Prop Shaft Anode

[DaveS]

That's an interesting possibility Perry.

It might be worth speaking with MGDuff, and see if the adapter-kit anodes will fit in place of the Beneteau type shaft anode that I have.

Looking at the amonut of exposed sahft that you have, you could have fitted a 'clamp on' type shaft anode, if the other option hadn't been available. I would have gone down that route if there had been some exposed shaft to attach it to.

Dave

[senator]

Dave you still have the issue of finding magnesium or Aluminium prop nuts which appear to be in short supply.

Not up on anodes for shafts as never had one but could you not just fit a shaft anode in front of your P bracket? Have heard they are not popular with fast seagoing boat owners but on the river I would have thought the disadvantages would be negligible.

[DaveS]

I'm not sure what a 'P' bracket is, but if its something that supports the shaft, as seen in Perry's picture, then I don't have one. The shaft comes straight out of the hull tunnel, and straight onto the prop.

Even if I managed to get an aluminiun type shaft-nut anode, there is nothing that I can do about the anodes on the thrusters, they just aren't made in aluminium.

I have also read a technical article that states Aluminium and Zinc anodes should never be mixed. Its either all zinc or all aluminium.

I am rapidly coming to the conclusion that I am going to be stuck with zinc anodes, albeit there is some 'pitting' evident on the shaft nut anode, so it must have doing something at some time, but of coures this may relate to the time when it was used in salt water. But then again, there is no evidence of any 'pitting' on the prop or rudder, both of which are bronze. So again this might indicate that the zinc anodes have been providing a degree of protection.

I will install a Galvanic Isolator, as this is will protect against any stray current when connected to shore power.

Other than that, if as it is, Vetus only supply zinc anodes for their thrusters, and MGDuff don't only make Zinc end of shaft anodes, I am truly stuck with the zinc.

All very strange really, as a large number of boats, especially those on the Broads, traverse fresh, Brackish, and salt water.

I wonder what type of anodes the hire companys fit to the boats, especially as these are shaft drive, and many have bow thrusters. Perhaps Clive could comment on this?

Dave

[mbird]

Hi Dave

Not sure if this would be of any use?

http://www.martyranodes.com/content/martyr-products-services/pleasurecraft-anodes/propnut-anodes.php

It might also be worth searching the Performance metals websites. I'm sure they also do Ali prop nut anodes, but I can't seem to find them at the moment.

As for the hire boats, I don't know what they do wi new builds nowadays, but certainly there were no anodes on my first boat (a 30+ year old alpha). I can only assume the proliferation of 240v on boats makes the corrosion issue a little more important than it used to be.

[pks1702]

Looking at the amonut of exposed sahft that you have, you could have fitted a 'clamp on' type shaft anode, if the other option hadn't been available. I would have gone down that route if there had been some exposed shaft to attach it to.

I have to say I don't like them Dave for two reasons.

With uneven wear running the shaft at relativity high speed could lead to vibration, and I also had a fear of the securing screw & nut slipping and it ending down near my P Bracket. Having said that I did have one fitted for a couple of months while I was trying to find a solution and Kiki was out for just a couple of days, I felt anything was better than nothing.

[DaveS]

Hi Mark

Martyr certainly do offer the correct size/type of prop-nut anode (their CMAN230), and I have e-mailed their European contact point to ask if there is an aluminium option. Also if they have a UK stockist.

If they do, then although that solves the problem with the prop-nut anode, it still leaves me the problem of the two thuster anodes, as apparently mixing anodes of different materials on a boat is a 'no-no'.

It may be that on the Broads galavanic corrosion isn't a significant problem with boats not using 'shore power', and those that do can be protected with a Galvanic Isolator. Albeit, as you said earlier, you have an isolator, and yet your aluminium anodes were eroded.

I now know that MGDuff don't offer an aluminium Beneteau type prop-nut anode, and accoding to one Chandlery that stocks MGDuff anodes, it appears that MGDuff are actually reducing the range of aluminum anodes that they produce, as there very little call for them.

Apparently, zinc will provide protction while it is clean, but the problem is, when used in fresh water, a coating of Zinc Hydroxide forms, which effectively insulates the anode.

One suggestion that has been made, is to install a hull mounted magnesium anode, and bond this to the boat's zero volt rail (ground), as this will electrolytically erode in the mainly fresh Broads water. However, I still don't know if the 'only one type of anode' rule is truly technically correct.

The whole subject of boat anodes is very much a case of 'no one type fits all'. If you reside in salt water there is a suitable anode, as is the case for both brackish and fresh water, but if you traverse all three, then you are stuffed.

Dave

[Hockham Admiral]

I will install a Galvanic Isolator, as this is will protect against any stray current when connected to shore power.

All very strange really, as a large number of boats, especially those on the Broads, traverse fresh, Brackish, and salt water.

I wonder what type of anodes the hire companys fit to the boats, especially as these are shaft drive, and many have bow thrusters. Perhaps Clive could comment on this?

but if you traverse all three, then you are stuffed

Barnes Brinkraft don't, Dave (or didn't).

Friday Girl (ex BB boat) is 17 years old this year and she's never had an anode, anywhere and we traverse all three most of the time!

When we had her out of the water last Autumn we specifically looked for signs on the prop (bronze) and the rudder (iron)... there weren't any...

[DaveS]

That is a very interesting indeed John, and would certainly seem to indicate that on the Broads, galvanic corrosion isn't a significant problem.

Conversly, Mark reports that in his case, it is significant.

My boat is 7 years old, and to the best on my knowledge has been kept and used on the Broad's, with the occassional trip out to sea, and yet there is no evidence of galvanic corrosion on either the Prop or the ruddder.

From day one it has had a zinc anode on the factory fitted bow thruster, and the stern thruster (same unit as the bow thruster) which was fitted in September 2011, will also have a zinc anode, because this is the only type that Vetus fit. I intend to remove the props from the thrusters and check the state of the zinc anodes fitted behind. As there is no alternative to the zinc ones fitted, all I can do here is to either clean then up, or fit shiny new ones.

I can see that the zinc prop-nut anode has been doing something, as it has a coating of zinc hydroxide, and some 'pitting' is evident.

I have read reports of where in some Brackish and also some Salty marinas, Zinc anodes have been eroded away is just 5 months. Of course these relate to boats being connected to shore power, and earth leakage current arising from from this.

These being described as 'hot marinas'.

Out of interest, I might just buy a Magnesium hull anode, connect it to the zero volt rail, and leave it dangled over the side, when the boat is moored up in the marina. That way, I can see if there is any erosion over time. I suspect, that when I install a galvanic isolator, it will likely be minimal.

Dave

[mbird]

I guess I've been quite lucky in being able to source Ali anodes for all of my various components. Don't forget though Dave, that being based in Reedham we are permanently brackish which is why we have seen a lot of anode erosion. The further upstream you go, the less effect I suspect.

One other thing to consider is do you have any electrical bonding across your propshaft coupling? At present, we don't, which effectively means the shafts and props are electrically isolated from the entire boat by the coupling, and this is why we do have evidence of slight electrolysis on the props. Once we have put the bonding in, they will become part of the whole ground circuit and therefor take some protection from the hull anodes.

[DaveS]

Maybe being based near to the Oulton Broad end of the Waveney, it is quite likely that we are in fresh water.

In fact, given the fresh water fish that we catch down there, it would seem to support this.

I will check to see if there is an electrical connection between the engine and the 'pop-shaft', although I suspect that there is.

Life is never simple is it

Dave