If the centerboard tube is not wired to the ground bus, you can see voltage 
between that and the engine. That would make sense.
In theory there should be 0 volts between the engine and the prop shaft. In 
practice, if you think about it, no part of the rotating mass of the engine can 
possibly be hardwired to the block or it couldn’t turn. There is oil and/or 
rubber seals between any part that turns and the block. Commercial vessels 
frequently use a carbon brush arrangement to connect the ground bus to the prop 
shaft. You also can loosen a prop bolt and make a jumper if the coupler is made 
that way where there is a nut on the backside. Another work around for this 
issue on powerboats is hull mounted zincs wired to the ground bus or you can 
get one of those zinc guppies and wire that to the ground bus when not underway.
The mast voltage is ??? Is the mast grounded in some way or connected to 
underwater metal? My mast is connected to a keel bolt with 4 gauge wire.
Joe
Coquina

From: CnC-List [mailto:cnc-list-boun...@cnc-list.com] On Behalf Of Persuasion37 
via CnC-List
Sent: Tuesday, August 14, 2018 4:03 PM
To: cnc-list@cnc-list.com
Cc: Persuasion37 <persuasio...@gmail.com>
Subject: Re: Stus-List [EXTERNAL] Re: Galvanic corrosion

Joe

I connected the ground from the multi-meter to the engine block.  Then I took 
the positive lead and touched it against various pieces.  Largest voltage shown 
was at the stainless tube for the centre board 0.25.  The mast and prop shaft 
showed 0.02V

Mike
PERSUASION
C&C 37 K/CB
Long Sault

On Aug 14, 2018, at 9:41 AM, Della Barba, Joe via CnC-List 
<cnc-list@cnc-list.com<mailto:cnc-list@cnc-list.com>> wrote:
Voltage readings between what and what?


Joe Della Barba
Coquina
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