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Anodes.... a yearly replacement or not...


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I'm getting half a bucket full of em. Every year a new anode goes on the shaft, when the old one is maybe 30 per cent wasted, at most. Does everyone replace theirs annually? Is there risk if you don't, and let it go to say 60% wasted, replacing every two years?

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First the easy question answered. Yes they can be sold as scrap, but Zinc is (the last time I looked) only 36cents per KG. Which makes me question why anodes are so expensive to buy. Someone is making a good profit in light of the price of scrap dropping like it is. 
Zinc is really easy to melt with a Temp just above Lead.
Be vary careful of using scrap Zinc from other sources other than scrap anodes. The difference with Zinc Anodes is that the Zinc is very pure. Most Zinc naturally contains Cadmium and as Cadmium is highly toxic, it is removed from Anodes so as we don't leach it into the Marine Environment.

Now the slightly harder one to answer. An Anode gives protection based on surface area, not based of what is left. So as the Anode depletes, the area decreases as well and the protection decreases. To know just exactly what level of protection an anode is giving, you need to measure the electrical current flowing. That requires a special metre that very few, apart from Marine sparkies, would have. So we work on a rule of thumb of getting 12months from an Anode. The trick is to have enough left at the 12months, that there is still a level of protection, but you don't want too large an Anode to start with either, because there is also a danger of over protecting and having the electrical current flow the wrong way. Or if the the Anode has a bad Hull connection and is not working, the Anode will go white, which means the Zinc is oxidizing and protecting the metal under the surface. Exactly like Galv does on brand new Anchor chain before that turns dark grey as Zinc Phosphate which is what protects the metal under the surface.
There are metres available, or at least they used to be in the days of Wooden Boats, as having the correct level of protection was most important and a very narrow window of not enough, just right and too much. So you could adjust a knob to alter the voltage level and thus maintain a more constant flow of Current. It also gave an advantage of knowing when the was no longer enough Zinc left.

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Yes wheels spot on with over anoding there was a steel boat here years ago that did that and the bottom paint kept falling off.my anodes on the back of the prop so wont rattle loose like shaft ones i can let it work a bit longer

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Im not sure what is the normal haul out period in NZ but in Australia it is usually every two years..

...Anodes are checked and/ or replaced at this frequency.

There are several  anode size and number calculators on line. 

Recasting block anodes from old anodes is easy. (....picking up all the dumped ones, often on the ground in a larger yard can save a lot of money) Shaft anodes are a bit harder..

On metal vessels the spacing is also important.

All other materials need too have good clean bonding straps to everything that you want protected.

Dont expect a bronze through hull in the bow to be protected by a anode on the skeg.

The arguments of electrically isolated and electrically bonded seem to be a another one of those endless "boat arguments"

...I have a steel boat so its not an issue. 

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