anodic question

[Freebee]

Whats the difference between a ground plate and an anode. Whilst preparing to put my american built boat back in the water I have noticed some 2 x 6 inch bronze or copper plates under the hull which are grounded inside the hull. These were covered in thick anti foul paint which I cleaned off thinking they were anodes. The outdrives have suffered from galvanic corrosion and my thinking was that the paint covered plates were the reason. Now I am told these are grounding plates not anodes-so what are they for?

 

[desG]

Ground plates and anodes perform completely different functions. The ground plate connects to the boat wiring ground and ensures that the latter is maintain at the surrounding water potential. (Like in the home the earth return connects to the ground so there is no potential difference between you standing on the ground and an earthed metal object.) To be effective the ground plate should not be covered by paint- however if it were I wouldn't expect it to cause your galvanic corrosion. It can help reduce electrical interference and, (possibly?), reduce damage in the event of a lightening strike.

Anodes are sacrificial metals to protect against local galvanic action whereby two dissimilar metals in water act as a battery in which one metal dissolves and deposits on the other. The intent is that the anodes act as the metal to be dissolved, so protecting your outdrives.
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Des

 

[Oldhand]

A grounding plate should not be painted and is usually intalled as a specific ground (earth) for MF/HF radio equipment.

 

[halcyon]

We have a large grounding plate on the Halcyon, connected to the mast, and installed as a lightening conductor by previous owners, for trans-atlantic runs, wonder if they had a near miss ?.
Have slight doubts, and one worry, the plate is between the heads input and output skin fittings, just contenplating a strike. For health and safety, should we fit a notice advising people to check for thunder storms before using!.


Brian

 

[dave36]

I am a total novice to boating, but there seems to be so much discussion on anodes I feel I must be missing something!
1. What does an anode do?
2. Should I as the idiot rebuilding a 12' speedboat from the '50s worry about anodes?
3. If I should be installing anodes - how why where?

 

[Chris771]

Dave,

You'll be glad to know that in addition to the Merurcy Engine I have now located the fuel tank, new hose and the remote controls, and, believe it or not, the reverse lock collar. The latter probably means that I have not properly tidied my desk in the last 12 years, as it was still where I put it when I bought it.

Regarding anodes I doubt that your boat will be in the water long enough, or have sufficent immersed dissimilar metals to set up any significant galvanic action.

Galvanic depletion is a bit like a battery in reverse. Where one has a number of different metals immersed in an electrolyte (sea water) then an electrical potential exists between all the different metals, based on their nobility or position in the periodic table. The more electronegative a metal is, the greater the potential existing between it and something like steel. So aluminium parts will start to erode and put a nice protective coating of aluminium chloride on any bare rusty steelwork you have immersed. Bronze propellors and brass fittings act in a similar way, the zinc in the alloy being eroded away and the metal adopts a bright golden crumbly porous appearance and becomes weakened and brittle and liable to fracture.

The reason for fitting zinc anodes is so that all immersed metal parts can be bonded together inside the hull (and be at an electro-equipotential) and bonded to the anode which then becomes the most "dissimilar" metal. The zinc readily gives up molecules in the galvanic process through the elctrolyte (seawater being a much more effective conductor than brackish or river water, in fact absolutely pure freshwater(never encountered except in laboratories) is not conductive) and zinc chloride deposits are laid onto other immersed metals , hence "protecting" them from erosion.

Also, for the above reason, boats used in only freshwater or canal water should normally have magnesium anodes to protect aluminium, brass and bronze. Narrowboats are normally all steel, so zinc will still provide adequate protection as only the two metals are involved.

I also wrote another quite long post on a question about anodes about 4 weeks ago. Either search it, or look back through all my posts to find it if you are interested to learn more.

At the end of the day, if you are trailing your boat to the water on a Sunday and taking it home again afterwards I would rather invest the anode money in fuel or beer and just hose everything off in freshwater when you get home - not forgetting to get the "muffs" which allow you to run the outboard for a couple of minutes whilst hosing through the colling system with freshwater to get rid of both salt and sand or mud which may lead to clogging.

That is one of the reasons why I keep my boats just inside the canal sealock. I use it in seawater and then at the end of the trip I come back into fresh canal water. This flushes out the engine/gearbox cooling system and galvanic action slows to a fraction of that in seawater. The seawater has killed the freshwater slime during the trip and the freshwater kills all the seawater beasties so antifouling lasts about 3 or 4 years, surprised that more people have not sussed the benefits of mooring in freshwater. The only problem I have found is that it freezes in winter......but my boat is tucked up at home by then anyway.

Chris

 

[Freebee]

Thanks to all I now understand the nature of my problem and will be fitting new anodes this week!

 

[DavidP]

Hi,
As Chris has said ,he posted an excellent answer to a question i posted regarding Anodes,well worth your time looking it up.

Re Anodes, posted on 14 May 02 by DavidP.

David.