Aluminium Anodes

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sheyes

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I moor my Sadler 34 in brackish water and sail mostly in salt water so I thought I would change to aluminium anodes to avoid the oxidation coating which happens with zinc.

I have been able to source the prop shaft anode and also one for my flexofold propeller (although the cost of the aluminium version for the propeller is twice what I pay for the zinc version). I have not been able to source an aluminium anode for my Beta engine. I shall ring Beta next week to discuss this with them but in the meantime I wondered whether any of the experts on this forum had some advice as I understand that anodes of different metals should not be used on the same boat


Thanks

Steve
 
There are no consequences in these circumstances. Mixed anodes on the drive train could raise issues but the engine anode is electrically almost isolated from the others. Electrons travel in straight lines in seawater and thus cannot pass along hoses.
 
vyv_cox said:
There are no consequences in these circumstances. Mixed anodes on the drive train could raise issues but the engine anode is electrically almost isolated from the others. Electrons travel in straight lines in seawater and thus cannot pass along hoses.
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That's going to require some explanation.

Yes, the field strength is less because of the distance. "Straight lines" is not a factor, unless you know some science I don't. DC curent goes around coiled wires quite well, and saltwater in a hose is just a poor wire.
 
vyv_cox said:
There are no consequences in these circumstances. Mixed anodes on the drive train could raise issues but the engine anode is electrically almost isolated from the others. Electrons travel in straight lines in seawater and thus cannot pass along hoses.
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Electricity is conducted though electrolytes, such as seawater, by the movement of cations, eg sodium ions, away from the anode, towards the cathode and by anions eg chloride ions, in the opposite direction.
 
thinwater said:
That's going to require some explanation.

Yes, the field strength is less because of the distance. "Straight lines" is not a factor, unless you know some science I don't. DC curent goes around coiled wires quite well, and saltwater in a hose is just a poor wire.
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It is well established that anodes need to 'see' the object they are protecting. Anode manufacturers are all agreed on this and regularly provide such advice. The conductivity of seawater is not particularly good and a hose of it, disregarding the presence of a vane pump along its length, can in no way be likened to a copper wire.
 
vyv_cox said:
It is well established that anodes need to 'see' the object they are protecting. Anode manufacturers are all agreed on this and regularly provide such advice. The conductivity of seawater is not particularly good and a hose of it, disregarding the presence of a vane pump along its length, can in no way be likened to a copper wire.
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Does that mean that if I filled a hose fitted with a couple of electrodes with seawater and measured the resistance between the electrodes I would get a much lower value if the hose is dead straight than I would if it was not straight (coiled, perhaps)?
Or am I reading too much into what the anode manufacturers say?
 
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vyv_cox said:
It is well established that anodes need to 'see' the object they are protecting. Anode manufacturers are all agreed on this and regularly provide such advice. The conductivity of seawater is not particularly good and a hose of it, disregarding the presence of a vane pump along its length, can in no way be likened to a copper wire.
Click to expand...

I am a chemical engineer, I've done considerable corrosion testing on fluid systems, including as automotive cooling systems and HVAC systems, in my day job (the coolant is an elelctrolye, much weaker than seawater) (coolant additive development and manufacture), and I am quite certain that current runs around hoses just fine. Auto manufacturers would be thrilled if it did not. It is the distance and the reduced cross section of the conductor (hose vs. open ocean) that reduces the current. So yeah, electricity can flow around hoses. Yes, a pump vane adds resistance... but obviously there is a metal pathway around that. This is stuff I know.

I agree that engine anodes are well separated from external anodes, but NOT because curent flows in a straight line. You misspoke, it happens.
 
VicS said:
Does that mean that if I filled a hose fitted with a couple of electrodes with seawater and measured the resistance between the electrodes I would get a much lower value if the hose is dead straight than I would if it was not straight (coiled, perhaps)?
Or am I reading too much into what the anode manufacturers say?
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Obviously, the resistance is the same in both cases.

The bottom line is simpler yet. The potencial of aluminum alloy (alloy is the key) and zinc anodes is very, very close. It's not like installing a mag anode. They won't fight each other if the spacing is sufficient.
 
"See" in this context means connected by a water pathway, much as we say a PC can "see" the printer it is connected to via wires and software. Neither means line of sight.

Just think about the physics and the chemistry. This electricity is NOT conducted through water by anything like radio waves that are line-of-sight.
 
thinwater said:
"See" in this context means connected by a water pathway, much as we say a PC can "see" the printer it is connected to via wires and software. Neither means line of sight.

Just think about the physics and the chemistry. This electricity is NOT conducted through water by anything like radio waves that are line-of-sight.
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Not true. See in this context means line of sight. MG Duff are clear about this.

There is also a limiting distance, due to the resistance of the seawater . For pipelines the limit is 5 x diameter. MG Duff show suggested layouts of anodes for steel vessels that take this into account.
 
thinwater said:
"See" in this context means connected by a water pathway, much as we say a PC can "see" the printer it is connected to via wires and software. Neither means line of sight.

Just think about the physics and the chemistry. This electricity is NOT conducted through water by anything like radio waves that are line-of-sight.
Click to expand...
I have always understood this to mean anodes should be in direct line of sight of what they are protecting although never understood why, but nothing to do with electrons .
I cannot now find anything on MG Duff's website that supports the idea but I am pretty sure there used to be.
 
Without getting into the science (there are some people who know this better than I), my experience has shown that in this type of application it is typically OK to mix anode types. Worst case, is that one anode will protect the other and erode a bit quicker, but for practical purposes it should be OK.

The reality is, as with most cases, different to the theory in that we should make do with what we have, rather than what we don't have.
 
From the discussion it is clear that I didn't need to worry if I couldn't find an aluminium anode for my Beta engine however I've now talked to Beta and they do have an aluminium engine anode available.
Thanks for all the inputs
Steve
 
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