Anodes on Lead Keel?

[Natsumi]

I have owned a Sunbeam yacht for the last three years. It is fitted with a lead keel and when I purchased her she had two disc shaped anodes, through bolted to the trailing edge of the keel.
When she was lifted after the first year the anodes had disintegrated completely and there was strange errosion to the paint on the keel, but no damage to the keel itself.
Second year i fitted larger anodes and a Galvanic Isolater, as I do run a 240 volt dehumidifier during the colder months. Again, most of the anodes had eroded and a the paint on the keel looking distinctly coloured and "erroded". I have spent ages checking all on board systems to see if there is a voltage leak, but proven nothing. The only other anodes are on the prop shaft and the propeller itself and always show typical errosion. The keel , P bracket and sea cocks etc are not bonded.
I spoke to someone at MG Duff and sadly all he could suggest was fit a pear anode, and bond that to the engine...........
I have come to the conclusion , that perhaps the Lead Keels does not need a Zinc Anode, and infact this is setting up a battery in the salt water. What do you think? Is there anything else I should try?
I assumed that the anodes as fitted when I bought the boat, were as intended by the builder, but worry these were added by a previous owner.

 

[Tranona]

Cannot see any reason to have anodes on a lead keel - it is all the same material and not in contact with any other metal under water. I expect it has stainless bolts through the hull, but these are (hopefully!) not in seawater! Don't replace it and fill the hole. Your prop and shaft are correctly protected. Just BTW the only time I have sen galvanic action on a lead keel was on a 1960s S&S where they had attached a stainless steel fairing piece to the aft end to get a very fine edge. Inevitably the stainless suffered. one of their bright ideas which did not work and was dropped after a year in favour of better keel castings.

 

[earlybird]

I had a lead keeled boat, no keel anode needed. However, if the primer/antifoul painting of the lead was not thoroughly done at the start of the season I found that the hull anode, which was connected through to the engine etc, eroded very rapidly. I concluded that this anode was being sacrificed to "protect" the keel.

 

[VicS]

I have come to the conclusion , that perhaps the Lead Keels does not need a Zinc Anode, and infact this is setting up a battery in the salt water. What do you think?

You are right . The lead keel does not need cathodic protection

Zinc and lead are approx 0.8 volts apart in the galvanic series so together will form a fairly active electrochemical cell, the zinc being quickly eroded.

Alkalinity produced at the cathode ( the lead ) will be detrimental to a paint coating. If you see any evidence that the paint coating is flaking off the it is probably due to hydrogen being produced under it.

 

[Graham376]

Cannot see any reason to have anodes on a lead keel - it is all the same material and not in contact with any other metal under water. I expect it has stainless bolts through the hull, but these are (hopefully!) not in seawater! Don't replace it and fill the hole. Your prop and shaft are correctly protected. Just BTW the only time I have sen galvanic action on a lead keel was on a 1960s S&S where they had attached a stainless steel fairing piece to the aft end to get a very fine edge. Inevitably the stainless suffered. one of their bright ideas which did not work and was dropped after a year in favour of better keel castings.

As you say, it shouldn't be in contact with anything but, I've seen problems with deterioration on steel/iron keels when they've been bonded to engine and other bits. Something's causing his anodes to waste so check the keel isn't internally bonded.

 

[LittleSister]

Something's causing his anodes to waste . . .

See post #4 for the explanation.

. . . so check the keel isn't internally bonded.

Good idea to check that, to eliminate other potential (arf, arf) problems, but not being connected won't stop anodes fitted to a lead keel wasting.

If the keel's not connected to anything else, the way to stop the anodes wasting is to not fit them!