prop patina

[Jim Hardesty]

My maxiprop doesn't get the brown, green patina that most props get.  After 6 months in fresh water it's splotchy looking and some what rough to the touch.  I've been polishing between seasons, the roughness is like a lime mineral that comes off pretty easy.  As the boat is in fresh water I use a magnesium anode from BoatZincs.com, also the docks are checked for stray electric current.   I'm thinking that the magnesium anode may be the cause of the weird build-up on the prop. 
Does that sound right?  Any one else have similar experience?
Jim

[KWKloeber]

Jim
So the roughness just the deposit there's no pitting or etching on the prop surface?
Does it react with LimeAway or CLR?

[Jim Hardesty]

No pitting.  Haven't tried any chemicals. 

[mark_53]

How many anodes are you using and where?

[Jim Hardesty]

One on the end of the prop, shown in pictures, plus one on the shaft both magnesium. 
Jim

[mark_53]

I keep two on the shaft between the strut and the boat and one on the strut.

[scgunner]

Jim,

How about trying a Zinc anode?

[GB]

Jim -

After a season's use in Lake Michigan w/Magnesium anodes, we get a similar deposit on the prop as shown in the picture.  The prop polishes up great in the Spring, so I haven't worried about it.

I came across this link (also of a freshwater boat in Lake Michigan) that indicates the deposit is a Calcareous Coating (https://www.corrosionpedia.com/definition/200/calcareous-coating):
https://jeanneau.proboards.com/thread/5256/prop-shaft-calcium-deposits

[Jim Hardesty]

we get a similar deposit on the prop as shown in the picture

Thanks GB,
That looks just like I get.  Got a name for it now, Calcareous. Great information.  I believe the deposits do hurt the performance of the prop, so unless more information comes up think I'll clean and polish the prop another season and go to zinc anodes. The problem with zinc anodes in fresh water is they get a build-up and loose effectiveness, so If the prop looks good after using zinc I'll clean them up each season.  Much less work.
Jim

[Robert Mann]

My prop, which was a Martec, showed nothing like this after 7 years in Lake Lanier. I use a magnesium anode on the shaft. It must be water chemistry.

[GB]

Jim -

Just did the annual prop cleanup, which reminded me of this thread. Did switching to zinc anodes make a difference for you?

Thanks,
Greg

[Jim Hardesty]

Did switching to zinc anodes make a difference for you?

Yes it did. After a season with the zinc on the prop (Maxiprop)  had no build-up.  I am using a magnesium shaft collar anode a few inches from the strut.   Because I had it, may try aluminum next time.
Jim