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Go Back   SailNet Community > On Board > Gear & Maintenance > Engines > Diesel
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Old 11-05-2010
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Diesel Anodes

On my 30 year old Yanmar 3GMD (sea water cooled) I tried to replace the three anodes. I got the one off the head and replaced. The two in the block will not come out. Ended up rounding off the head of the plug so will need to drill the anode plug out.

Is there another way to protect the engine from corrosion until I can drill the anode plugs out and replace? I was thinking of doing a fresh water flush at the dock after every use but I read the fresh water causes a coating on the one anode I did replace and will render it useless.

Will the one anode I replaced in the head protect the entire engine? Could I clip on one of the zinc fishes to the engine block and drop iit into the water to protect while at the dock?

Thanks, Casey
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Old 11-05-2010
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Quote:
Originally Posted by casey1999 View Post
On my 30 year old Yanmar 3GMD (sea water cooled) I tried to replace the three anodes. I got the one off the head and replaced. The two in the block will not come out. Ended up rounding off the head of the plug so will need to drill the anode plug out.

Is there another way to protect the engine from corrosion until I can drill the anode plugs out and replace?

I was thinking of doing a fresh water flush at the dock after every use but I read the fresh water causes a coating on the one anode I did replace and will render it useless.

Quote:
What? Who told you that?
Will the one anode I replaced in the head protect the entire engine? Could I clip on one of the zinc fishes to the engine block and drop iit into the water to protect while at the dock?

Thanks, Casey
Casey. I had a 30 year old Volvo with no anodes, so I wouldn't panic. The one you have should work fine if it is not electrically isolated from the rest of the engine. A fish anode over the side wouldn't hurt but is probably not needed. How bad were the anodes you are replacing? Understand there is a lot of metal inside an engine so any electrolysis will be spread out to where it might not ever cause a problem.

The big problem with RWC engines is the salt line. That is the horizontal line where the sea water level (outside the boat) equalizes inside the engine when you shut it off. Water will drain back through the water pump (slowly) until the water level in the engine is equal to the water level outside the boat. The problem is very moist air in the engine above the water line causes rust. Depending on the boat this rust can plug water passages and hurt cooling and water flow. Anodes do nothing for this problem.

So, you can flush with fresh water and close the cooling intake valve to prevent water from draining out of the engine back into the sea. Only really needed if you are leaving the boat for a few days or more. If you hang the key on the valve you won't forget to open it!

Gene
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Gene,
Thanks for reply. The anode I replaced in the head had been totally eaten away. Now it has a new anode. I assume the two anodes in the block have been eaten away. I bought this boat used and the anodes probably were not regulary replaced and I imagine the engine would go for years with no anodes.

My engine sits below the water line so it will not have the salt/air interface.

I now always shut the water intake valve in order to prevent boat flooding in case of a cooling water leak while boat is at dock.

If I do the freshwater flush will that coat the one zinc I now have and render it useless as the Wikipedia article on zincs anodes states?
Casey
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i would like to read the article as that doesn't make any sense to me.

Gene
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Sacrificial anode - Wikipedia, the free encyclopedia

Above is article on anodes. Look towards the end- "Choosing the Correct Anode"
Thanks, Casey
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Their source implies oxidation layer (which is passivation) after several weeks in fresh water. Maybe, but we are talking about a closed system here. I would be more concerned about rust than electrolysis myself. And I am not so sure that flowing hot salt water would not overcome any passivation pretty quickly.

You can also run white vinegar through your engine to remove any built up salt deposits. But fresh water fill will do that over time too.

Whatever you do I don't think there is not a lot to worry about one way or the other. Me, I would flush with fresh water.

Gene
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