Aluminium and Zinc Shaft anodes

[DAKA]

I continue to have problems with the rapid dissolving of Zinc shaft anodes and I am concerned that as the shaft anode wears out it slips down onto the cutlass bearing before it drops off.

My plan

To fix an Aluminium shaft anode behind the Zinc anode.

Theory is the zinc anode will protect the shaft , the propeller and the aluminium anode until the Zinc anode wastes away and drops off (8-10 months), the aluminium anode will prevent the zinc one slipping down and then the aluminium anode will be as new and will take over from the Zinc one and protect the shaft and prop until the annual lift out.

Sounds simple, but I am concerned I have missed something, has anyone tried this and anyone foresee any problems ?

 

[Bandit]

Jubilee Clips my boy

I have seen anodes work their way down to impede the water flow to the p bracket and up to touch the hull or stern gland.

I use a stainless jubilee clip on the shaft above and one below the anode so that when the prop is turning in the forward direction the tail of the clip is trailing.

It works well.

 

[miket]

Bondage!!

I have never used shaft anodes and never found the need.
Surely if the bonding for the shaft (brushes or coupling bridge) is properly maintained, shaft anodes are not required?

 

[Trundlebug]

Flawed plan

I'm afraid the idea of bolting on aluminium and zinc may not work as you intend.

My understanding is that the aluminium one will erode first, followed by the zinc, so it won't quite work as intended.
Always best to stick to just one common material throughout for all the anodes on the boat - otherwise the odd ones out will erode very fast.

An alternative to jubilee clips (which may unbalance the shaft) is to wrap insulating tape round the shaft, several times to build up the thickness, forward and aft of the shaft anode. That stops it sliding onto the cutlass bearing.

 

[Tranona]

Shaft anodes are used because they are convenient and quick to fit. They also directly protect the shaft and prop. However you can also use hull anodes near the prop and bonded internally to the shaft. You may then find the shaft anodes redundant.

Similar strategies are used on saiboats with saildrives when a bronze prop is fitted and the normal saildrive anode is overwhelmed.

 

[pissativlypossed]

I continue to have problems with the rapid dissolving of Zinc shaft anodes and I am concerned that as the shaft anode wears out it slips down onto the cutlass bearing before it drops off.

My plan

To fix an Aluminium shaft anode behind the Zinc anode.

Theory is the zinc anode will protect the shaft , the propeller and the aluminium anode until the Zinc anode wastes away and drops off (8-10 months), the aluminium anode will prevent the zinc one slipping down and then the aluminium anode will be as new and will take over from the Zinc one and protect the shaft and prop until the annual lift out.

Sounds simple, but I am concerned I have missed something, has anyone tried this and anyone foresee any problems ?

Sorry to have to tell you this, but the aluminium anode will disolve very quickly and will most likely exacerbate the rate the zinc one corrodes.

I have no idea of your mooring circumstances, but will suggest fitting a galvanic isolator and or hanging a sacrificial anode (Zinc) over the transom of your boat, it will need to be bonded to the earthing system on your boat but should increase substantially the life of your shaft anodes.

 

[DAKA]

Thanks to everyone who has replied, all read and understood.
My shafts are bonded but as the shaft zincs disappeared so quickly they have to be working well I thought they were worth perceiving with, I am a bit concerned the jubilee clips may slip down the shaft or snag debris.

 

[VicS]

Your shaft anodes waste away quickly because of the proximity of the prop, which is presumably a copper alloy, That' s the electrochemistry at work! It does not necessarily mean they are doing anything useful.

As said an aluminium anode (actually an alloy containing a small percentage of indium) located close to a zinc one is likely to waste more quickly than the zinc.

 

[markdj]

AFAIK, the noble table of metals goes like this:

Magnesium
Zinc
Aluminium
Mild Steel
Iron
Aluminium Bronze (your props)
Naval Brass
Tin
Copper
410 S/S passive
304 S/S passive
316 S/S passive
Titanium
Platinium
Graphite

So the least noble is Magnesium and the least noble will corrode the quickest, therefore, the zinc anode will waste first, then the aluminium, contrary to previous posts.

