Prop Shaft Anode

[DaveS]

The survey report on my boat, which I bought last September, stated that the wrong type of Anode was fitted at the end of the prop-shaft.

Apparently it is a Magnesium Anode, which while ideal for salt water, is next to useless in fresh water, and that an Aluminium Anode would be suitable.

Today I went down to the boat, which has been out of the water since November, to look at removing the Anode so that I could use it as a size and fitting guide, when buying a new Aluminium one.

After scraping off the bulk of the crud that had built up, due to the Anode not working in fresh water, I found what appears to be an Allen Key Headed bolt in the hole at the back end of the Anode.

I'm assuming that this is what secures the Anode to the prop-shaft, is that correct?

Excuse my ignorance on this, but this is the first shaft-drive boat that I have had.

I have attached a couple of photos that I took with my phone's camera, and the close-up is way out of focus, as I had to site the camera between the rudder and the end of the prop shaft, so it was very close.

Dave

[MBA Marine]

Thats the one, go for it using the correct size allen key/socket looks like someone may have had a chew at it before!

[Guest]

The survey report on my boat, which I bought last September, stated that the wrong type of Anode was fitted at the end of the prop-shaft.

Apparently it is a Magnesium Anode, which while ideal for salt water, is next to useless in fresh water, and that an Aluminium Anode would be suitable.

If the surveyor said that magnesium was ideal for salt water, then I would be quite worried about his knowledge level.

Magnesium is the correct material for fresh water, not salt water, where it would fizzle away very quickly.

The correct usage is :

Magnesium for fresh

Aluminium for brackish

Zinc for Salt

I keep my boat mostly at Horning at the moment, so I changed the zincs for magnesium, the water being completely fresh up there.

Though I believe people on this form have said they favor Aluminium in the lower reaches of the Broads rivers, so you're intended change to alluminium is probably the correct choice though for the lower Waveney, being somewhat brackish down there.

[Palmtree]

Hi,

I am in the process of changing my anodes from zinc to magnesium as we are based at Ranworth. Just one question what type of bolt, Mild steel or stainless, is best for attatching the anode?

Ian

[senator]

Have always bought mine as a kit and the bolts that come with them are stainless if that helps.

[Guest]

Yes, I always use stainless as well.

Make sure it's A4 as well, not A2, as is sold in many places like Norfolk Marine.

(The fastenings specialists Beardshaws are on the small trading estate next to Jewsons at Hoveton, and they stock a very wide range of A4 bolts, and are happy to sell just one bolt... )

[MBA Marine]

Yes, I always use stainless as well.

Make sure it's A4 as well, not A2, as is sold in many places like Norfolk Marine.

(The fastenings specialists Beardshaws are on the small trading estate next to Jewsons at Hoveton, and they stock a very wide range of A4 bolts, and are happy to sell just one bolt... )

But take cash as they are a decade or 2 behind they will sell retail but do not have a card machine.

[Guest]

But take cash as they are a decade or 2 behind they will sell retail but do not have a card machine.

One bolt ? pay by card ?

I thought I was tight....

[MBA Marine]

One bolt ? pay by card ?

I thought I was tight....

I pay by card wherever possible as it makes the accounting easier, and I would not buy just one bolt, I would rummage through a bucket full and if I didn't have a one suitable I would go and get a quantity along with a box of penny washers (can never have enough washers!) but as I do not buy enough to warrant a credit account with beardshaws I would need to go to the cash machine first. Unless i have been paid in cash and not banked it yet I will not normally have any cash on me (oh slight lie, i always have £1.50 in my vehicle in cash if need to use a car park in town).

[DaveS]

Strowager, you are indeed correct with regards Magnesium for fresh water use.

It was my mistake, as I should consulted the survey report before 'posting'.

I hadn't looked at the report since last August, and for some obscure reason I had Magnesium fixed in my mind, instead of Zinc.

I quote from the Survey Report:

One anode was fitted to the vessel: a cone anode on the prop shaft, aft of the propeller. This was of Zinc construction, and suitable for seawater use. This had furred over, as commonly happens in fresh water, and may not be providing protection against galvanic attack.

