13 Oct 2016 – Contracted Underwater Services to check/replace shaft anode. Their report was: “We have changed the anode on Halo. There was still life in the old anode but it was loose on the shaft and it couldn’t be tightened.”
I then measured the Hull Potential from the prop shaft (inside the engine room) using the new silver reference electrode. http://www.boatzincs.com/corrosion-reference-electrode-specs.html
Measured-970 mV which is within the recommended range according to boatzincs.com : Fiberglass w/ Inboard Engine(s): -750 to -1000 mV
Update April 2017
Measured -870 mV
The same test from the Engine Block gave reading of -340 mV. According to Bob Olsen of Boatzincs.com:
“Since your engine is not attached to a bonding system that connects to underwater metals, and since your engine is not submersed in water, you do not need to be concerned with the reading from the engine block“.
Update: 9 Aug 2024
I tested the corrosion potential on the prop shaft today using the silver reference electrode.
It was indicating around -715 mV which is just below the recommended range according to boatzincs.com :
Fiberglass w/ Inboard Engine(s): -750 to -1000 mV
Previous measurements have given -970mV after fitting a new anode and -870mV with a partially worn anode.
I’ll fit the new shaft zinc anode before we go to Waikawa and test it again.
Halo Log Book 