I got my boatzinc reference probe and did some testing. Now I need some help with the data. For background, I've been loosing a pair of shaft zincs in 3mo and have had significant pitting in the lead keel
the past 2 winters requiring epoxy
, refairing, repaint. more background in previous thread here:
Stray Current Isolation Testing?
Testing was done at the mooring
with the probe hung over the side near the prop. The bonding wire runs from the negative side of battery
, to engine
bolt (disconnected as stated below), keel to mast
. Here’s the results….
Test conditions. “Fish” zinc on shroud
(connected to keel) and 1 shaft zinc:
______________Bat sw OFF _____ lots ON _____ Keel Bond remove dc off
Shaft__________ -962mV________ -965mV________ -945
Engine_________ -810mV________ -885mV________ -815
Keel bolt_______ -827mV________ -858mV________ -780
Bnz Thru ______ -146mV________ -146mV_________ na
___________ keel bond removed lots On _____ Eng@1100rpm
Engine_____________ -931 _________________-903mV
Keel bolt____________ -802 _________________ -850mV
Bnz Thru_____________ na _________________ -146mV
Partial Repeat with fish
zinc out of circuit, only 1 shaft zinc:
_________________Bat sw OFF____________ lots ON
Shaft______________ -903mV_____________ -875mV
Engine_____________ -686mV_____________ -663mV
Keel bolt___________ -678mV_____________ -675mV
Bnz Thru ___________ -145mV____________ -144mV
_____ Keel Bond Removed batt off ___ keel bond removed lots On
Shaft_________ -891 ______________________ -874
Engine________ -703 ______________________ -862
Keel bolt______ -648 ______________________ -701
Voltage between Engine and Keel when bond is lifted:
Lots DC ON: 149mv and 135mV (fish zinc out and in respectively, keel +)
Batteries off: 68mV and 53mV (fish zinc out and in respectively, keel +)
Testing bilge water
running/not running had no effect on voltage in bilge water
Bilge water voltage would start out high (more neg like -660) then reduce over a few min to -520mV… maybe because I’m using el cheepo DMM?
I would love people to weigh in on some conclusions. Here are some observations I have...
1) The shaft and engine are not well connected electrically showing 150mV-200mV+ difference. poor connection could explain loss of lead keel, but I would think it would slow the shaft zinc loss, right?
2) Turning on DC or the Engine On reduces the difference between shaft and engine, but not consistently. In general DC circuits ON results in a higher negative potential on the engine bonding system. Per the Boatzinc manual, this more negative value means better protection... hmm.. electrical
leakage is good!?
3) When keel bond is lifted the difference between engine and shaft potential generally reduces to a few dozen mV.
4) There's approx 100mV difference between DC ON and DC Off at the keel to engine bond. There is no specific circuit causing this. As I turned DC panel switched on, each had a % impact... issue must be on common wire, not out in branch circuits downstream of panel.
What do I do now???? Leave the keel to engine unbound?
Thanks for your comments/suggestions and sorry for the long post. Lots of data to communicate.