Anode bonding resistance check

[Plum]

Before launching I always check, while under the boat, the electrical resistance between hull anode and prop is less than 1 ohm.
Now with boat in water while checking engine I measured the resistance between anode stud and propshaft at 30ohms. Not good. Suspected my "electro eliminator" MG Duff Electro Eliminator for Shafts up to 50mm so cleaned shaft and brushes but still 30ohms. All connections cleaned, no change.
Disconnected bonding wire from anode stud and measured between wire stud terminal and shaft 0.2ohms!
Measured voltage betwee that wire terminal and anode stud as .027v. Obviously I would expect a current flow if the anode is doing its job but did not expect it to be measurable. So was that current flow affecting the resistance reading of my Fluke multimeter?

(Batteries were completely isolated and no mains connection)

www.solocoastalsailing.co.uk

 

[VicS]

I dont think you can test the resistance of a circuit while it is " live" . The current flowing will produce small voltages across any points of resistance which will affect the meter reading.
You will probably find that if you reverse the meter leads you will get a different reading.

Maybe an instrument that uses AC will give a valid reading ....??? (I'm thinking of the conductance meters I've used in the (distant) past for measuring conductivity of water samples.)

I'm surprised that you only see 0.027 volt between the end of the bonding wire and the anode stud. I would have expected something in the region of 0.7 to 0.8V, measured with a high resistance meter, depending on the prop alloy.

FWIW in a YouTube video Anode Outlet suggest the resistance between the prop and the anode should be only 0.2 ohms.

 

[Bran]

Your multimeter probably has a very high input impedance, most of mine are around 10M ohms. That means they are very sensitive to secondary voltages when measuring resistance.
make sure you are not touching any of the metal parts and the metal probe tips you are measuring with as the body produces enough voltage to distort the measured resistance. Try reversing the leads and then measure the resistance, ie positive to anode negative to shaft, take a measurement then positive to shaft and negative to anode and take a measurement, I bet one of the resistance values will be negative which proves it’s a voltage causing the issue. That’s a quick test I always do if there is any chance of a secondary voltage being present.

 

[Forty_Two]

I keep a good old analogue multimeter for exactly that purpose.

 

[Bran]

I keep a good old analogue multimeter for exactly that purpose.

Avo 8, best bit of kit for that. Bit big to store on a boat!