Cathodic protection voltage

[Patrick A]

By what voltage increase is it required to protect an aluminum bronze prop and a 2205 prop shaft?

-Wooden boat so I need to keep the voltage to a minimum.
-With no anode fitted the shaft voltage = .120
-Increasing shaft/prop voltage increases voltage in stuffing box and stern tube although only electrical path is via the sea water in the stern tube.
-Measuring with a silver/silver half cell
-Using a variable resister pot between anode and shaft so I can control the voltage
-Shaft is isolated from coupling and engine

So ....will a shaft voltage of .300 protect the prop or just slow the electrolysis?

[tkeithlu]

I would expect a bit more, maybe 0.5V. I expect just over a volt aluminum to steel hull, and pulled the boat recentlywhen it dropped to 0.9v. You have a wooden hull, much less, but still 0.5v.

[Patrick A]

Thanks - all info is useful on this seemingly esoteric subject

[GordMay]

Destruction of wood fibers (alkali delignification) occurs around metal fittings made more negative than -650 mV.

[Patrick A]

Originally Posted by GordMay

Destruction of wood fibers (alkali delignification) occurs around metal fittings made more negative than -650 mV.

Indeed and this is my concern to keep the voltage as low as possible but still protect the prop. My thinking is that the Monel keel bolts holding the lead ballast in place read - .430 so if I can keep everything below that voltage all should be well. eg use this as a safe voltage. These bolts are not bonded to anything else so that is their 'natural' voltage from day 1, 40 years ago and as designed.