[C320-list] Strange Coating on Prop and Shaft

[Michael Leschisin]

This past October, when we hauled the boat out for the season, there was a thick hard coating, mostly on the bronze prop, but also heavily on the stainless shaft tip and speckled across the zinc anode, the stainless shaft and bronze strut.  My first reaction was, “Oh crap I just cooked a thousand dollar prop with galvanic corrosion” but on closer inspection it seemed to be some sort of yellowish/ off-white coating on the underwater drivetrain.  Some areas of the prop were up to an eighth inch thick but after an aggressive power washing and scraping with a metal putty knife I was able to remove the hard crusty substance.  I’m guessing it was calcium carbonate or something similar, but I found there was no pitting or metal lost at all.  In fact the narrow threads of the stainless shaft were untouched, even the cotter pin was 100% intact and the bronze prop showed no loss of metal.  (I can email before and after photos if anyone has any thoughts on assessing the issue).  The drivetrain was clean as a whistle when the boat was splashed in May of 2022, so the buildup occurred after just one season in the water.  There was never any buildup or anything odd in any previous season and nothing has really changed on the boat other than the installation of a new fail safe isolator in 2020 (no buildup following installing it in 2020 or 2021).  In 2018, after I had first purchased the boat, I put a fresh zinc anode on the shaft, come to find out I should be running a magnesium one in fresh water (rookie mistake), but again no buildup in any of the previous years.

Here’s the weird part; the boat is slipped in Menominee, MI. (Great Lakes, fresh water) and after talking with the two boatyards that service our marina, out of the couple of hundred boat that they pulled, mine was the only one with this odd coating on the drivetrain.  I’m wondering if anyone has experienced anything similar or has any thoughts on how to start chasing down the source of the issue.  

I did some research online and I’m guessing I have cathodic coating happening, which is sort of the opposite of galvanic corrosion.  Rather than your drivetrain acting as the negative pole for the stray current coming off of someone else’s boat and giving up its metal, I think I’m pushing a positive stray current into the water and attracting minerals out of the water.  I’m not sure if this assessment is correct, if my isolator would allow a stray current from the dock to be passed onto the boat via the ground or how and where to start to digging for the source of a stray current on my boat.

Any thoughts or ideas would be much appreciated,



Michael Leschisin

Wild Blue Yonder
C320 995
Menominee, MI