Do galvanic isolators reduce anode wear?.........some facts

[dkm]

6 years ago I bought a Targa 34 and lifting it for the first time noticed how rapidly the anodes had worn, particularly on the anti cavitation plates and that there was small spots of corrosion on the outdrives in various places. I fitted a galvanic isolator and my perception is that this made a real improvement though it's always subjective unless you actually measure it.
Last year I bought a new Targa 38 and launched it in May. 5 months later the boats was lifted and once again I noticed anode wear faster than I would have expected. This time I weighed the anodes as they came off and found that those on the anti cavitation plate had lost 58% of their new weight, those on the trimtabs, cross bar and bow thruster lost in the region of 15-25% and the anode on the transom shield had lost a massive 67% of its weight. At this rate I was looking at lifting the boat every 5-6 months just to change a few anodes. At this point I had a galvanic isolator fitted and I've just lifted the boat again after another 5 months.
The boat has been moored since new at Swanwick for both periods ( with and without a galvanic isolator), each period being 5 months, the boat was at the same marina on the same berth and the only difference is the season and the fact that a galvanic isolator was fitted for the second period.
There are no signs of corrosion anywhere and the props look like new, except a different colour, so the anodes are clearly doing their job and all have the classic look of anodes that are dissolving at the appropriate rate.
The difference in anode wear is startling and I can only put it down to the galvanic isolator. Anode wear on the anti cavitation plate is reduced from 58% to 12% for the same period ( and these are exactly the same anodes, VP specific); trim tab, cross bar and bow thruster wear is redced to around 10% and the transom shield wear reduced to 25%.
The cost of the galvanic isolator at less than £200 fitted was well worht it and paid for itself in the first season.

 

[kashurst]

a very scientific test - I have had an isolator on my last two boats and found similar results.

 

[Hurricane]

IMHO
Galvanic isolators are not intended to slow your anode wear down.
They are purely to provide a safe way of isolating between the shore power's earth and your boat's earth.
Of course, if there is a stray current running along the shore supply's earth wire, you will get extra wear and in that case, a galvanic isolator would help.
A galvanic isolator will "insulate" between the shore earth and the boat's earth for voltages less than 1.5v (the voltages generated by the galvanic protection system)
But WILL conduct safely back to the shore if there was an electrical failure.

So, in your case, it seems that there is a problem in the shore supply because you now have less wear with the isolator than without.
So - it seems that there is something wrong with the marina's earth or some other boat on the same earth leg is continually leaking electricity down the shore supply's earth.
Remember, where there is a current flow, there will be a resultant voltage which, I believe in this case, has raised the voltage on your boat's earthing system thus forcing the anodes to dissolve quicker.

All IMHO of course.

 

[VicS]

IMHO
Galvanic isolators are not intended to slow your anode wear down.
They are purely to provide a safe way of isolating between the shore power's earth and your boat's earth.
Of course, if there is a stray current running along the shore supply's earth wire, you will get extra wear and in that case, a galvanic isolator would help.
A galvanic isolator will "insulate" between the shore earth and the boat's earth for voltages less than 1.5v (the voltages generated by the galvanic protection system)
But WILL conduct safely back to the shore if there was an electrical failure.

So, in your case, it seems that there is a problem in the shore supply because you now have less wear with the isolator than without.
So - it seems that there is something wrong with the marina's earth or some other boat on the same earth leg is continually leaking electricity down the shore supply's earth.
Remember, where there is a current flow, there will be a resultant voltage which, I believe in this case, has raised the voltage on your boat's earthing system thus forcing the anodes to dissolve quicker.

All IMHO of course.

The trouble with your HO is as you have said the galvanic isolator will "insulate" the boat from the shore supply earth for low voltages typically less than 1.5 volts that are of galvanic origin. It will not isolate if the origin is a higher voltage source. It wont therefore offer any protection if the source is a defect in the marina's power distribution system, nor for the same reason will it protect against defective 12 or 24 volt systems on board nearby vessels.

The fact that the OP's Galvanic isolator has lead to a reduction in the rate of loss of anodes indicates that the source of the previous high rate of loss of anode material was in fact low voltage source, almost certainly "galvanic" in origin.

The classic, but perhaps rather theoretical, example is that of a large steel hulled boat nearby, (with no electrical defects) connected directly to the shore-power earth.
If your own boat has the shore-power earth connected directly to the anodes, without a galvanic isolator, you have a situation in which your anodes will be connected to and attempting to protect the large nearby steel hull. A high rate of loss of material from your anodes will be the result.

A similar situation could arise from the steel piling and other structural steel work around the marina.

 

[Beamishken]

if its the case that fitting a galvanic isolator reduces anode wear will leaving the boat unplugged from the shore power do the same thing or do you still get the same effect from the 12v systems?

 

[GrahamHR]

Fit an active corrosion protection system. They really do work; the anodes on my drive last at least 2 x 6 month seasons, usually more ( with no outdrive corrosion).

Also, ensure there is electrical continuity between all the outdrive parts and the anodes; there are bonding wires. No continuity = no protection from the anodes. Certainly on the DPS by design there is no continuity between the stainless props and the outdrive. So, the stainless props so mounted do not induce corrosion in the outdrive, despite popular myths to the contrary.

