Why do we need anodes?

[VicS]

I take your point about the polarity.

However, the rate of corrosion suggests very strongly that there is an impressed current across the anode and the shaft bridge seems to be the only possible source. What is needed is information about the voltages at various points, so I don't agree that disconnecting a few wires and waiting to see what happens is the correct approach. At the very least the OP needs to replace the anode cable with a meter, checking with and without the engine running. My suggestion of checking the voltage across the flexible coupling also seems valid to me.

Yes ceratinly carry out some checks rather than waiting to see what happens but what little we know suggests that things may not have been wired as they should be.
Firstly, therefore, get everything wired correctly and then check for current flows.
I'm don't know off hand what sort of current flows would be normal but at least with a measured figure one can calculate a likely rate of loss of zinc etc. ( polarity of flow also being important to know)

OTOH some measurements on the system as it is a present might reveal some other issues to resolve.

Measurement of potentials relative to a silver /silver chloride reference electrode is probably the right thing to do. Any deviations from listed values then requiring investigation.

 

[pappaecho]

Impressive physical chemistry in this thread, but the equation perhaps do not tell the whole story.

"If you metal gear on the boat corrodes, it will cost megabuck to repair and replace. If you spend a small amount on a sacrifical anode or two you will prevent the spending of megabucks"

Your call

 

[VicS]

Impressive physical chemistry in this thread, but the equation perhaps do not tell the whole story.

"If you metal gear on the boat corrodes, it will cost megabuck to repair and replace. If you spend a small amount on a sacrifical anode or two you will prevent the spending of megabucks"

Your call

Use corrosion resistant materials below the water line where possible.

Avoid where possible mixing metals ... That's where many fancy folding props seem to come unstuck.. Nickel aluminium bronze blades (one of the most corrosion resistant alloys available) but mixed with stainless steel bits and pieces with the result that they have to be fitted with anodes which are inevitably going to have a pretty short life.

Avoid unnecessary bonding .. it can easily create return circuits for galvanic corrosion cells. There was case on the Mobo forum a while back where a boat sank in its berth when a skin fitting failed..Almost certainly because of an unnecessary connection between it and a bilge pump, probably coupled with a bad connection in the wiring to the pump. If that connection had not been made the boat would almost ceratinly not have sunk

A "small amount" on anode or two often involves an much larger cost to have the boat lifted to replace the anodes. Far better to avoid the corrosion in the first place whenever possible.

In Nimbusgb's case there is evidence of previous excessive corrosion of the iron keel........ A small anode fitted to protect the keel is being lost at an unexpectedly high rate ... raising suspicions that there is a problem, the cause of the excessive corrosion, there somewhere that requires a solution.

On reflection a single 0.5kg anode is too small Two much larger anodes, one either side and each several kg, would be more appropriate .. but there is still a problem to be solved, I feel.