Zinc anode erosion

[ianat182]

A friend of mine who keeps his Pageant at Keyhaven has just inspected the zinc anode after 1 year on a halftide mooring. The anode has disappeared except for the metal strap and bolts. This sounds excessive erosion to me ; my own anode lasts at least 2 years.
The water pump is the only item aboard that is permanently 'on'. We're going to scrub off next weekend and would appreciate any ideas why the erosion appears to be so great - no mains or other connections. The boat hasn't been used more than 3 times last year owing to illness . I understand that one connection is to the bilge keel as an 'earth'.
Any ideas out there please?


ianat182

 

[boatmike]

Actually I would say this proves it's doing it's job. If it's on a swinging mooring you might be being affected by something on the bottom as it dries out. Whatever it is it's better this way than not working! It's not unusual to deplete an anode in a year. In fact one that lasts more than 2 is probably not doing it's job properly....

 

[VicS]

I'm rather surprised there is an anode on a Pageant. It is unlikely that it would have had one originally unless specified as an "optional extra"

Do you know what it has been fitted to protect? Stern gear?

What is it bonded to?

About all you can do is check the bonding is correct. make sure the bonding is independent of any current carrying wiring.

make sure no non ferrous through hulls are connected to it.

I'm not sure I see the point of the connection to one of the keels but it does mean that the anode will be "protecting" that keel and that may be the reason for the high rate of loss of the anode.

Any reason why the water pump is permanently on. Id expect everything to be disconnected by the battery isolator.

Id want to check for stray currents within the boat's wiring that might be the cause of the trouble although if there were any the anode would probably be lost in weeks rather than 12 months.
Worth checking if there is any measurable current in the bonding of the anode itself and nvestigating to find the cause if there is.

There are areas where exceptionally high and unexplained rates of corrosion occur particularly where boats are drying into mud. If this one is drying with its anode siting in mud that may be the reason.

Sadly you cannot conclude that the rate of loss proves that the anode is "doing its job".

 

[Ex-SolentBoy]

Sounds like he needs to be happy his anode is working and just fit a much bigger one.

 

[VicS]

Sounds like he needs to be happy his anode is working and just fit a much bigger one.

You cannot conclude that the rate of loss is due to the fact that it is working. It may be suffering excessive corrosion due to some other cause.

 

[ianat182]

Thank you VicS.
He's visiting me today and I'll show him your comments when he arrives.
Of course, the bilge keel will be ferrous won't it, that sounds a possibility/cause.
As this is only the first year of ownership he has yet to wade through the circuitry on board in full detail but did mention the waterpump setup.

ianat182

 

[Tranona]

Not sure why a waterpump would be running all the time - or is it just hard wired to the battery? Anyway, there should be no need to bond the anode to the keel as the keel does not need protecting. The anode may well be bonded with a wire to the engine as a path to protect the stern gear, but most Westerlys don't need that.

 

[Ex-SolentBoy]

You cannot conclude that the rate of loss is due to the fact that it is working. It may be suffering excessive corrosion due to some other cause.

True, but as we don't know the size of the original we do not really know if the wear is "excessive". If its one of those little pear ones it's not going to do an harm to fit a bigger one and get a handle on the wear rate whilst the electrical trouble shooting continues surely?

 

[Bobobolinsky]

Is it in a marina, shore power connected?

Earth fault on shore power?

Stray current from another boat (liveaboards)

Change in water acidity/salinity?

Nearby boat overprotected

Impressed net on the quay

Lots of reasons really

An impressed current unit may be enough to protect it in the mean time, or auxilliary anodes

 

[ianat182]

Bobolinsky.
No I'm afraid none of the above would appear to be appropriate in this case. I suspect that the Keel connection may be the culprit.
I passed on all the Forum opinions to my friend, so down to a fuller investigation and removal of the keel connection after a voltage check, mentioned below.
He's lifting out shortly anyway to tighten up and seal off the bilge keels that were producing a small leakage, however I believe that the keel bolts themselves are stainless in a cast iron keel, another mix of metals if so.
Where should the voltage measurement be made and how? We've got multimeters to use, and need to know an acceptable value to look for as well.
Thank you.

ianat182

 

[ianat182]

VicS



Do you have any information on the latter question please?


ianat182

 

[VicS]

VicS



Do you have any information on the latter question please?


ianat182

Sorry I'd not seen the question.

what do you want to do exactly.

The galvanic series tells us that in seawater there will be approximately 0.6 volt between cast iron and passive 316 stainless steels but once in contact of course there wont be any measurable voltage .
If the stainless steel becomes active then the volts difference will only be approx 0.2 volt or so.

 

[ianat182]

Yes, sorry I didn't word my question properly Vic!

Assuming we were rewiring the anode and its two studs through hull,what would be the basic connections needed; I assume that connections to all Bronze fittings in Heads and cockpit drains are not all necessary. Is it a -ve from engine to one terminal of the anode to start with? and then where to, sterngear,shaft or other item.As mentioned previously there is a connection already to the keelbolt,does this sound right?. Then, where to take the other voltage measurement from( a +ve source ).

ianat182

 

[Tranona]

Yes, sorry I didn't word my question properly Vic!

Assuming we were rewiring the anode and its two studs through hull,what would be the basic connections needed; I assume that connections to all Bronze fittings in Heads and cockpit drains are not all necessary. Is it a -ve from engine to one terminal of the anode to start with? and then where to, sterngear,shaft or other item.As mentioned previously there is a connection already to the keelbolt,does this sound right?. Then, where to take the other voltage measurement from( a +ve source ).

ianat182

The normal way is to the engine block - usually a bell housing bolt if the objective is to protect the prop. This relies on there be a good path through the coupling, so if there is a flexible coupling it will need bridging, or alternately you can use a brush on the exposed shaft - MG Duff sell them. If it has a bronze stern tube the other stud can be connected to that. However, it may be the boat does not need an anode at all - it is unlikely one was fitted when it was built. No point in connecting it to a keel bolt.