In practise, I would think the zinc will go very fast as it will be trying to protect the aluminium anode first with little protection given to the shaft or propeller.

Really, your should calculate the surface area of the metals to be protected and the surface area of your anodes to give a negative potential of between -0.55 and -0.9 volts using a silver/silver chloride reference electrode; but the manufacturers have done this already and the bolt fittings on the boat for protecting the props etc. are adequate protection.

Overprotection can cause issues too as the zinc forms a crust, gets fouled and stops working. Increased water flow, temperature, and salinity can all speed up anode depletion.

If you are keeping the boat in the water for a long time, the correct way of using additional zincs hanging in the water is to only use them after the hull anodes have wasted away sufficiently so as no to cause over-protection.

Personally, I would not use shaft anodes if at all possible but check with a multimeter that your hull anodes are connected with less than an ohm resistance and use additional zincs after 6 months if required.

Hope this helps

Mark
www.boatdoctorni.com

 

[VicS]

the zinc anode will waste first, then the aluminium, contrary to previous posts.

But you miss the point that aluminium anodes are made of an aluminium /indium alloy to a US military specification not pure aluminium!

Read my earlier post.

Typical potential figures quoted are -1.05 volts for a zinc anode and -1.09 volts for an aluminium/indium alloy anode,relative to a silver/ silver chloride electrode. (Other aluminium alloys lie in the range -0.7 to -1.0 volts.)

Therefore an aluminium anode will waste in preference to a zinc anode!

 

[markdj]

Whether the aluminium anode has a small percentage of indium still puts it in the more noble category according to the chart I have for metal nobility:

zinc has a range of -1.05 to -0.98
Aluminium and its alloys -1.0 to -0.76

mmm.

 

[VicS]

But your chart is just the general chart for the galvanic series. It does not cover every metal and alloy and in particular does not cover the aluminium indium alloy used for anodes.
Repeat they have a quoted potential of -1.09 volts relative to Ag/AgCl

 

[hlb]

Thanks to everyone who has replied, all read and understood.
My shafts are bonded but as the shaft zincs disappeared so quickly they have to be working well I thought they were worth perceiving with, I am a bit concerned the jubilee clips may slip down the shaft or snag debris.

Ah but. I had the arguement with the boat yard earlier this year. I said put anodes on the shafts, they said you dont need them cos the shafts are bonded. I said, well put them on anyway. So they said, you dont want anodes do you. Arrgg!!! So it got anodes.

A few weeks ago, we took the boat out for a scrub. It had lost an anode.

Last week we discovered that the bonding had come adrift.
Dunno how long we'd gone with no anode and no bonding though.

 

[DAKA]

Thanks to Vic and Mark, you have clarified my confusion.

I was working on the basis that I used to use Zinc anodes to protect my Aluminium legs so zinc had to erode before the leg.

It hadn't occurred to me that the Aluminium anode was not pure.

If I understand you correctly Vic, you are also saying that my anode is not protecting much, all it is doing is creating its own little battery circuit, in effect wasting away because it is there, once removed the circuit is removed.

Thanks.

Pete


Hi Haydn,

Was there any evidence of the zinc rubbing on the P bracket or did it just spin off without damage ?

 

[VicS]

If I understand you correctly Vic, you are also saying that my anode is not protecting much, all it is doing is creating its own little battery circuit, in effect wasting away because it is there, once removed the circuit is removed

Not quite. The anode may well be doing a useful job ... if the prop is brass it will be. What I am saying is that it does not follow from the fact that it is wasting that it is doing something useful. A good quality manganese bronze prop should not suffer any corrosion so an anode is not necessary. If an anode were fitted though, being close to the prop it will waste away!

Unfortunately the only way you can tell is by leaving the anode off. If you suffer no corrosion/dezincification of the prop it was unnecessary. If without the anode the prop rapidly dezincifies then it was necessary but that's an expensive way of finding out!

 

[hlb]

Dont think there was any damage, but did not get time to look much.