Apologies for getting it wrong, and thank you for correcting me.

Dave

[DaveS]

I'm guessing this is what I want:

http://www.norfolkmarine.co.uk/shop-onl ... 19016.html

Albeit it doesn't state whether they are Zinc, Magnesium, or Aluminium.

Also need to try and find what size shaft fitting I need.

Hopefully, if I give them the dimensions of the anode that is in situ (52mm L, 42mm Dia at one end. and 37mm at the other, with a 13mm dia hole for the bolt), they might be able to identify it.

Dave

[DaveS]

Sorted!!

Half an hour ago, I sent off an e-mail to the boat's maufacturer, asking for the details of the anodes that they fit to this particular model.

Within half an hour, I had a reply from the MD, giving me the info that I needed.

Pretty impressive service!!

Dave

[Guest]

Yes, very good service.

I seem to remember the numerous full seagoing guys on here had quite a discussion about zinc versus aluminium anodes a few months ago.

I think some preferred to stay with zinc, for full salt water protection, rather than just brackish.

Do you know where your boat was used before you bought it ?

Zinc seems to be fitted by default everywhere, even for full fresh water use, and if your zinc anode has been in that for many years it would indeed have furred up, reducing it's effectiveness and stopping it from eroding.

It's just possible, that if cleaned up to bare zinc again, it may start to work properly again, on the brackish Waveney and salt Breydon.

A bit of a gamble though, I suppose, requiring a costly lift to check and/or change them again.

[DaveS]

The boat was new in 2005, and I am the third owner.

I know that the last owner had the boat moored in a Marina at Brundall, and given just how furred the anode is, I suspect its main use was on the Braods.

I suspect that the first owner, liked to venture out onto the 'salty side', as the boat is fitted with the 100HP diesel engine option. The options are 56HP for inland water, 75HP for estuary, and 100HP for coastal.

Plus the fact that in addition to a depth finder, it also has a dedicated 'fish finder' installed, and a hefty Danforth anchor.

My guess is that the second owner didn't change the anode from Zinc to Aluminium or Magnesium.

I have to wonder that as we travesre the whole of the Broads network, including Breydon, if an Aluminium Anode might be a good compromise.

There is a possibility that I might even want to try a trip out to the 'salty side', but it would be an out and back venture, so it shouldn't be much of a problem in having the wrong anode.

Dave

[Guest]

I have to wonder that as we travesre the whole of the Broads network, including Breydon, if an Aluminium Anode might be a good compromise.

There is a possibility that I might even want to try a trip out to the 'salty side', but it would be an out and back venture, so it shouldn't be much of a problem in having the wrong anode.

Dave

I faced the same thoughts when I replaced mine last year, to go for aluminium or magnesium. Uncertain of how much time would be spent on or near Breydon, compared with way up at Horning/Wroxham.

I'd read of instances of magnesium fizzing completely away in just two weeks in seawater, but I did go for it because my boat has a VP leg, and I can easily and frequently check the prop ring anode without needing a lift, though the bigger transom shield anode is way down out of reach and sight at the bottom of transom.

[jillR]

a couple of anode threads i found in the forum search

viewtopic.php?f=68&t=10775&p=130098&hilit=anode#p130098

viewtopic.php?f=68&t=7552&p=89755&hilit=anode#p89755

[mbird]

We have aluminium throughout, being moored at Reedham and regularly visiting the salty stuff it's the only choice.

Secret Lady had been used on inland waters mostly by the previous owner. However, he hadn't bothered changing the anodes from zinc, and these had completely calcified to such an extent they were providing no protection at all.

Do remember there may be other anodes to change too, such as pencil anodes in the heat exchanger, or one fitted to a bow thruster if you have one.

Sent from my iPad using Tapatalk HD

[DaveS]

Indeed Mark, I have both a Bow and Stern Thruster, and each has a Zinc anode behind the prop.

However, following some interesting and helpful conversations today with French Marine (Norwich), and Vetus UK, it seems that I now have to take a different route to provide effective galvanic protection for my boat.

