Frigoboat water-cooled grounding

[Karanga]

I have got a Frigoboat water-cooled fridge. The heat-exchange block, with anodes, is attached through a fibreglass cored hull.
I have owned the 2006 boat for 2 years
The unit has an electrical terminal loosely attached to the inside, but with no wire ever crimped into it.
At last haul out in March, there was no anode material remaining - just the retaining screws.
Recently I read in the Frigoboat installation manual that the terminal is supplied for connecting to the negative buss of the battery.
I am really surprised that mine has never been connected, because ALL the rest of the boat has absolutely exemplary wiring installed by a perfectionist professional (but still human).
I have put a multitester between the negative terminal of the compressor (almost the negative buss of the battery?) and the electrical lug on the heat exchange block, and got a potential difference of 0.924 volts. Nearly a whole volt of difference sounds huge to me.
Is it that big a deal?
More importantly, can I connect the heat exchange block to the negative terminal of the compressor (cos it is so close and easy) or must I run a wire all the way to the battery connections?

At that haul out, I also replaced the prop anodes on the 2 Volvo SD130 saildrives. I then spent about 3 weeks in a marina.
Soon afterwards, I noticed significant erosion of the prop anodes, but nothing at all of the saildrive anodes. Is there anything I can or should be looking for, or is this not too abnormal.

Thank you for any advice

[boatpoker]

Originally Posted by Karanga

I have got a Frigoboat water-cooled fridge. The heat-exchange block, with anodes, is attached through a fibreglass cored hull.
I have owned the 2006 boat for 2 years
The unit has an electrical terminal loosely attached to the inside, but with no wire ever crimped into it.
At last haul out in March, there was no anode material remaining - just the retaining screws.
Recently I read in the Frigoboat installation manual that the terminal is supplied for connecting to the negative buss of the battery.
I am really surprised that mine has never been connected, because ALL the rest of the boat has absolutely exemplary wiring installed by a perfectionist professional (but still human).
I have put a multitester between the negative terminal of the compressor (almost the negative buss of the battery?) and the electrical lug on the heat exchange block, and got a potential difference of 0.924 volts. Nearly a whole volt of difference sounds huge to me.
Is it that big a deal?
More importantly, can I connect the heat exchange block to the negative terminal of the compressor (cos it is so close and easy) or must I run a wire all the way to the battery connections?

At that haul out, I also replaced the prop anodes on the 2 Volvo SD130 saildrives. I then spent about 3 weeks in a marina.
Soon afterwards, I noticed significant erosion of the prop anodes, but nothing at all of the saildrive anodes. Is there anything I can or should be looking for, or is this not too abnormal.

Thank you for any advice

If you have a decent multimeter buy one of these ... It comes with a pretty straight forward set of instructions and will track down any source of galvanic corrosion or stray current corrosion on a boat. https://www.boatzincs.com/corrosion-...ode-specs.html

[Karanga]

Following up on my own original question...
I have run a wire from the frigoboat in-water heat exchange unit on the keel, up to the negative buss of the battery.
I haven't actually connected it yet, because I am hesitating connecting my battery directly to the outside salt water.
Yikes - it feels like such a scary thing to do!
Can anyone confirm that it is the correct procedure?

[GordMay]

Yes, connect equipment case ground to boat ground bus [DC Neg].

ABYC Dc Negative & Grounding Diagram
➥ https://www.cruisersforum.com/galler...r&imageuser=79

[SailingHarmonie]

Originally Posted by Karanga

Following up on my own original question...
I have run a wire from the frigoboat in-water heat exchange unit on the keel, up to the negative buss of the battery.
I haven't actually connected it yet, because I am hesitating connecting my battery directly to the outside salt water.
Yikes - it feels like such a scary thing to do!
Can anyone confirm that it is the correct procedure?

In most boats (NOT all!) the correct system is to connect the corrosion bonding circuit to Battery Negative in ONE PLACE. The reason for that is to be sure that all of the bonding circuit is at the same potential and there are not small stray currents running around generated by voltage drops in various DC circuits connected to it.

This is usually done by connecting the bonding circuit to the main DC Negative bus. It is certainly possible (even likely) on an older boat that there are other connections between the bonding circuit and the DC system that have been made by ignorance or accident.

Be sure you know what you are doing here. Fridgoboat keel coolers come in two flavors, with a zinc and without a zinc. The one WITH the zinc is designed to NOT be connected to the boats bonding system, the one WITHOUT the zinc needs to be connected to a bonding system with a separate zinc.

If your boat is already bonded, you don't want a separate, unbonded, piece of hardware connected to the battery negative and NOT connected to the bonding system That is a surefire way to set up stray currents between the bonded hardware and the the unbonded.

[Karanga]

Originally Posted by SailingHarmonie

In most boats (NOT all!) the correct system is to connect the corrosion bonding circuit to Battery Negative in ONE PLACE. The reason for that is to be sure that all of the bonding circuit is at the same potential and there are not small stray currents running around generated by voltage drops in various DC circuits connected to it.



This is usually done by connecting the bonding circuit to the main DC Negative bus. It is certainly possible (even likely) on an older boat that there are other connections between the bonding circuit and the DC system that have been made by ignorance or accident.



Be sure you know what you are doing here. Fridgoboat keel coolers come in two flavors, with a zinc and without a zinc. The one WITH the zinc is designed to NOT be connected to the boats bonding system, the one WITHOUT the zinc needs to be connected to a bonding system with a separate zinc.



If your boat is already bonded, you don't want a separate, unbonded, piece of hardware connected to the battery negative and NOT connected to the bonding system That is a surefire way to set up stray currents between the bonded hardware and the the unbonded.

Thank you so much for your reply.

