On 31 March 2020 at 10:34 "karkauai via Groups.Io" <karkauai@...> wrote:

Gary and Danny,

I am certain that you are right about the WOB acting as the sacrificial anode.

If this had gone on any longer, might have had catastrophic failure of the WOB.  What damage would have occurred if that had happened?  I think that the shaft zinc might be a good solution. Serving the same purpose as the prop zinc.  With added protection I think the stainless WOB would be OK. Thoughts?

Kent

Kristy

SM 243

On Mar 30, 2020 1:33 PM, "Gary Silver via Groups.Io" <garysilver@...> wrote:

On Sun, Mar 29, 2020 at 11:55 AM, karkauai wrote:

bottom line for me is that while hull potential is a good measure of overall bonding efficacy, we must REGULARLY measure continuity at ALL metals exposed to sea water to confirm that they are protected.  It goes on my quarterly maintenance list today.

Hi Kent:  What a great find and a great discussion along with Joel's post.  Sooooo valuable.  Thanks for sharing.  Glad you got that WOB (wearing out bearing) out.  It is still a bit of a mystery to me as to why only that corroded.  The color (salmon like) looks like de-zincification.  I am only speculating but, since brass is copper and zinc, the zinc in the brass was the least noble element in that vicinity (i.e.the prop, c-drive shaft etc.) Everything must have all been more noble metals, so with no continuity of the C-drive to the normal sacrificial anodes (on the rudder), the WOB became the sacrificial anode for your C-drive.  I guess it only corroded where it was in contact with sea water and hence the lack of corrosion "inside" the lip seals etc. 

Again, thanks for you invaluable reminder about preventative diagnostics on our bonding system. 

Gary S. Silver
s/v Liahona    (Puerto Del Rey Marina, in-accesible due to Puerto Rico's COVID-19 quarantine)
SM 2000 # 335