My boat is a huge battery!

[Guest]

My wooden boat suffered from some galvanic corrosion to some bronze stem band fittings last season. In an attempt to see what might be happening, I took a few multimeter readings between various fittings. To my horror, the meter read 0.4 volts between the keel and some copper nails and 0.3 volts between the keel and the propshaft and the same wherever else I tried. I immediately disconnected the batteries and shore power but the voltage remained the same. In my panic I sneaked a couple of reading from nearby wooden boats and found a similar pattern with voltages between 0.1 and 0.3 volts. My boat and presumably other wooden boats seem to be huge batteries.

Now of course I wish I had not taken the measurements. Ignorance is bliss and all that.... but can anyone shed a light on the subject?


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[VicS]

You don't state the polarity of the voltages you have measured. One would expect copper and copper alloys to be positve and iron and steel to be negative. Corrosion of the the iron or steel would then occur if it were bonded to the copper alloys. For galvanic corrosion of the bronze to occur it must be connected to something significantly more electropositve. You don't have a gold plated ground plate do you? Small potential differences (small fractions of a volt) will not lead to significant corrosion, a fact exploited by galvanic isolators.

I assume you have used a high resistance digital voltmeter to measure the voltages you have quoted.

<hr width=100% size=1><font color=purple>Ne te confundant illegitimi.</font color=purple>

 

[Keith]

where are you moored, i know of some marinas where there is a weak electrical field, due to lack of maintenance of phone lines, shore power, etc......keith

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[Guest]

The boat is on the hard and the multimeter a standard multimeter. The voltage is positive when the negative is to engine bearer bolts and the positive on the iron keel.

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[Tekkie]

Sounds more like agenerator to me, battery only store power.
Tis a well know problem. Do ensure that your anodes are "up to scratch" in a bid to stem your problems.

Regards
T

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[Tekkie]

Sounds more like agenerator to me, battery's only store power.
Tis a well know problem. Do ensure that your anodes are "up to scratch" in a bid to stem your problems.

Regards
T

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[Guest]

Sorry can you explain please? ..... preferably in words of one syllable as this is all muck and magic to me

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[Mirelle]

....er...um...er

I wonder if I dare to try mine! We don't have any discernible corrosion, despite a happy mixture of iron keel, bolts and floors, copper nails and bronze fittings.

<hr width=100% size=1>Que scais-je?

 

[MainlySteam]

But you will only have a galvanic problem if you connect the different pieces of metal together (both immersed in seawater)with a piece of wire. The multimeter (in volts mode) will measure the galvanic voltage between the two different metals in seawater but very little current will flow on account of the high resistance of the multimeter. If you connect the metals together with a piece of wire then a current will flow (leading to erosion of the most anodic metal) but the voltage between them will drop (on account of being shorted).

It is interesting trying different metals (including the probes themselves of the multimeter) in a bowl of salty water.

If your stem band fitting is not electrically connected (piece of wire or other metal) to another metal component and it has suffered from corrosion, then it is a deficiency in the bronze alloy that is the problem. If it is connected to a different metal, the fasteners perhaps, then there may (or may not) be galvanic corrosion due to the connected disimilar metals.

John

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