 

[VicS]

As Tranona says

No point in the connection to a keel bolt.

Don't bond seacocks etc either to each other or the anode ( despite what MG Duff may say) and definitely not to any other part of the electrical system.

Bonding between the skin fittings and the anodes was judged to be one of the contributing factors to the near loss of the FV Random Harvest off Brighton a few years ago.

A couple of years ago another forumite had his boat sink on its mooring due to what appeared to be electrolysis of a skin fitting. Almost ceratinly due to a connection to the negative supply to a bilge pump.

There is a lot of useful information on MG Duffs website http://www.mgduff.co.uk/

There are no voltage measurements you need to make. Just make sure that every thing that needs to be bonded including bridging the flexible coupling if the anode is to protect the stern gear is effectively bonded by good low resistance connections ( MG Duff recommend 4mm² cable but it seems a bit OTT to me). The bonding must be independent of any cable carrying current..

Before any one decides to tell me about it The level of protection can be measured but it needs a special silver/silver chloride electrode (or a copper/copper sulphate half cell) to do it

Finally, to support Tranona's suggestion that an anode my not be necessary,
The Westerly I sailed for many years has no anode and there is no evidence to suggest that one is necessary. The boat still has its original prop and shaft and original seacocks etc.

 

[vyv_cox]

Finally, to support Tranona's suggestion that an anode may not be necessary,
The Westerly I sailed for many years has no anode and there is no evidence to suggest that one is necessary. The boat still has its original prop and shaft and original seacocks etc.

Supporting that, when I bought my Sadler in 1995 a previous owner had fitted a pear anode bonded to the engine and various skin fittings. I took it off and disconnected all bonding wires. I now have only a shaft anode and the one on the back of the Autoprop. All of my skin fittings are original (1985) with no evidence of corrosion anywhere.

 

[fisherman]

You cannot conclude that the rate of loss is due to the fact that it is working. It may be suffering excessive corrosion due to some other cause.

In Newlyn anodes go at a terrific rate, supposedly due to nearby welding on boats without strapping (earthing) them to the quay.

 

[ianat182]

Interesting comments re whether to fit or not and where. I'm glad I asked first before connecting them up.
Seems the general opinion is that they are not always necessary, but perhaps an anode hung over the side would be a 'halfway house' when the boat is left on its moorings; a shaft anode wouldn't be possible in this instance, no room at shaft end or between cutless and prop.
Thanks to all responders.

ianat182

 

[VicS]

Interesting comments re whether to fit or not and where. I'm glad I asked first before connecting them up.
Seems the general opinion is that they are not always necessary, but perhaps an anode hung over the side would be a 'halfway house' when the boat is left on its moorings; a shaft anode wouldn't be possible in this instance, no room at shaft end or between cutless and prop.
Thanks to all responders.

ianat182

Further thought

You have two holes in the hull and the studs already there so you might as well fit another anode and just bond it as explained to the stern gear via the engine /gearbox.
( assuming its fitted to the hull near the stern gear)

Follow the fitting instructions on MG Duff's website

 

[ianat182]

Following on from my recent Anode erosion thread, I was able to go to Keyhaven yesterday where the Pageant was moored ; we took her to the quay at High Water 0800 and set her alongside there. So far so good. After usual wait for the dropping tide,we did some small sail cover and rigging jobs, then tackled the anode problem.
This was a surprise when we checked; the earthing connections to the forward stud had been made by the previous owner using an A4 stainless steel A4 bolt, nuts and washer plus a mild steel one(unsealed inside or out), and the normal mild steel nuts on the inside securing the connections; the aft stud had no connections and was of a mild steel material throughout. Releasing the mild steel nuts was done by hand! Not surprisingly the external thread of the aft stud had been eaten away,the washers used both rusty.
Our first idea was that probably the metal mix of mild steel and zinc and stainless at the anode had been the cause of the zinc anode depletion to zero,so for the moment have not altered any of the connections found,1 to the keelbolts,via a connection to the water inlet filter and a third wire taken from here we think to possibly the -ve busbar somewhere behind the switchboard panel,but were not sure so did not check further. new plated studs with integral flat 'washer' nuts and washers were fitted, this time with Sikaflex sealant beneath the surfaces inside and out,and all connections left as found. Job done- or so we thought!
Having completed the antifouling, just in time, we waited until the water level rose so that we could ensure a flow from the engine, a Beta 10. Turned on the ignition, and no ignition or lights on the panel.
All that we had done was to use the stop /start button to close the engine down when alongside, and turn off the ignition.
Panic stations! We were not expecting to stay there overnight!

We've recapped on every wire we touched or connected/disconnected. All other lighting and power items, radio, etc are working correctly, and solar panels kept the batteries topped up OK.
The boat has an Off,1,2, Both, isolating switch. But there doesn't appear to be any power to the ignition panel when the key is 'On' or turned to the glowplug position. I wondered if the stop /start button could be faulty or causing this loss of starting function. Edit: my mate also checked flat fuse which he reckons was still functioning OK (possibly from the engine??)

Any other suggested causes would be appreciated for a check during next week.

Incidentally I rigged our inflatable to the rear of the yacht and with the 3.5 4stroke Suzuki managed to motor her back to her 1 mile distant mooring at about 2 knots and against the incoming tide! my mate didn't believe it would do it!!

ianat182