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Old 21-07-2012, 19:38   #16
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Re: The Blonde Zinc Replacement

Fstbttm's is right. The zinc corrodes to the point where the screw will sometimes loosen. Even with over 2/3 the zinc left...

The cone zinc's are just zinc no collar and then tend to wear from the inside as well as the outside. I actually do check my zinc's btw... and have found them loose sometimes after about 1/3 loss
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Old 21-07-2012, 19:41   #17
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Re: The Blonde Zinc Replacement

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I disagree unless the boat is not maintained.
I'm sorry but 18 years of underwater zinc installation experience tell me different.

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Not sure how cone zincs work but think they also have a load sleeve.
If a prop (cone) zinc depletes sufficiently, the screw can easily be completely unloaded and free to move in either direction. In fact, it is not uncommon for these zincs to deplete in such a way that the main body comes completely off, leaving nothing but the screw and a small disk of anode material where the screw went through the zinc. Nothing securing the screw to the prop nut but threads.
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Old 21-07-2012, 19:43   #18
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Re: The Blonde Zinc Replacement

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HUM. on the galvanic chart, aluminum is more noble (Ie lower voltage potential) then zinc, IE less current inductance, so zinc protects aluminum.
I'm no physicist or electrical engineer. I just know what the zinc manufacturers tell me.
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Old 21-07-2012, 19:56   #19
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Re: The Blonde Zinc Replacement

I really should look in to a magnesium anode too I'm thinking, if I hang around the delta more. Zinc does have issues in fresh water plus it pollutes more. I'm not sure aluminum really protects the zinc in bronze prop's, It works for copper though. Me I would use zinc for salt and magnesium in fresh.

Power boaters could get away with aluminum I'm thinking... But then I've only studied corrosion a little and am not an expert. I do get feisty sometimes.... Feisty sounds better
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Old 21-07-2012, 20:43   #20
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Re: The Blonde Zinc Replacement

Sailorchick,

Do you know exacally what alloy is in the prop? Sadly it seems most manufacturers of marine parts can't figure out the difference between bronze and brass, and use the terms interchangeably. Technically bronze doesn't use zinc as an alloying agent, and instead tin is used, while brass is a copper/zinc alloy.

Of course a lot of marine 'bronze' really has a good bit of zinc in it, but so long as there is less than about 15% zinc the alloy self isolates the zinc from saltwater. On a microscopic level basically the zinc corrodes away, but it is such a small part of the whole that there is a boundary layer of copper surrounding the rest of it. Once the electrolyte can't get to the zinc, no electrolysis.

On the other hand if you have a true bronze prop (and many of them are), then there is no zinc in the alloy to begin with, and what you are protecting is the stainless-bronze connection. Most likely actually the stainless, but it depends on exacally which grade of stainless shaft you have, and what bronze alloy you have. Most combinations are actually pretty close galvanicaly. It is typically the Mid alloy steel in the engine that really need protecting, not the prop-shaft connection.
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Old 21-07-2012, 20:57   #21
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Re: The Blonde Zinc Replacement

Your right.

Bronze most of the time has tin in it. Though now a days it can have zinc or any of a few other metals in it too. Odds are mine has tin as an alloy. Hey blonde moment. Sounds better then a senior moment anyway....

As rosanna rosanna danna would say..... never mind.

So with a tin alloy aluminum would work for bronze props. Though one would need to check to make sure.
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Old 21-07-2012, 21:02   #22
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OK, I admit that I scrolled down and did not even read all the responses. I am like...both arms in the air and all the way down to the floor. I admire you for...1 Knowing to check this, 2. Diving to check it yourself, 3. For being a do it yourself kind of gal. Kudos to you. I think I am way to accustomed to being "taking care of". I would be so lost and truly admire you. I am a tough girl and have survived being held captive in Russia. No lie...but just don't have that mechanical instinct. Thumbs up to you!!!
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Old 21-07-2012, 21:05   #23
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Re: The Blonde Zinc Replacement

So with the mild steel issue, do you think my zinc in the sea strainer idea, electrically connected to the engine works??? Well at least better then no zinc I'm assuming.
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Old 21-07-2012, 21:07   #24
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Re: The Blonde Zinc Replacement

What can I say, I learned most of the do it yourself from my dad. I also have a son how did two custom engine swaps (cars of course) before he was 19. So maybe it's in the blood.
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Old 21-07-2012, 21:14   #25
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Re: The Blonde Zinc Replacement

Aluminium has a galvanic potential of -.8, to -1.

