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Old 01-14-2015, 06:04 PM   #1
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Aluminum or zinc anodes

My search function isn't working, so can someone point me towards a discussion of Al vs. Zn anodes or care to weigh in? Salt/brackish water boating in Narragansett bay.
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Old 01-14-2015, 06:07 PM   #2
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Aluminum are better in salt and brackish than straight zinc. I boat in both also and will be changing my boat over probably next year. The one thing you must do is run all zinc or all aluminum - not a mix.

BoatUS – BoatTech – Sacrificial Zincs by Don Casey

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Old 01-14-2015, 07:51 PM   #3
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I was under the impression that zinc was much better but the switch to aluminum is a green thing. I made a point of getting all zinc before launch this fall.
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Old 01-14-2015, 10:28 PM   #4
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Check the archives and here's a good link but remember there is more than one correct answer:

BoatZincs.com Frequently Asked Questions

Take a look at question 6 and 7. Also click on the charts in 7.

And maybe we'll here from Fastbottoms, a forum member and diver who cleans bottoms professionally. He's good at this stuff.
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Old 01-15-2015, 03:38 AM   #5
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I am replacing all my zincs with aluminum. They are side-by-each on the galvanic scale and al is cheaper and is supposed to last 20% longer. At least it is for now, until demand goes up, then I'll switch back to zinc!

When you look at the scale, note the one that's on the top at 1.0 - graphite. Maybe you might re-think putting graphite-impregnated stuffing in your bronze stuffing box rubbing on a stainless shaft?
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Old 01-15-2015, 07:09 AM   #6
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I was under the impression that zinc was much better but the switch to aluminum is a green thing. I made a point of getting all zinc before launch this fall.
Got nothing to do with green, it's all about voltage potential.

Zinc = -1.05v (salt water).
Aluminum indium = - 1.10v (brackish water, although some thruster) mfg's recommend for all water).
Magnesium = -1.60v (fresh water).

Basically salt water is more conductive than fresh so a lower voltage potential is required with brackish water in between. Either under or over protecting below the water line metals by use of incorrect anodes can cause damage.

These voltage potential differences may seem small but the effect can be considerable. Zinc used in fresh water will quickly (a couple of days) develop a calcereous scale which makes the anode useless. Magnesium in salt water will get eaten rapidly and again aluminum falls somewhere in the middle.

This is an extremely complex issue and if anyone is interested I am hosting the four day ABYC Corrosion Analysis course at Port Credit Yacht Club in March.
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Old 01-15-2015, 09:01 AM   #7
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Got nothing to do with green, it's all about voltage potential.

Zinc = -1.05v (salt water).
Aluminum indium = - 1.10v (brackish water, although some thruster) mfg's recommend for all water).
Magnesium = -1.60v (fresh water).

Basically salt water is more conductive than fresh so a lower voltage potential is required with brackish water in between. Either under or over protecting below the water line metals by use of incorrect anodes can cause damage.

These voltage potential differences may seem small but the effect can be considerable. Zinc used in fresh water will quickly (a couple of days) develop a calcereous scale which makes the anode useless. Magnesium in salt water will get eaten rapidly and again aluminum falls somewhere in the middle.

This is an extremely complex issue and if anyone is interested I am hosting the four day ABYC Corrosion Analysis course at Port Credit Yacht Club in March.
For some one planning to do the Great Loop and starting on Lake Michigan, how would you recommend they configure the anodes?
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Old 01-15-2015, 09:57 AM   #8
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For some one planning to do the Great Loop and starting on Lake Michigan, how would you recommend they configure the anodes?
I'd suggest aluminum. They won't last as long in salt water but they will do the job. There are too many variables to predict their lifespan.

The vast majority of anodes are improperly installed. For a little bit more about this go to Marine Survey 101 and scroll way, way, way down to the photo of an anode and a multi-meter.
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Old 01-15-2015, 10:57 AM   #9
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boatpoker,
Thanks very much for your input. I needed it.
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Old 01-15-2015, 01:34 PM   #10
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I'd suggest aluminum. They won't last as long in salt water but they will do the job. There are too many variables to predict their lifespan.
We keep our boat on a river well up the Chesapeake that ranges between fresh and brackish for months at a time. Whenever we're away from port, we'll be in water that's brackish to salty. Does this sound like aluminum would be the right pick for us?

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Old 01-15-2015, 01:37 PM   #11
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We keep our boat on a river well up the Chesapeake that ranges between fresh and brackish for months at a time. Whenever we're away from port, we'll be in water that's brackish to salty. Does this sound like aluminum would be the right pick for us?

Thanks, BP!
That would be my choice.
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