Outboard engine anode gone in 15 days of use - why?

Ru88ell

Ru88ell

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I bought a new Mercury 6hp o/b this year, and it came with a small aluminium anode. After 15 days use, 12 of used shore power at some stage, the anode has gone to nothing.

I have an electrical installation of instruments, VHF, lights, etc, powered by a 100ah battery. I have shore power and a trickle charger. The outboard isn't connected to the boat beyond being clamped to the wooden transom.

The engine dealer says that the anode was aluminium. Is that the problem? Should it be something else? I don't have a galvanic isolator as this didn't seem to be an issue with my old engine, a Yamaha 4hp.

Advice gratefully received.
 
Aluminium is for fresh water, for salt it should be zinc. Although I don't know if this would cause the rapid erosion.

Pete
 
Ru88ell said:
I bought a new Mercury 6hp o/b this year, and it came with a small aluminium anode. After 15 days use, 12 of used shore power at some stage, the anode has gone to nothing.

I have an electrical installation of instruments, VHF, lights, etc, powered by a 100ah battery. I have shore power and a trickle charger. The outboard isn't connected to the boat beyond being clamped to the wooden transom.

The engine dealer says that the anode was aluminium. Is that the problem? Should it be something else? I don't have a galvanic isolator as this didn't seem to be an issue with my old engine, a Yamaha 4hp.

Advice gratefully received.
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Aluminium should be suitable. Its preferred to zinc for brackish water but is generally considered suitable for salt water too

No electrical connection to the outboard ? No battery charging facility or electric starting ?

If shorepower or even electrolysis due to some problem with the boats electrical system were to blame Id expect signs of corrosion of the outboard itself , at least once the anode has gone.

No chance it had a magnesium anode I suppose....... Not actually heard of them as available for outboards but they might be.

Magnesium is for freshwater
 
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As PRV says, zinc is normally used in salt water.

BUT, when I had a Mariner 8hp (which is similar engine), I found that the OEM anodes usually only lasted a few weeks. I ended up bolting a larger one onto the anti-cavitation plate. The big one lasted a few years and it meant that the OEM anode lasted well over a season which, at the price that Mariners charged, was a good saving. (I assume that your engine has a thin rectangular anode with an angled screw hole?)

Incidently, isn't your engine also connected to the boat via the battery charging circuit?

As an aside; I wonder why they have aluminium anodes when the engine is also aluminium? I thought the idea was that you had a sacrificial metal which went before the metal you were trying to protect? Somebody may be along to explain.
 
As an aside; I wonder why they have aluminium anodes when the engine is also aluminium? I thought the idea was that you had a sacrificial metal which went before the metal you were trying to protect? Somebody may be along to explain.
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Aluminium anodes are in fact an aluminium /zinc / indium alloy ( usually to a US military spec).

The have a potential, relative to Ag/AgCl, of -1.09 volts whereas zinc has a potential of -1.05 volts. Very marginally, therefore, more active than zinc.
 
prv said:
Don't think Zephyr has any charging from the engine. Don't remember seeing any wires when I was sitting on her eating crisps the other day, anyway :)

Pete
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Correct Pete - no wires, just an engine fixed to the transom. I'm off to the Morbihan festival on Saturday so can pick up a new one on the way. I'll have to keep an eye on it when I'm away.

Guys - do you think a galvanic isolator is needed?
 
Ru88ell said:
Guys - do you think a galvanic isolator is needed?
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Nothing in what you have told us to suggest that it is

If you had hull anodes bonded to the shorepower earth or if there was a connection between the earth and the engine via say a charging circuit the answer would be yes.
 
Ru88ell said:
Guys - do you think a galvanic isolator is needed?
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Surely your shore power isn't connected to the water in any way? No bonded skin fittings or prop shafts - it's basically an extension lead with a breaker or two in the end. So I can't imagine you'd need an isolator.

Pete
 
VicS said:
Aluminium anodes are in fact an aluminium /zinc / indium alloy ( usually to a US military spec).

The have a potential, relative to Ag/AgCl, of -1.09 volts whereas zinc has a potential of -1.05 volts. Very marginally, therefore, more active than zinc.
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Cheers Vic. I thought you would explain it.
 
sailorman said:
Why ?????
Does it not "tilt"
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It's in a well at the aft end of the cockpit. Obviously Russell knows his boat better than I do, but it wouldn't surprise me to learn that the leg hits the transom if you try to tilt it instead of lifting out vertically. Tilting it would also bring the top of the engine further forwards into the cockpit, taking up space.

Pete
 
prv said:
It's in a well at the aft end of the cockpit. Obviously Russell knows his boat better than I do, but it wouldn't surprise me to learn that the leg hits the transom if you try to tilt it instead of lifting out vertically. Tilting it would also bring the top of the engine further forwards into the cockpit, taking up space.

Pete
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In the good ole daze outboards never had anodes
 
If you have a small aluminium anode electrically connected to a large zinc anode that is quite near and in line of sight, the aluminium anode will get consumed and will actually protect the zinc one. The nearer the two anodes, the faster the aluminium will get used up.
Aluminium is ideal as anode in seawater and it's Ok to use Al and Zn on the same boat so long as you dont have them too close together.
 
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