Vetus do not offer an aluminium option for this anode. A magnesium version could be ordered from their supplier in Italy, but is not stock item.

Its is possible that MGDuff would be able to make me one as a 'special', as it is a standard 30mm Beneteau type cone shaped, shaft end anode.

It seems, looking around the chandlery web sites, that there isn't an aluminium option with this type of anode from any outlet.

Even if there was, it would still leave me with a problem, as each of the Vetus thrusters is fitted with a 'custom fit' zinc anode, with no aluminium option availabe.

I'm guessing that giving its better conductivity, galvanic corrosion is a more significant problem in salt water, than in fresh water. Hence Vetus fitting Zinc anodes as standard to their thrusters, and the greater proliferation and availability of Zinc anodes.

So, the only sensible option would seem to be to leave all the ineffective Zinc anodes in-place, and install a Galvanic Isolator to prevent any leakage current from flowing in the first place. This way, the anodes will become dormant, and surplus to requirements.

A galvanic isolator is not the cheapest of options, but the most practical for my particular boat. I might even look into obtaining suitable plus an enclosure, and building one.

A 'plus-point' is, as the only way to check the depletion of any anodes fitted to my boat, would be to have the boat lifted, with a galvanic isolator installed I won't to worry about the state of the anodes.

Dave

[mbird]

Hi Dave.

I have a QL bow thruster with the same problem of not being able to get aluminium anodes. However, I did find that Sidepower do make aluminium anodes for their thrusters. A quick visit to JPC at Brundall and I found they had one of a similar size. All I had to do was counterbore the fixing hole and Bob was indeed my uncle.

Be wary of thinking that a galvanic isolator will stop any corrosion issues; it won't. I have a galvanic isolator and even so, after one season the bow thruster anode and the pencil anodes in the heat exchangers are all but vanished.

Galvanic isolators will only stop stray currents from your neighbours boats using your anodes, not stop you own boat using them!

Sent from my iPad using Tapatalk HD

[DaveS]

Hi Mark

Thanks for that info, I clearly was 'barking up the wrong tree' here.

Equally as worrying is that you say that your aluminium anodes only lasted one season

This effectively means that the boat would have to be lifted, and the anodes replaced every year.

Although my boat has zinc anodes which I'm guessing have been in situ for a few years, and which are apparently ineffective in fresh, or even brackish water, I couldn't see any signs of galvanic pitting on either the bronze prop or rudder.

So, have these zinc anodes been offering some sort of limited protection, I wonder.

The whole subject of galvanic protection seems to revolve around zinc anodes, with magnesium and aluminium being very much the poor relation. Especially, when manufacturers such as QL and Vetus, seem only to offer zinc.

I will look into what JPC have to offer, albeit the Vetus thruster anodes are bit more complex in shape than just a disc with holes in them. From what I can see from the diagram in the Vetus manual, the anode is hollowed out at the rear, two large cut-outs in the rim, and a machined step in around the rim on the front. So very much a 'custom fit'.

Even if the JPC ones could be modified, I would still be left with the problem of finding an anode to fit the end of the drive shaft.

Life is never easy, is it

Dave

[DaveS]

Just had a look at Sidepower's website, and I can see that their thruster anodes are very different to Vetus, in that the Sidepower anodes fit on the front of the prop, where as the Vetus ones are at the rear of it, and fixed to the thruster body, as opposed to the prop.

So, it looks like its back to 'square one' on this option.

Dave

[DaveS]

Here's the Vetus Thruster anode, that 'sits' behind the prop.

As you can see, very much a 'custom fit'

Dave

[Palmtree]

If you want to stop the anodes going too fast just give them a coat of antifoul just like mine were when we got the boat....

[pks1702]

Designed to prevent growth and make for long life span Ian

[pks1702]

Not sure if you saw this link in one of Jill's resurrected thread links Dave.

http://www.mgduff.co.uk/mgduff-product- ... ut+Anodes/

I had the same issue with Kiki being Turkish built and had to make an adjustment to the prop nut so that it would accommodate UK sourced Prop Anodes

Old Prop Nut