Do I need a galvanic isolator?

[cmedsailor]

My boat is 15 years old (10 years in my possession) and I have always kept it connected with the shore power when at the marina. The only zinc of the boat, a cone type at the end of the shaft gets eaten every 8-9 months and I usually replace it earlier than this. I have changed all throughulls recently and despite slight pink colour on some of them they were relatively OK. Neither the propeller, nor the shaft shows any corrosion due to electrolysis. Same with the keel. A galvanic isolator was never installed in my boat. Should I bother installing one?

 

[Mistroma]

It sounds as if things have been OK so far. However, there could always be a fault or you move to a different marina with dodgy electrics and surrounded by boats without anodes. If nothing changes then you can probably get away without a galvanic isolator.

I fitted a galvanic isolator years ago and test at least once each year. It does give some peace of mind and I'm not on shorepower very often anyway. I found a source for relatively cheap isolation transformers a few months after fitting the galvanic isolator, typical. I'd fit an isolating transformer now if the GI blew up and I could get one from this company (or similar price elsewhere).

http://www.airlinktransformers.com/boating_transformers/BT1631

I've never dealt with this company but it does give an idea of possible price and I think they were mentioned in a fairly recent thread.

 

[rogerthebodger]

My boat is 15 years old (10 years in my possession) and I have always kept it connected with the shore power when at the marina. The only zinc of the boat, a cone type at the end of the shaft gets eaten every 8-9 months and I usually replace it earlier than this. I have changed all throughulls recently and despite slight pink colour on some of them they were relatively OK. Neither the propeller, nor the shaft shows any corrosion due to electrolysis. Same with the keel. A galvanic isolator was never installed in my boat. Should I bother installing one?

My question would be . Is there a connection between the incoming mains earth and the shaft anode. If yes I would disconnect it. If you are unsure as some may be fit an GI in the earth line.

What is the boat hull made of if its GRP and all metal is insulated anyway only connect the mains earth to any metal cased mains powered appliances. If the hull is metal (steel or aluminium) it becomes a different ball game.

 

[cmedsailor]

What is the boat hull made of if its GRP and all metal is insulated anyway only connect the mains earth to any metal cased mains powered appliances. If the hull is metal (steel or aluminium) it becomes a different ball game.

It's GRP (a Beneteau).

 

[lpdsn]

All a GI allows you to do is to connect the shorepower earth to the boats ground without any current flowing at low voltages. I think they work up to 1.4V - I definitely had leakage last year when there was a 4V difference (glad I bought the little LED monitor to go with mine). I traced the leakage back to a nearby rarely-used MoBo.

The main reason for connecting up the shorepower earth to the boat's ground is to provide a supplementary path in case of a short. The earth route back to shore is much longer than would be typical at home, so if you do end up with a short the voltage on the earth wire can get pretty high (typically approx 50% of the live voltage) until the RCD trips. So having the supplementary path would reduce the voltage anyone is likely to experience.

It also helps provide a better gnd/earth if for any reason there's a short between the mains live and 12V DC wiring.

That's what you gain. If you don't have a connection between the shorepower and the boat you don't actually need a GI.

The usual practice is to insert the GI in the incoming shorepower earth. It's not what I've done as I prefer straight through copper for the earth connection. So the GI sits between the shorepower earth and the boat's gnd - which is also connected to the boat's -ve. I've connected the water heater earth on the GI side in case of current flow between the heater circuit and the engine block via the cooling water circuit - freshwater but not distilled water.

 

[cmedsailor]

I know it's more expensive, but any other disadvantages if I decide to use one and buy an in line one? I mean one of those you connect with the actual shore power cord.

 

[lpdsn]

I know it's more expensive, but any other disadvantages if I decide to use one and buy an in line one? I mean one of those you connect with the actual shore power cord.

I've never used one but they should work as well as any. Can you get one that indicates when there's leakage? My experience as recounted above is that you can get leakage even with a GI if there's a fault on another boat. I'd never have known but for the little monitor I got with my Safeshore one.

 

[pvb]

I know it's more expensive, but any other disadvantages if I decide to use one and buy an in line one? I mean one of those you connect with the actual shore power cord.

No, that would work as well as an installed galvanic isolator.

For safety, your shorepower earth should be wired to the boat's ground - this is required by legislation for new boats. Your Beneteau may already be wired that way, and you'd be wise to ignore the recommendation to disconnect it!

The other thing to consider is whether you need to leave the shorepower connected when you're not at the boat. If you visit it every few weeks, the batteries won't go flat in that time.

 

[VicS]

My boat is 15 years old (10 years in my possession) and I have always kept it connected with the shore power when at the marina. The only zinc of the boat, a cone type at the end of the shaft gets eaten every 8-9 months and I usually replace it earlier than this. I have changed all throughulls recently and despite slight pink colour on some of them they were relatively OK. Neither the propeller, nor the shaft shows any corrosion due to electrolysis. Same with the keel. A galvanic isolator was never installed in my boat. Should I bother installing one?

The important question one must know the answer to before saying "yes" or "no" to your question, "should I bother installing one?" Is the question Rogershaw asks namely: " is there an electrical connection all the way from the anode to the shorepower earth"

The answer is probably "no" because firstly if the boat does not have a hull anode there is no reason for the flexible shaft coupling to have been electrically bridged ( assuming of course that there is a flexible coupling which would otherwise electrically isolate the shaft from the engine and gearbox)
Secondly when your boat was built it was not a requirement of the ISO13297, the standard for AC installations in small craft, for the shorepower earth to be connected to the DC system negative provided a "whole craft" RCD is fitted as part of the shorepower installation. Since 2012 it has been a requirement of that standard to bond the shorepower earth to the DC negative ( except where the DC system is fully isolated)

My suggestion then is that you check for electrical continuity between the prop and its anode and the shorepower earth. If there is no continuity then there is no need for a galvanic isolator, at least as far as the prop anode is concerned.

If you do find continuity then IMHO the sensible way forward is not to disconnect the shorepower earth from the DC negative but to install a GI ! Your choice whether to opt for the "plug play" type" such as Safeshore Marine offer, to to opt for a permanently wired in one.

Hopefully there is no bonding between the thruhulls or between them and anything else

 

[KellysEye]

I would suggest fitting a galvanic isolator on all boats that use shore power, a bad marina earth can cause stray current corrosion, it doesn't happen often but a GI gives peace of mind.