My keel cooler is the version WITH zinc anodes.
My understanding of the installation manuals says this keel cooler must be connected to the negative of the battery (but not to any other zincs, which is a requirement of non-anoded keel coolers).
I just have this horror of connecting my battery directly to a huge electrolytic solution!

But I am a bit confused by the "bonding circuit".
I previously had an Icom HF SSB radio installation, which I removed as redundant. This included 2 heavy duty wires to an underwater grounding plate. Was that grounding plate an SSB requirement, or was it part of the bonding circuit? Nothing else was connected to that grounding plate.

I don't think I have any other bonding circuits that I know of (cored fibreglass hull).

Just to repeat... the wiring on this boat is immaculate and appears perfect, which is why I am reluctant to do anything different!

Thank you again

[GordMay]

A SSB Radio will have 2 “grounds”:
The DC Power return cable, connected to battery negative.
The RF ground counterpoise foil, connected [directly or capacitively] to a seawater ground plane [plate, strip, or screen].
These grounds are both separate from the boat’s lightning ground, and galvanic bonding [anodes] systems.

[Karanga]

Originally Posted by GordMay

A SSB Radio will have 2 “grounds”:
The DC Power return cable, connected to battery negative.
The RF ground counterpoise foil, connected [directly or capacitively] to a seawater ground plane [plate, strip, or screen].
These grounds are both separate from the boat’s lightning ground, and galvanic bonding [anodes] systems.

Fantastic answer - thank you.
So the underwater plate was the counterpoise, and is now redundant.

As far as I know, there are no other grounding circuits, unless the 2 volvo saildrives (D2-50 motors plus SD130) act as groundings (I have heard that volvo have an anti-corrosion circuit)?

[SailingHarmonie]

Originally Posted by Karanga

Thank you so much for your reply.

My keel cooler is the version WITH zinc anodes.
My understanding of the installation manuals says this keel cooler must be connected to the negative of the battery (but not to any other zincs, which is a requirement of non-anoded keel coolers).
I just have this horror of connecting my battery directly to a huge electrolytic solution!

But I am a bit confused by the "bonding circuit".
I previously had an Icom HF SSB radio installation, which I removed as redundant. This included 2 heavy duty wires to an underwater grounding plate. Was that grounding plate an SSB requirement, or was it part of the bonding circuit? Nothing else was connected to that grounding plate.

I don't think I have any other bonding circuits that I know of (cored fibreglass hull).

Just to repeat... the wiring on this boat is immaculate and appears perfect, which is why I am reluctant to do anything different!

Thank you again

It sounds like you do not know what a bonding circuit is and how it differs from a "ground".

In Caldor's book, Boatowner's Mechanical and Electrical Manual the why and how of bonding probably takes over 50 pages, so this is necessarily extremely abbreviated.

In a bonded boat, all of the underwater metals are connected together (usually with a green insulated wire) and connected to one or more zinc anodes. The rational here is to be sure that all the metals are at the same potential and that the only current that flows is from the zinc out to everything else, so the zinc corrodes, and protects everything else. This protects against GALVANIC corrosion, which is caused by electric currents generated by two different metals connected together and in an electrically conductive liquid. Galvanic corrosion is slow, but insidious and should not be ignored.

This is different than STRAY CURRENT corrosion which is typically caused on a boat by a current flowing from the the battery positive back to battery negative that has escaped from wires, and is flowing from one metal part to another through seawater. The metal bit that is positive relative to the other is the one that corrodes, and in some cases VERY fast.

I have no idea how your boat is wired, and what is connected to what. You need to be especially careful with saildrives. They are very susceptible to stray current corrosion. If they are somehow even a little positive relative to the keelcooler that is connected to battery negative, they can dissolve rapidly. If you want an idea of the potential magnitude of the issue do a google image search for "stray curret saildrive". I am not arguing that you should not connect the keelcooler to battery negative, just be aware that if things are not as perfect as they might look, your concern about this connection might be justified.

Again, this is the very, very short version. Bonding for corrosion control is complicated by issues around DC power, lightning protection, and shore power safety.

[Ween]

Originally Posted by Karanga

Thank you so much for your reply.

My keel cooler is the version WITH zinc anodes.
My understanding of the installation manuals says this keel cooler must be connected to the negative of the battery (but not to any other zincs, which is a requirement of non-anoded keel coolers).
I just have this horror of connecting my battery directly to a huge electrolytic solution!

Just to repeat... the wiring on this boat is immaculate and appears perfect, which is why I am reluctant to do anything different!

Thank you again

Hi Karanga,

Were you able to resolve this issue? I too have stray current from the same setup verified as Boatpoker suggested by using the silver chloride electrode test.

When my fridge is on I get a 16mV increase at the test electrode. This happens in spite of my coolers being well connected already to the bonding circuit.

I've been losing prop zincs quickly and need to find a fix for the stray current being introduce to the water via the copper fridge lines from the compressor/system.

Thanks.

[Karanga]

Originally Posted by Ween

Hi Karanga,

Were you able to resolve this issue? I too have stray current from the same setup verified as Boatpoker suggested by using the silver chloride electrode test.

When my fridge is on I get a 16mV increase at the test electrode. This happens in spite of my coolers being well connected already to the bonding circuit.

I've been losing prop zincs quickly and need to find a fix for the stray current being introduce to the water via the copper fridge lines from the compressor/system.

Thanks.

I don't know if resolved or not.
I don't have a reference cell to check what is happening (can't get anything in Pacific Islands - unless a fellow cruiser is nearby for a loan?)
I have bonded the keel cooler to negative bus of battery, cos that's what Frigoboat instructs clearly.
I don't know if it has had any effect.
Feels a bit uncomfortable...