I am still not clear on how the voltage is being measured and between what 
points.
I have bronze thru-hulls that are not wired to anything. If I measured between 
them and the lead keel or the engine with the stainless shaft, I am sure I 
would read a voltage, they are dissimilar metals in salt water. Absent me 
wiring up the voltmeter, the thru-hulls have no connection to anything else and 
would not corrode. If you do have all these thru-hulls wired to ground, as is 
done on some boats, you now have a battery. You need to be sure there is a zinc 
involved in there somewhere and it had better have a good connection so that 
the zinc is the part of the battery corroding.
IMHO and also the article referenced in another post, I like to make sure my 
seacocks and thru-hulls are NOT grounded. This eliminates the issue of poor 
contact to zincs and prevents the boat wiring from being a path for leakage 
between boats on one side of you and boats on the other, which is something a 
zinc will have a hard time saving you from.
Joe
Coquina

From: CnC-List [mailto:cnc-list-boun...@cnc-list.com] On Behalf Of Michael 
Brown via CnC-List
Sent: Tuesday, August 14, 2018 1:53 PM
To: cnc-list@cnc-list.com
Cc: Michael Brown <m...@tkg.ca>
Subject: [EXTERNAL] Re: Stus-List Galvanic corrosion (Della Barba, Joe)

You could try reading current with your multimeter instead of voltage. Small 
stray voltages
likely exist when measuring between lead and SS even in fresh water but I doubt 
they will
sustain much current.

Just as an example of magnitude very roughly if there was a current of 1 amp 
continuously
in normal soil it would corrode about 20 lbs of steel in a year.

If you are reading less than 0.5 milliamp I would say those two surfaces are 
not going to have
much corrosion between them. If you are seeing over a milliamp then there is an 
issue.
The circuit requires two paths, the dissimilar metals in water and then 
something on the boat
connecting them together. Some advice has been to connect the mast, standing 
rigging and
keel together but do not ground them to the boats electrical system. That may 
be difficult
if items like the VHF antenna is mounted to a metal bracket on the mast. You 
can test for
this by measuring the resistance between the mast and ship's ground. Anything 
under
10 ohms would indicate a connection.

Michael Brown
Windburn
C&C 30-1


Date: Tue, 14 Aug 2018 13:41:43 +0000
From: "Della Barba, Joe" 
<joe.della.ba...@ssa.gov<mailto:joe.della.ba...@ssa.gov>>


Voltage readings between what and what?


Joe Della Barba
Coquina

Sent: Monday, August 13, 2018 7:25 PM

Hi Alan

The readings I posted are at anchor in the Thousand Island area.  I almost 
never plug in during the cruising season.  I only use shore power to charge the 
batteries during winter layup when the solar and wind are removed, every 6 to 8 
weeks

So today I did more testing.  I disconnected the positive cable for the start 
battery and I still saw 0.02V at the prop shaft.  So I ruled out that as an 
issue.  I disconnected the battery charger and still saw voltage.  Next I took 
the wind generator out of the equation and the results were the same.  Next I 
disconnected the windlass, same results.  Next I moved inside and checked for 
voltage on the tube for the centre board pendant.  Saw 0.25 V.  I removed the 
positive cable to the house electrical panel.  Still saw voltage.

Am I doing this right.  My memory tells me that the way I?m testing might be 
creating some kind of battery but my memory might be tricking me.

Could really use some help from any electricians or corrosions specialist out 
there.


Mike
C&C 37 K/CB Shoal draft
Persuasion
Stormont Yacht Club
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