Zincs disappearing, prop shiny

[Han]

Hi All,

We have a puzzling problem on our boat. We're burning through a shaft zinc every 2-3 months and we also have a very shiny prop, which looks nice but also seems to be indicative of a problem. Reading and research points to a stray current issue, but tracking it down has been very difficult. We're full time cruisers, so it's not an issue with marina power. We did repower in the spring which is when we first noticed this problem, but our old engine was a very unique setup (a Norwegian Sabb with integral prop), so it's hard to be sure the issue didn't exist before the repower. In any case, we've tried testing each circuit on the boat for a ground fault following Don Casey's procedure in Sailboat Electrics Simplified (i.e. disconnect the main positive from the battery and put an ammeter in line. Turn all breakers off. Disconnect the negative from each circuit one at a time and flip that breaker on. Watch for any sign of current flowing which indicates a ground fault). So far no luck finding anything and we're stumped. Any suggestions greatly appreciated!

[boatpoker]

Originally Posted by Han

Hi All,

We have a puzzling problem on our boat. We're burning through a shaft zinc every 2-3 months and we also have a very shiny prop, which looks nice but also seems to be indicative of a problem. Reading and research points to a stray current issue, but tracking it down has been very difficult. We're full time cruisers, so it's not an issue with marina power. We did repower in the spring which is when we first noticed this problem, but our old engine was a very unique setup (a Norwegian Sabb with integral prop), so it's hard to be sure the issue didn't exist before the repower. In any case, we've tried testing each circuit on the boat for a ground fault following Don Casey's procedure in Sailboat Electrics Simplified (i.e. disconnect the main positive from the battery and put an ammeter in line. Turn all breakers off. Disconnect the negative from each circuit one at a time and flip that breaker on. Watch for any sign of current flowing which indicates a ground fault). So far no luck finding anything and we're stumped. Any suggestions greatly appreciated!

If your description is accurate it is unlikely that you are suffering from "stray current corrosion. SCC is a very fast acting corrosion and I've seen propllers disappear in a matter of days. It sounds to me like a case of galvanic corrosion. The only surefire way to determine this is with a and silver/silver/chloride half cell such as this ... https://www.fisheriessupply.com/pro-...st-meter/20008

you can also buy the half cell on it's own without the display and use your multimeter ... Like this one https://www.marine.com/products/11-7...ll-agagcl-2039

[Rumrace]

By chance is your prop bronze ?

[Han]

Originally Posted by Rumrace

By chance is your prop bronze ?

It is ..

[mlydon]

The rate at which you go through zincs is highly dependant upon a large number of factors, a large one of which is (water) current flow…..

I moor on the Hudson River, in brackish water, and typically have to change my zincs every 3 months, as a ‘normal’ rate. People is marinas have to worry about improper grounding of OTHER peoples boats as well as their own.

I don’t know what to tell you about the shiny prop - bronze subject to galvanic corrosion goes dull and pink as the zinc in the alloy is lost…..

Matt

[Han]

Originally Posted by mlydon

The rate at which you go through zincs is highly dependant upon a large number of factors, a large one of which is (water) current flow…..

I moor on the Hudson River, in brackish water, and typically have to change my zincs every 3 months, as a ‘normal’ rate. People is marinas have to worry about improper grounding of OTHER peoples boats as well as their own.

I don’t know what to tell you about the shiny prop - bronze subject to galvanic corrosion goes dull and pink as the zinc in the alloy is lost…..

Matt

Thanks for your thoughts. I guess it could be regular prop/shaft galvanic corrosion that's eating our zincs, but it does seem very fast to me. Our previous boat went through a zinc every year or so. I know that no two boats are the same, but this seems a bit much.

The shiny prop is also strange, on other boats it's been a fight to keep growth off, but this one still looks brand new after six months. No other boat around has such a gleaming prop! An old fisherman we met says the shiny prop indicates a positive current and is from plating onto the prop (maybe with copper from bottom paint?). I'm not an expert, but the prop does seem suspicious.

As for location, we're currently cruising, so we're constantly in new locations (we've traveled Alaska to California since the spring) and we're at anchor most of the time.

It's definitely a puzzle, and I'd be much happier not buying zincs constantly, not to mention worrying about having some stray current somewhere.

[fstbttms]

This is what bronze looks like when attacked by stray 12-volt current:

[Bowdrie]

I do not know the assemblage of your Sabb engine between engine and prop.
But I have always advocated for the installation of a "Drivesaver", (or similar device,) to electrically isolate the shaft and prop from the engine and any and all electrical connections, (and devices such as alternators,) that may be on the engine.