 

[halcyon]

The classic, but perhaps rather theoretical, example is that of a large steel hulled boat nearby, (with no electrical defects) connected directly to the shore-power earth.
If your own boat has the shore-power earth connected directly to the anodes, without a galvanic isolator, you have a situation in which your anodes will be connected to and attempting to protect the large nearby steel hull. A high rate of loss of material from your anodes will be the result.

A similar situation could arise from the steel piling and other structural steel work around the marina.

First met galvanic corrosion around 1992, got a phone call from Frank at Sealine, he was sending me a bill for new props on a new 450, corroded away in a month, must be a faulty battery charger. The charger was okay, turned out the badly corroded prop was near the pontoon pile.

Now, or at least then, marina mains earth was a spike in the ground by the marina office, by the time you reached the boat you had a lot of volt drop. While the pile driven into the marina mud had a good earth. So as the bonded ships earth was higher than the pile, we had a current flow.

So you do not need a fault to get corrosion, just a little voltage which is what the isolator blocks.

Brian

 

[VicS]

First met galvanic corrosion around 1992, got a phone call from Frank at Sealine, he was sending me a bill for new props on a new 450, corroded away in a month, must be a faulty battery charger. The charger was okay, turned out the badly corroded prop was near the pontoon pile.

Now, or at least then, marina mains earth was a spike in the ground by the marina office, by the time you reached the boat you had a lot of volt drop. While the pile driven into the marina mud had a good earth. So as the bonded ships earth was higher than the pile, we had a current flow.

So you do not need a fault to get corrosion, just a little voltage which is what the isolator blocks.

Brian

If it was just case of a steel pile and props , which were presumably a copper alloy of some sort, brass, bronze, Ni Al bronze or whatever, linked via the earth conductor galvanic corrosion would cause the steel to corrode, not the props. Must have been some other factor that caused electrolysis and corrosion of the props.

My money would have been on a defect in the 12 volt installation on board that made the props anodic relative to the earth conductor and the pile.

 

[VicS]

if its the case that fitting a galvanic isolator reduces anode wear will leaving the boat unplugged from the shore power do the same thing or do you still get the same effect from the 12v systems?

If the shorepower is not connected there is no circuit between the anodes and other boats or the structural steel work so no corrosion of the anodes caused by that route.
It does not mean that some obscure fault or incorrectly wired equipment on board could not result in electrolysis and severe corrosion.

Case on here a couple of years or so ago where what was almost certainly electrolysis of a skin fitting due to probably a bad connection coupled with an incorrectly connected wired bilge pump caused a boat to sink in its berth..

 

[halcyon]

If it was just case of a steel pile and props , which were presumably a copper alloy of some sort, brass, bronze, Ni Al bronze or whatever, linked via the earth conductor galvanic corrosion would cause the steel to corrode, not the props. Must have been some other factor that caused electrolysis and corrosion of the props.

My money would have been on a defect in the 12 volt installation on board that made the props anodic relative to the earth conductor and the pile.

It was the difference in earth voltage on the pontoon feed into the boat, due to cable run and connections, and earth voltage of the pile which is driven into the earth, so is common to the earth spike at the marina office.

Brian

 

[VicS]

It was the difference in earth voltage on the pontoon feed into the boat, due to cable run and connections, and earth voltage of the pile which is driven into the earth, so is common to the earth spike at the marina office.

Brian

So there must have been a current flowing in the earth conductor due to a fault on the system or on one of the other boats. Do you know if it was AC or whether it was DC due to a fault on the DC system on another boat? I guess there must have been some poor connections in the earth conductor to have given rise to a significant voltage difference ...... or a high fault current.
A galvanic isolator would probably not have been effective. The only thing that would have saved the props I guess is an isolation transformer.

 

[halcyon]

So there must have been a current flowing in the earth conductor due to a fault on the system or on one of the other boats. Do you know if it was AC or whether it was DC due to a fault on the DC system on another boat? I guess there must have been some poor connections in the earth conductor to have given rise to a significant voltage difference ...... or a high fault current.
A galvanic isolator would probably not have been effective. The only thing that would have saved the props I guess is an isolation transformer.

Prop and pile are different material, the Earth completed the loop allowing a small current to flow, a GI breaks the loop, have you seen an elastic band cut a sheet of stainless by electrolytic corrosion.

Brian

 

[VicS]

Prop and pile are different material, the Earth completed the loop allowing a small current to flow, a GI breaks the loop, have you seen an elastic band cut a sheet of stainless by electrolytic corrosion.

Brian

But know you have gone back to where we started this discussion in posts #7 and 8

If the pile was steel and the props were any of the copper alloys normally used for props ( brasses or bronzes including NIBRAL) the props would have been more noble and galvanic corrosion would have caused corrosion of the steel pile not of the props.

For corrosion of the props to have occurred there must have been some other factor driving the current flow in the opposite direction to that which the galvanic effect would have caused. This could have been a fault on the vessel, on another vessel using the same shorepower circuit or perhaps on the shorepower installation itself.

Since this would have been from a higher voltage source, 12 volts, 24 volts or maybe 240 volts, than the galvanic difference between dissimilar metals it is unlikely that a galvanic isolator would have prevented the corrosion.