Zinc anode life, use of hanging zinc?

[JamesG161]

Your Drop Zinc must be electrically connected to your Shaft Zinc.

Using an engine block clamp for the Drop Zinc may not work.

Why?

Some shafts have a non-metal disc between the shaft and engine.

Set you Digital Volt Meter to OHMS. Then touch one lead to the Clamp on the Drop Zinc. Then other lead to the shaft.
You should show Zero Ohms resistance.

If you show Infinite Ohms, then you are NOT protected by your Drop Zinc.
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Your Shaft Zinc loss will NOT stop, but slow down by the Ratio of the surface Areas exposed per @Ralph Johnstone .

Jim...

 

[fullbore]

Thanks Jim. Anode is grounded but I don’t know about shaft and prop. Will need to check that.

 

[fullbore]

I’m trying to better understand galvanic corrosion, So please forgive me if my understanding and/or questions show gnorance.

My understanding is that a complete (2-way) circuit is needed for galvanic corrosion to occur. If so, and if shaft (and thus attached anodes) are electrically isolated from boat ground - then there shouldn’t me galvanic corrosion, and shouldn’t need sacrificial anode.

(Per DVM, the mast seems to be electrically connected to shore power ground, but not the engine block.)

 

[JamesG161]

Per DVM, the mast seems to be electrically connected to shore power ground, but not the engine block.)

I was guessing you connected your Drop Zinc to the mast.

You need to connect that Drop Zinc Clamp to your Shaft or Engine Block per my post#21 above.
______

My understanding is that a complete (2-way) circuit is needed for galvanic corrosion to occur.

True in Salt Water. Sometimes Fresh water but normally Zinc Sacrificial anodes are for Salt water.

The Galvanic circuit is completed through water you boat is floating in.
Jim...

 

[fullbore]

Thanks Jim!

 

[Davidasailor26]

(Per DVM, the mast seems to be electrically connected to shore power ground, but not the engine block.)

I’m surprised by that. Usually the engine is grounded to the DC negative, and the DC negative should be connected to AC ground at one point on the boat. In any case you're trying to protect your prop shaft, so attaching to the engine block seems a better choice than the mast, as Jim says.

The Galvanic circuit is completed through water you boat is floating in.

As I understand it the water provides an electrolyte path for ions but you also need the metals to be connected electrically for electrons to pass between them. With only the electrolyte you have a battery with no load - it won't self discharge and cause any corrosion.

 

[fullbore]

Thanks David!
(If engine block and shaft/prop are not electrically connected to boat bond or shore power ground, not clear of return path, or that anodes even needed…)

 

[Ralph Johnstone]

then there shouldn’t me galvanic corrosion, and shouldn’t need sacrificial anode.

Galvanic corrosion is the result of two (or more) dissimilar metals contacting one another in the presence of an electrically conductive liquid (electrolyte). Your prop and shaft are such an example. Without the lowly (galvanically speaking) zinc collar on your shaft, your shaft (likely SS) would eat your bronze prop.

Even if both your prop and shaft were identical grades of SS, there are always small currents floating around your boat. And if that doesn't cause enough of a problem, even SS is not 100% homogenous. All sorts of microscopic little bits floating around in the SS, some of them cathodic (protected) and some anodic (corroded). Corrosion may be slow but eventually it's gonna get you. Therefore, you always want an electrically connected zinc in the water to act as a supply of electrons to the part(s) which need protection.

 

[JamesG161]

For a long thread but very informative on Boat Corrosion see this...
Sailboat Owner's Guide to Corrosion

Broken down by Chapters of a good Corrosion Expert.
Jim...

 

[fullbore]

Lot of great info! I took time out from reading thread referenced by Jim to order the referenced book.
Thank you also Ralph!

Now back to the thread…

 

[JamesG161]

order the referenced book.

You might see a few of the people on SBO referenced in that Link too like @Davidasailor26 and many more who contributed.
Jim...