filler to get a smooth surface. If the pitting is extensive worth considering blasting and epoxy coating, but keeping a cast iron keel completely free of rust is very difficult, even if epoxy coated from new.

[Kibo]

I had similar issues. Search "Kibo" in all forum and look for my post "Keel electrolysis Bav Vision 46" from about 2.5 years ago.
There were some folks saying it wasn't electrolysis and others saying it could be due to marina current from poor grounding etc.
Sorry I am sure there may be a way to link the thread here but it escaped me....

I took the "belt and braces" approach. I had my keel completely stripped and sanded, faired etc. 2x Epoxy coated after priming and then anti fouled. I had two bolts welded to either side of the keel and anodes fitted.  After 2 plus years of 4 months in water per year (I live-aboard in the Caribbean winters) there was a small amount of corrosion to the anodes seen and no rust.

There are several factors that may have lead to the problem being solved so I cannot hand on heart say that the original issue was galvanic corrosion. Obviously stripping, fairing and coating the keel is probably the main solution to keep out the rust as mentioned by others. I also moved from the marina that the boat was kept in previously so that may have been a factor too if a poorly grounded marina supply was a contributor. Finally it was no real great expense at all to add the anodes and since there is some small degradation evidenced now perhaps there was some leakage current absorbed there too....

My advice is to ensure the keel is thoroughly stripped, faired (filled and sanded) primed and epoxy coated at least with two coats as a minimum. I would add the anodes too personally from my experience....

[Kibo]

Here are my before and after photos....

[Irmascherkid]

Hi Kibo & Symphony thanks for the input regarding this,

Symphony, i was under the same thought as yourself with the fact that the keel is cast iron just seems strange that the anode for the sail drive has very little damage to it and i have seen two bav 30`s up in the marina with extra anodes, one has them bolted on both sides and the other has one just aft of the keel, i thought this was because they were copper coated !
Anyway the course of action for this time has been to clean out and treat the pits and then fill them with liquid steel, this is only as i am due back in the water next week and to strip the keel back would be to long at this time, however i have planed for the next haul out that if the problem is still persistent then i will strip it back to bare metal then drill and thread some bar for the anodes then give it 4 coats of interprotect then 4 coats primocon then the antifoul.

Kibo, Was there a particular reason that you welded the bolts instead of getting the holes drilled and then taped out to receive the studs?     

[Kibo]

Hi Dave,

No big reason to weld v drilling and tapping. I have a guardianage company who takes care of my boat on the hard in the off season and their yard guys doing the job suggested welding on the studs so I let them get on with it. If you are DIY I think drilling and tapping might be easier if you don't have access to welding equipment etc. Would think either method would be fine.

Good luck and safe sailing.

[Spirit of Mary]

To me welding is not a good idea. The keel is cast iron. welding on cast iron doesn't give a good strength. Maybe to hold an anode it is OK. but be carefull at tightening. Drilling and tapping to me is better.

[Symphony]

Hi Kibo & Symphony thanks for the input regarding this,

Symphony, i was under the same thought as yourself with the fact that the keel is cast iron just seems strange that the anode for the sail drive has very little damage to it and i have seen two bav 30`s up in the marina with extra anodes, one has them bolted on both sides and the other has one just aft of the keel, i thought this was because they were copper coated !
Anyway the course of action for this time has been to clean out and treat the pits and then fill them with liquid steel, this is only as i am due back in the water next week and to strip the keel back would be to long at this time, however i have planed for the next haul out that if the problem is still persistent then i will strip it back to bare metal then drill and thread some bar for the anodes then give it 4 coats of interprotect then 4 coats primocon then the antifoul.

Kibo, Was there a particular reason that you welded the bolts instead of getting the holes drilled and then taped out to receive the studs?     

The saildrive is completely unconnected to the rest of the boat. The anode should erode, but the rate varies from boat to boat and location to location. when I had a boat in the Med the anode lasted more than 5 seasons in the water. In the UK with the boat in the water all year round the anode lasts just about 3 years.

Just because other boats have anodes does not mean they are right as many (most?) people do not really understand what they do and how to use them.

If you do fit one to your keel it may erode slowly but will not prevent rust from forming if the cast iron is exposed to seawater.

BTW Coppercoat is essentially inert and does not promote galvanic action.