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Old 28-05-2013, 23:05   #1
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Losing antifoul from metal hull fittings

My copper based antifoul is stripping off the metal fittings on rudder and P bracket on my sailboat, and I am told I probably have stray current. Pictures attached. A galvanic isolator is fitted. Is this a DC or AC problem?

Nigel Calder in a magazine letter response explained how to test for stray current on the DC side, but I have no idea where to start if it is an AC issue.

I have one piece of kit, a chart plotter squeezed into a tight space in the cockpit which 'hums', is this a possible source of the problem? The (?) electrolysis problem was identified when we lifted after the long trip from Dubai to the Adriatic, the humming chartplotter was fitted just beforehand.

We use the boat for a couple of months each summer, the rest of the time it is on hard standing, in Croatia. How dangerous is the problem given the level of use? is the rudder going to drop off?

Anyone have any advice or suggestions?

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Old 29-05-2013, 08:10   #2
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Re: Losing antifoul from metal hull fittings

Dangerous? With that level of activity I could see a bolt or two dissolving.
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Old 29-05-2013, 09:32   #3
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Re: Losing antifoul from metal hull fittings

It is difficult to say what the problem is (or if a problem at all). Metal requires particular prepping and priming for paint adhesion. It is possible that the paint just isn't sticking to it because of prep.

Our chartplotter has a hard drive in it and it "hums" while the drive is spinning. Other than that, I can't think why a CP would be humming unless it is so old it uses a CRT.

The one bronze rudder fitting looks like it may be losing zinc, which would surely point to an electrolysis problem if that is so.

The galvanic isolator is not going to protect you from yourself - only from stray DC shorepower current.

Mark
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Old 29-05-2013, 09:51   #4
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Re: Losing antifoul from metal hull fittings

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Originally Posted by colemj View Post
It is difficult to say what the problem is (or if a problem at all). Metal requires particular prepping and priming for paint adhesion.

Our chartplotter has a hard drive in it and it "hums" while the drive is spinning. Other than that, I can't think why a CP would be humming unless it is so old it uses a CRT.
Thanks Mark. The hull was antifouled with cuprotect, and they pointed to stray current when I sent them a set of photos,they advised prepping with Primokon and antifouling over it. I did this and problem repeated.

The CP was new in 2010 and hums all the time the circuit is on.

You think the problem is likely to be stray DC from inside the boat?

Wendy
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Old 29-05-2013, 09:58   #5
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Re: Losing antifoul from metal hull fittings

What you are seeing is called "burnback." Your metals were improperly primed.
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Old 29-05-2013, 10:02   #6
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Re: Losing antifoul from metal hull fittings

Hi fstbttms, Can you point me to the correct method of priming then?

Wendy
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Old 29-05-2013, 10:06   #7
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Re: Losing antifoul from metal hull fittings

I had the same problem on my strut when I put an extra zinc on it--no amount of priming solved the problem--took off the zinc and the problem went away.
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Old 29-05-2013, 10:12   #8
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Re: Losing antifoul from metal hull fittings

Ah, cuprotect - I agree with fstbttms on improper priming.

Yes, it is electrolysis, but only between the copper antifoul and the metal fittings - the rest of your boat isn't in play here. This is going to be much more critical and evident with cuprotect than regular antifouling paint.

Our Furuno CP "hums" most of the time due to the hard drive, but it really isn't noticeable unless you put your ear to it. Do you feel any vibration through your CP body when it is humming? Can you audibly isolate where the humming is on the CP?

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Old 29-05-2013, 10:13   #9
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Re: Losing antifoul from metal hull fittings

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Originally Posted by wendy_hs View Post
Hi fstbttms, Can you point me to the correct method of priming then?

Wendy
Cuprotect has its own primer and priming system. Most likely, it just wasn't done correctly in those areas.

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Old 29-05-2013, 11:24   #10
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Re: Losing antifoul from metal hull fittings

Hi Mark, regarding CP, I am returning to the boat next month and will do a bit more diagnosis on the CP, including checking for stray current from it and listening harder to the hum. The hum starts as soon as the circuit is on, even before the unit itself is powered up. I have a larger version of the same CP at the nav station and that does not make any noise at all.
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Old 29-05-2013, 12:16   #11
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Re: Losing antifoul from metal hull fittings

Ah, then it is not a hard drive if it doesn't make noise when not on! I don't know what would cause the noise, unless it is running through a power converter or transformer of some sort. Good luck with it and keep us posted as to the problem.

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Old 30-05-2013, 13:06   #12
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Re: Losing antifoul from metal hull fittings

To check for an AC problem, measure the AC voltage across your galvanic isolator with all the normal AC items running. AC voltage across the isolator can reduce its electrolytic blocking by up to 50%. If the AC voltage you read is over about 0.5 volts you need to first find the source if possible and second, bypass it with a Galvanic Capacitor.

First make sure that the AC ground and the AC neutral are not joined together on your boat, this is a common mistake. You should not measure a low resistance between the ground wire and any other leads in the shore power cable when unplugged from the dock.

While watching the AC voltage on the isolator, turn off each of the AC loads to see which one drops the voltage, then trouble shoot the wiring on that item to see that the neutral and ground are isolated.

Although many Galvanic Isolators have a capacitor to bypass AC voltages they are often way too small. But you can add a galvanic capacitor in parallel with the isolator that will decrease the AC voltage.
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Old 30-05-2013, 16:51   #13
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Checking across the isolator will not reveal anything , persay,

I wonder if that is impressed current corrosion , maybe its just paint , primer , seawater reaction.

Dave
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Old 30-05-2013, 17:03   #14
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Re: Losing antifoul from metal hull fittings

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Originally Posted by goboatingnow View Post
Checking across the isolator will not reveal anything , persay,
SNIP
Dave
HUH? Reading across the galvanic isolator will reveal considerable information.
If it reads ZERO voltage the isolator is shorted or bypassed somewhere.
If it reads more than about 1 volt DC it indicates that DC current is flowing that is not being blocked by the isolator and causing electrolytic corrosion.
If it reads more than 1.2 volts it indicates that the diodes in the isolator are burned out.
If the AC voltage is greater than about 0.5 volts it indicates that DC current can be flowing on top of the AC cycles and reducing the isolator efficiency up to 50%.
It is not an ultimate test but certainly the easiest test to make.
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Old 30-05-2013, 17:26   #15
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Re: Losing antifoul from metal hull fittings

Even though I never have to plug my boat into shorepower, I had a similar problem a year ago although nowhere near as nasty.
The copper paint appeared blistered around every underwater metal bit.
The problem was I had installed a large zinc to the underside of the boat and had bonded it to the bonding system.
The size of the zinc put too much voltage on the bonding system and ate the paint. Voltage between a silver electrode and the bonding system was almost 1 volt !
The big zinc went away just as fast as a small one while it did the damage.

A couple of months ago, I inquired on an electronics message board if anyone could come up with a circuit I could build to automatically limit the zinc voltage and solve both problems at the same time without having to spend a thousand bucks.

The electronic guys were great, although it took some time to get them to understand what I was trying to do.

I wound up being served with two different schematics, easily built by me.
I built the first one and it worked great, and was inexpensive to build.

My bonding system voltage is not only adjustable to whatever I desire (-0.550 is ideal) and it's rock stable whether the tidal water salinity goes up or down.
(We're on a salt canal with fresh inflow during rains)

The paint problem has not reocurred.

If anyone wants to build it, here's the diagram I used.
A BIG thanks to Ron on the allaboutcircuits.com forum !
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