Prop nut anode

[Heckler]

I think that the fixing screw is still there - assuming it has an allen key type end to it? There are tiny fragments of the remaining zinc attched. I wonder if I can do a photo .... ooh yes .... July and now should be below ....

Thats knocked my theory on the head, the screw is still there! I have always thought that the anode was too small, coupled with the VP "keep the gearbox electrically isolated" that anode is protecting the gearbox as well! There is no room for a shaft anode with you having a cutter. The prop isnt pink (on my first 351 it was!) so no de zincification, as you have an isolator I would say feck it and carry on with the status quo!
Stu

 

[Tranona]

that anode is protecting the gearbox as well! There is no room for a shaft anode with you having a cutter.

Not sure how, or why, it is doing anything about "protecting" the gearbox! It is the shaft and prop that are dissimilar metals in seawater (electrolyte). The gearbox is only used as a path to connect an external anode, assuming any flexible couple is bridged,

 

[Dockhead]

I agree with previous comments. I rarely get 6 months out of them. I fitted not a galvanic isolator, but a much more expensive and supposedly 100% effective isolation transformer. Didn't make the slightest difference -- were still generally gone in 4 months (my usual lift & scrub interval).

Fit a shaft anode as well to be safe and don't leave your boat in the water as long as six months.

 

[Robin]

Unless I've missed the point somewhere this is a shaft drive so add a pear shaped hull anode wired to the engine or the gearbox. If there is an insulated connection between the gearbox and shaft couplings, like flexible rubber bushes, then connect a jumper lead across. The prop nut anode lasting 6 months seems about right but it still probably falls off rather than wears away. It might be worth painting the inside thread of the anode before fitting to prevent this bit vanishing first and the anode falling off. Brunton cone anodes respond very well to this trick and ours were still there after 12 months afloat, whereas before they always vanished.

 

[VicS]

I think its pretty inevitable that if you connect a little bit of zinc to a copper alloy prop with such an enormous surface area and immerse them in sea water that the little bit if zinc is going to disappear pretty quickly.

Electro-chemistry, dissimilar metals , galvanic corrosion or what ever you like to call it!

What else do you expect to happen?


The only hope may be is to use a hull anode rather than the prop nut anode, unless there is some obscure complication along the lines that Stu Davies mentions in post #5.
Always assuming of course that the prop does actually need cathodic protection and is not made of a corrosion resistant alloy.

Worth perhaps considering checking that there is not a problem with the galvanic isolator. The question then is how would you test a GI for effectiveness as a galvanic isolator?

 

[nigelmercier]

I think its pretty inevitable that if you connect a little bit of zinc to a copper alloy prop with such an enormous surface area and immerse them in sea water that the little bit if zinc is going to disappear pretty quickly...

Quite. So that's a good thing then