The most galvanicaly active brass/bronze that should ever be on a boat for any reason, no matter how bad sits between -.4 and -.3

Unless you have some seriously high zinc brass (some artistic brass goes up to 50% zinc). Theoretically it also is -.4 to -.3, but there is so much surface zinc it never passifies, at least not until there are holes in it. But this stuff is usually used for decorative lamps, and interior architectural detail in houses.

Stainless is tricky, since different alloys can vary massively, and it has different galvanic potential in and out of the water. But generally marine stainless ranges from -.38 to -.58 depending on alloy under water, and 0 to -.1 above the water.

The problem is right at the water-non-water interface for stainless where you can get inter granular pitting. At least without some annode elsewhere in the system to take the load. But it only needs to be more cathodic than the below water cathode to work. So from an electrical standpoint a cast iron anode would be fine, but would obviously rust away in no time.
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Old 21-07-2012, 21:17   #26
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Re: The Blonde Zinc Replacement

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So with the mild steel issue, do you think my zinc in the sea strainer idea, electrically connected to the engine works??? Well at least better then no zinc I'm assuming.
It's certainly better than nothing. But drive shafts are expensive, and the closer to the galvanic cell the better. I thik you are fine for a few days, or a couple of weeks. But I would go ahead and order the proper anode in aluminium.

Alternatively you could replace the drive shaft and prop with titanium ones and not worry about it anymore.
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Old 21-07-2012, 21:22   #27
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Re: The Blonde Zinc Replacement

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So with the mild steel issue, do you think my zinc in the sea strainer idea, electrically connected to the engine works??? Well at least better then no zinc I'm assuming.
Do you have any kind of synthetic spacer in the driveline? If you do, your zinc is likely protecting nothing except maybe the strainer.
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Old 21-07-2012, 21:42   #28
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Re: The Blonde Zinc Replacement

It's a little depressing the way new ideas get dismissed on forums like this. Whoever suggests them is automatically considered to be a neophyte, and whoever dismisses them generally portrays themselves as the fount of all knowledge.

Generally, some of the commenters will be in such a hurry to cement their reputation that they don't even read the thread properly.
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Old 21-07-2012, 22:22   #29
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Re: The Blonde Zinc Replacement

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It's a little depressing the way new ideas get dismissed on forums like this. Whoever suggests them is automatically considered to be a neophyte, and whoever dismisses them generally portrays themselves as the fount of all knowledge.
You know what? It doesn't take a rocket scientist to determine that not every new idea is a good idea. The OP came here looking for opinions about her ad hoc zinc arrangement, and she got them. In this case there were no raves of enthusiasm about what she is doing. Does that distress you? Gosh, I'm so sorry about that.
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Old 21-07-2012, 22:34   #30
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Re: The Blonde Zinc Replacement

Manganese bronze which is commonly used to make propellers is actually a brass, it contains 39% zinc. A friend recently lost a blade, previous owner didn't maintain zincs. You can see the pink of dezincification where the blade fell off.

Bronze types:
Types of Bronze

Here's why not to use zinc in fresh water.

From: Anode FAQs
Zinc anodes can become inactive after only a few months due to the build up of an insulating film of zinc hydroxide.

Seems to me a scrub of your zinc every few months would work.

From same site above, why aluminum alloy works. It's :
Navalloy™ (aluminum/zinc/indium alloy) with -1.1 V on the galvanic table.

(For myself 8 years in fresh water with zinc, took the maxprop in for reconditioning a couple of years ago as the blades get some slop over time shifting back and forth, they didn't find anything else wrong, as in no corrosion issues. Also I have no dock power which might influence what's happening.)


If you really want to read more than you ever wanted to:
Metal Corrosion in Boats
The prevention of metal corrosion on hulls, engines, rigging and fittings
by Nigel Warren

He writes about wiring zincs to nearby metals to be protected.

John
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