Thru hull bonding advice?

[Alasgun]

I recently read an article in Passagemaker where the author was extolling the virtue of bonding (grounding) to the thru hull. It got me thinking that I had seen a screw on the side of my thru hull fitting. I went and looked, sure enough there is a screw in the side of the sea cock that I had no use for previously. So I present the question, is there any benefit or need to consider the additional ground? I assume the galvanic isolator is grounded back to the battery. Would additional grounding be beneficial?
Non Electrical Mike!

[james]

[thataway]

I'll have to double check my thru hull, but I believe it is bonded--the Armstrong bracket is bonded.

The bonding is most helpful in marina stored boats. I have owned boats bonded and unbonded and not really seen much difference ref electrolysis. There is a way to measure differential voltage using a silver chloride electrocde and volt meter. This should be done if you are having an electrolysis problem.

Agree that most electrolysis problems are related to the stray current from mis-wired boats or docks in the vacinity--and this is a fairly common problem.

[Minnow]

This is worthy of a whole book, but I'll try to touch on a few points.
Galvanic corrosion is a DC problem of dissimilar metals in contact with one another, either directly or through an electrolyte. The galvanic isolator on your boat will block DC currents, up to only about one volt, so that your neighbor's boat and yours cannot complete a galvanic DC circuit through the shore power connections.
But the galvanic isolator will allow AC fault current to pass, so that if your grounded microwave shorts out, it can carry enough current back to the panel to trip a breaker. The shore power ground must be bonded to DC negative. Mine was done from the AC panel ground bus to the cigarette lighter negative on that panel. If you don't have, or remove this jumper, it's likely you'd have less corrosion issues, but you can't do it for safety's sake.

The bonding, (not grounding) system on a boat is there for life safety.
Say, you're working with a drop-light, drill, whatever, and the frayed cord touches some metal thing on the boat. What you want to happen is that the high fault current flows through the thing, through the bonding jumper, to the AC panel ground, and trips the breaker.
If the thing wasn't bonded, then you could receive a shock if you got between it and another surface that was grounded (AC appliance), or bonded (motor, etc.). This isn't likely on a C-Dory, but is a real issue in the bilge of a big boat.

None of this has anything to do with stray current corrosion. Let's say your boat has a bilge pump whose splice is sitting in a puddle in the bilge. It isn't a dead short, so it doesn't blow the fuse, but it is creating a 12 volt circuit to a bonded piece of equipment, your through-hull. The galvanic isolator isn't going to help, because the circuit is contained on your boat, and if it were from your neighbor's boat it still wouldn't work because it can't block more than about one volt. Stray current corrosion can happen very quickly, hours or days, while galvanic corrosion takes time.

I know this is confusing, I read and re-read that excellent article in Passagemaker a few times, and it's still sinking in. And I'm even an electrician. If I've misinterpreted any of this, I hope I'll be corrected.

IMHO, don't bother bonding the through hull.

[mikeporterinmd]

Opinions on galvanic isolators? I installed shore power on a CD-22.
I bonded the AC grounding to DC neutral like you are supposed to.

The only metal in the water is the portion of the outboard bracket that
is always in the water...no throughhulls on a CD-22!

The boat will be kept in a slip. Should I go to the expense of
adding a galvanic isolator?

Mike