Annodes - AGAIN!

[Robert Wilson]

T

An engine can never be given corrosion protection by an external anode.
Galvanic couples, such as the prop on its shaft, need an anode for protection.
Folding and feathering props may contain different materials and need an anode for protection.
P-brackets may benefit from an anode, as they are usually manganese bronze (=brass) but coating with an impervious paint or epoxy is an easier solution.

So my boat, for example, has a shaft and a prop anode, nothing else. I have owned it for more than 15 years with no corrosion problems.

This depend on boat, and engine/shaft arrangement, should be designer's job to decide. Normally engine block, shaft/prop/sterntube (may be connected through engine or separately), rudder fittings. Sometime even keel. But for iron keel and steel rudder plate better to put separate anodes right onto them.

Seacocks/thru-hulls should not be connected - made of corrosion resistant material and left alone - so they will not be affected by electrolysis should something go wrong. There were systems where they had an anode - but separate one, only for them. No connection to engine or any of electrics.

Now I'm being either thick or I've misread something.
I have highlighted the above because I am confused :-
a. I thought my hull lozenge protected my engine, and other bits connected internally by a wire "trail". If it doesn't protect the engine, what is it protecting? Do I need it?
b. With anodes on my prop-shaft I thought that would protect the shaft and the prop. Should I affix an anode to the prop too, and if so how?
As I stated earlier, when I bought the boat three seasons ago, the anodes were fixed; hull lozenge and two shaft anodes.
I hope I'm not the only forumite getting even more confused !!
Thanks for your input.
RW

 

[vyv_cox]

Your a. No, an engine cannot be protected by an external anode. This would be expecting electrons to travel between the anode, along the hull, into the skin fitting, up the hose, past the pump vanes somehow, along another hose and into the engine. It just isn't going to happen - in seawater electrons travel in straight lines, hence the instruction that anodes should be able to 'see' the object they are protecting. The hull anode may be protecting the P-bracket, provided it can see it and is connected to it, but my preference was to coat the P-bracket and remove the anode.

Your b. Many folding and feathering props come with their own anode, whereas fixed ones rarely do, although there are types of anode that will screw onto the end of the propshaft. In general I would say that a shaft anode is sufficient for a fixed prop.

 

[Robert Wilson]

Your a. No, an engine cannot be protected by an external anode. This would be expecting electrons to travel between the anode, along the hull, into the skin fitting, up the hose, past the pump vanes somehow, along another hose and into the engine. It just isn't going to happen - in seawater electrons travel in straight lines, hence the instruction that anodes should be able to 'see' the object they are protecting. The hull anode may be protecting the P-bracket, provided it can see it and is connected to it, but my preference was to coat the P-bracket and remove the anode.

Your b. Many folding and feathering props come with their own anode, whereas fixed ones rarely do, although there are types of anode that will screw onto the end of the propshaft. In general I would say that a shaft anode is sufficient for a fixed prop.

Thank you, kindly.
That helps a lot and clearly explained, and even I can follow the "science" of it all !
I shall now go back to the vendor and as him why he thinks the hull anode was there in the first place. He looked after the boat in a practical manner, so he may have attached one for some other reason.
I must admit, it would be pleasing to get rid of the spider's web of wiring running throughout the bilges to various hull items!

It appears that the "fog of ignorance" may be lifting from my eyes
Thanks again.
R