Alcyone
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Reged: 09/04/2008
Posts: 351
Loc: Bridgend, Wales, UK
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I need to fit a new anode to my boat (Cobra 850). Am I right in thinking I need one of the narrow shaft mounted ones - the gap seems very tight? Also, should it touch the prop, or sit 'in the middle' of teh gap I have. From what I've read I think I need aluminium for Cardiff bay.
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AvastMark
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Reged: 28/11/2006
Posts: 304
Loc: Nottingham
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you may squeeze a shaft collar in there if you're lucky. Though you're right, there doesn't look like there is much room. I use a collar that's about 25mm wide. They may come in other widths.
I don't think it will matter if it touches the prop. There needs to be an electrical connection between the prop and anode anyway, which is made by the prop shaft they are both bolted to. If you have a multimeter, then once you've fitted the anode, use it to check for continuity between the prop and anode. That's what I do, just to make sure everything is ok.
The collar anodes aren't as big as other types, such as the ones that bolt on the boat and mine dissolves very quickly. if you get the opportunity to dry out, then check it in say August and see if this is an issue. You may need to supplement it with a type that hangs over the side of the boat, which you can lower in the water when you leave the boat on its mooring. That's what I plan to do anyway. Of course if your boats a bilge keeler you can easily keep an eye on it and replace when necessary.
As for the material, I thought they were all Zinc. Different types, like magnesium, are available, but I thought they were for use with different boat materials, not sailing areas. Though I may be wrong.
Oh and, you may be well aware of this, so appologies in advance, but don't antifoul the anode or it won't work properly.
Edited by AvastMark (16/05/2008 09:24)
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npf1
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Reged: 09/10/2004
Posts: 1082
Loc: SE
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FYI: might be worth speaking to Zincsmart who also have have prop nuts with Anodes on them
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Alcyone
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Reged: 09/04/2008
Posts: 351
Loc: Bridgend, Wales, UK
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Great, thanks.
Re: material. From what I've read, zinc is for sea water, magnesium for Fresh. Cardiff Bay is fresh but clearly when you lock out you are in salt water, and aluminium is best for that environment.
Please feel free to correct me, I'm very new to this.
Oh, if the gap is just too small for the anode, nothing to stop me grinding a mil or two off it is there? Bearing in mind that I might have to change smaller ones more often, of course.
Edited by Alcyone (16/05/2008 09:33)
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VicS
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Reged: 13/07/2002
Posts: 9003
Loc: Home: Kent. Boat: Chichester
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Quote:
I need to fit a new anode to my boat
the first question is why and the second is what is it to protect. The Cobra 850 is hardly a new boat so what has been fitted before?
Assuming you do have a need to protect the stern gear then It may just be possible to fit a slim collar type. This page from MG Duff discusses the selection of anodes for different environments and This pageshows the types of shaft anode available I am not sure if a suitable anode is available in aluminium though. Shifting between salt and fresh or brackish water is a problem when it comes to choosing the anode material as the link will explain to you.
The closer it is to the prop the faster it will be consumed and if it touches the prop then it will be eaten away locally even faster. (somewhere on the MGDuff website i think there is a recommended minimum distance for the spacing of the anode from the prop but I cannot find it this morning. Maybe it is on another site.
An anode combined with the prop nut as suggested may be a possibility but it will be smallish and may require frequent replacement.
It is well worth looking at the MGDuff website in general.
The more satisfactory solution could be a hull anode fitted as close as possible to the prop but it will have to be wired to the engine/gear box and any flexible coupling will have to be bridged to complete the electrical connection. If the corrosion of your stern gear is truly excessive then an electro eliminator, a system of brushes, to make contact with the shaft may be more effective. You will find them on the MGDuff site as well.
I still ask however if cathodic protection is really necessary especially being moored in water with a low salinity.
Although i have made several links to the MGDuff site I suggest you also look at others and compare what is available from them and their prices
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Old Chemists never die, they just fail to react
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Tranona
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Reged: 10/11/2007
Posts: 1084
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Whatever you decide on anodes from the protection point of view and like Vic i question the need if the boat does not have it at present - don't fill that gap in front of the prop. It is there for a purpose, primarily to let water flow in and out of the cutless bearing and, if your engine is on flexible mounts to allow min +/- 3mm (total 6mm) to cope with fore and aft movement when you engage gear.
On the anode side, a properly bonded external anode - the teardrop shape as suggested by Vic is the most common solution.
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VicS
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Reged: 13/07/2002
Posts: 9003
Loc: Home: Kent. Boat: Chichester
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Quote:
don't fill that gap in front of the prop. It is there for a purpose, primarily to let water flow in and out of the cutless bearing and, if your engine is on flexible mounts to allow min +/- 3mm (total 6mm) to cope with fore and aft movement when you engage gear.
Two very good points!
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Old Chemists never die, they just fail to react
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Alcyone
regular
Reged: 09/04/2008
Posts: 351
Loc: Bridgend, Wales, UK
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OK, I've just bought the boat and the survey mentions the lack of an anode. It also mentions some very slight pitting on the prop which could be related. I also have the previous 2001 survey, however, which mention the same points, so I assume that there can not have been that much erosion if the pitting was slight in 2001 and is still slight now.
However, since the boat is out of the water, and the anodes are not overly expensive, I thought why not just do it?
Thanks for your point about the gap though, I guess that rules out a shaft anode anyway.
I'll have a look at the link, but I'm guessing now I'll need to fit one elsewhere.
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boatmike
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Reged: 30/06/2002
Posts: 4342
Loc: Solent
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Another even better point is that depending on the type of gearbox, the shaft may well move longitudinally from forward to reverse and needs clearance to do so. Don't put a shaft anode on. Fit one adjacent to the prop on the hull and connect through either a shaft slip ring or more commonly through the engine block. If the latter make sure there is a metal flexible strap across the flexible coupling to maintain electrical continuity.
Edit.... Oops! That was one of the earlier points. Should have read it properly before posting. Doesn't hurt to say it again though... Don't do it!
Edited by boatmike (16/05/2008 13:09)
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VicS
regular
Reged: 13/07/2002
Posts: 9003
Loc: Home: Kent. Boat: Chichester
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Quote:
I thought why not just do it?
more to the point why do it. From what you have said there appears to be no reason to fit one certainly not to go to all the bother of fitting a hull anode and wiring it up. Then there's the issue of the material. Both zinc and aluminium will rendered ineffective if the boat is moored in fresh water while magnesium is too reactive to be used in salt water. Cardiff bay though is presumably brackish and therefore Ok with aluminium.
If you have a prop made of a suitable alloy it should not suffer any corrosion. Pitting is a bit odd though as normally dezincification is the problem if any.
I think this is a case where the rule "If it ain't broke don't fix it" should apply and why drill two holes through the hull for no good reason. You say your surveyor has pointed out that there is no anode. That's a fair comment I suppose since there is not one but if he recommends that you fit one then you are rather caught between a rock and a hard place!
You can always keep the situation under review and if there are any signs of the prop going pink, indicating dezincification, it would be the time to fit an anode. Difficult to see when its painted blue though!
I assume you are on a mooring and not in a marina with shore-power connected permanently otherwise we will be into discussions about galvanic isolators!
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Old Chemists never die, they just fail to react
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