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Old 15-05-2013, 13:19   #1
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Corrosion contemplation

Hi all I have a fiber glass monohull powered by a Volvo Penta engine and aluminum sail drive.

I have some corrosion around my bonded through hulls. In conversations with a few folks more experienced than I have already identified a few issues.

1. My bonded through hulls (Conbraco Apollo) should be tied to an anode and are not currently. Are there bolt on anodes for sailboats? If so where are they usually installed? I don't want my sail drive tied in to this loop but it does have a collar anode. I don't want to mess with a fish over the side more than short term.

2. While I'm thinking about the anode I was contemplating a surveyor suggested ground plate trying to completely isolate the engine and sail drive. (Current ground is typical engine bolt.)

(I will be going back through my electrical system a 3rd time trying to locate any stray current as well as a few recommended fixes.)

The bonded rudder skeg and all bonded through hulls look like this to some degree.

How do other boats with sail drives handle these issues? Mono or multi feedback welcome.
Thanks!
SC


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Old 15-05-2013, 15:49   #2
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Re: Corrosion contemplation

It is extremely unlikely that your corrosion has anything to do with "stray current".

With an aluminum sail drive and bonded through hulls you are creating a situation where the sail drive is the anode and it is trying to protect your through hulls. It has become the sacrificial anode.

My advise is you only bond through hulls if they show evidence of electrolytic corrosion. An isolated piece of metal in the water is only ONE pole of a battery. The moment you connect it with bonding to other metal in the water that is not the same metal (same electrical potential) you create a battery and current flows from the anode to the cathode in the water and back through the bonding.

I would remove all the bonding, in particular that connects to the sail drive.
If necessary I think you can get special anodes to protect the aluminum sail drive locally, perhaps magnesium?? - check with the sail drive manufacturer.

Then monitor your through hulls on each haul out and worry about adding a zinc anode to protect them only if you see trouble.

If the aluminum on the sail drive is a GOOD alloy and is not connected to anything else it will survive without any protective anodes. Aluminum hull boats that are TOTALLY isolated from all other metal don't even need protection. But if it has bronze or stainless prop and hardware you may need to follow manufacturer's recommendations.

Finally, thoroughly clean the existing corrosion and repaint with a Coal Tar Epoxy or some similar underwater paint that will seal and electrically ISOLATE the metal from the water. Within 36 hours of the epoxy setting, apply any anti-fouling if you are going to use it, if you leave it too long the epoxy hardens so much that subsequent paint will not adhere.
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Old 15-05-2013, 16:38   #3
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Re: Corrosion contemplation

Volvo saildrives are electrically isolated from the engine by design and should never have anything bonded to them. If fitted with bronze props, they are rubber hubbed and isolated from the saildrive. The collar zinc is to compensate for the stainless prop shaft and bearings, as well as for the stainless fasteners required. It should not be experiences a rapid loss of zinc.

If you have electrical continuity between the engine and saildrive, or are experiencing a rapid loss of collar zinc, you need to find out why and correct it.

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Old 10-06-2013, 08:21   #4
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Thank you for the replies. I've been actively attempting to correct this issue and have added barrier coat (Interlux) and repainted after that.

The boat is now in the water (fresh water) and I was finally able to use the silver chloride reference electrode. The bonding system was installed by a PO and apparently bonded 3 seacocks and the rudder skeg. It ties back to the negative side if the electrical panel.

With all that said what I found was a hull potential of 1280 millivolts unplugged and batteries off. The book states I should be in the 900-1050 millivolt range for type of boat and water.

My question is this: is it possible this reading is off because these bonded items are not tied to an anode? The only anode on board is protecting the saildrive. It is magnesium as recommended.

After cycling through all the breakers one issue was identified but is rarely powered on at the panel so not thought to be the main problem but also actively being addressed.
Thanks!
SC
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Old 10-06-2013, 11:10   #5
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Re: Corrosion contemplation

Analyzing the results of voltage measurements from a reference electrode is attempted by experts and results are still subjective and empirical, especially in fresh water. The characteristics of the meter can make a big difference at those low voltages and currents.

From memory I think the voltage of magnesium is about 1.1 volts anyhow so I suspect everything is OK.

If you are seeing any voltages higher than 1.0 across the galvanic isolator, or seeing anything over 0.4 volts AC across the galvanic isolator you might want to consider using two galvanic isolators in series if the electrolysis problems persist.
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Old 11-06-2013, 12:17   #6
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Quote:
Originally Posted by Andina Marie View Post
Analyzing the results of voltage measurements from a reference electrode is attempted by experts and results are still subjective and empirical, especially in fresh water. The characteristics of the meter can make a big difference at those low voltages and currents.

From memory I think the voltage of magnesium is about 1.1 volts anyhow so I suspect everything is OK.

If you are seeing any voltages higher than 1.0 across the galvanic isolator, or seeing anything over 0.4 volts AC across the galvanic isolator you might want to consider using two galvanic isolators in series if the electrolysis problems persist.
There is an isolation transformer on board and tested correctly. I suspect the difference may be the lack of an anode but hoping I can confirm that with others with more experience in this area.
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