Anodes

[jax]

We spend three to four months of the year cruising the east coast of the USA, down the intercoastal to Miami and then across to the Bahamas (tough life!) I have a Yanmar sail drive and by the time I return to the St Johns river I find the anode on the said sail drive is completly 'shot' The two 'tear drop' anodes on the hull seem to last very well (these are bonded to the engine) We have traced and we believe eliminated stray elec leaks. The question is why the anode dosn't last longer, if life gets tougher and we cruise for a few extra months we are going to be without an anode or have to haul out to replace it, sometimes not an easy task on a Bahamian island. I could, I know dive and replace but the folding prop ect could finish up deep sixed

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[snowleopard]

after 2 years afloat including 6 months in the caribbean our yanmar saildrive anodes are about 50% eaten. you are obviously getting far more wastage than that.

have you by chance had any antifoul splashed onto the zinc?

i did notice that the anode nearest the ssb earth plate was a lot more wasted than the other one so that may be a factor.

and no, i wouldn't want to take off the folding props underwater!

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[jax]

Thanks for your interest. There is no anti-foul on the anode and this problem has persisted for the past three seasons. I usualy fit a new anode and a-foul the leg also the hull when needed before launching. There is no ssb aboard.

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[snowleopard]

questions...

what is your prop made of?

do you have seacocks bonded to earth?

what's the boat made of?

it certainly sounds as though you have a galvanic cell

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[Sinbad1]

Jax

Not too familiar with saildrives but assuming that the drive is bolted direct to the engine and that between the engine and the propeller there are no plastic/rubber drive absorbers then you should have a continual protection from all the anodes.

There is the poss that the bonding wire from the pear hull anodes to the engine is no longer creating a circuit. Remove and clean up with emery paper and replace.

Also, get yourself a multimeter. Read the instructions. Set it to the resistance circuit at its most sensitive reading, then check for electrical continuity in your bonding circuit. I know this sounds tricky but its dead easy. let me know if you need some help with this.

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[jax]

Re: questions...

Prop is Flex-Fold, Bronze I think
Seacocks are not bonded, are isolated
Have multi-meter and knowledge of use

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[jax]

Got my repies a bit mixed up with Snow lepoard- sorry. However, you may have hit on the problem as the prop is isolated from the drive and engine by a shock absorbing bush. I will contact Flex/fold to clarify Thanks again all.

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[Sinbad1]

Jax

Take your multimeter and put onto ohms. Place one probe on engine and other on shaft beyond shock absorber bearing. You will have no reading I would gues. So the anodes are only protecting the engine and not the stern gear...oops!. Get some 4AWG wire and connect across the shock absorber bearing from one flange bolt to a DIFFERENT flange bolt/nut.

Check with multimeter. Problem solved.

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[HaraldS]

Wondering whether you talk an anode on the prop nut, or one attached to the sail drive shaft. Or are there two aside of the tear drop hull ones?

Would guess that if it is the prop nut one, that it is consumed towards the prop. That is for sure if the prop is isolated. This night not necessarily pose a problem when gone as an isolated prop would be relatively safe.

If all your anodes are electrically well connected, then the anodes closer to the obejct they are protecting are going away faster, but you may still have protection when one is gone.

Finally if the anodes are of somewhat differnt material (say magnesium, vs. zink) the lower on the scale one would get eaten first, possibly towards the higher ranked one.

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[craigbalsillie]

Anodes - newby question

I hope you don't mind me stealing your thread for a bit of education...

Anodes..?

I uderstand they are consumables and need replacing every so often cause they provide an area on the hull for electric charge to eat away at ( I think thats the theory anyway..)

so do all hull's require them regardless of material, GRP and Steel I can understand, do wooden hulls need them..
Are they just fitted to hulls or do you need them elsewhere..
What is the electrical phenomenon that makes them a requirement?

cheers


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[BrendanS]

Re: Anodes - newby question

No hull needs them if there is no metal in contact with water.

All hulls potentially need them if metal makes contact with water.

Usually they are required if you have a propellor, shaft or outdrive, on the hull.

The reason is, in very basic terms (there's all sorts of complications about batteries and onshore electric supplies connected to the boat that make it far more complicated) that different metals in close proximity when connected together by an electric conductor like water (sea water is much more conductive of electricity than fresh water as in rivers or lakes) create their own electric charge.

The movement of electricity, ions, between the two different metals means that bits of the metal can move from one to the other.

The anodes are called 'sacrificial' anodes. It means they move their bits of metal more easily than the expensive props, outdrives, shafts or metal nails in the hull.

What this means is that rather than expensive bits of your machinery move bits of metal between each other, the anode metal is carefully chosen so that it moves instead, and no harm occurs to any other metal

Clear as mud?

Just ask questions?

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[tcm]

Re: Anodes - newby question

For dummies like me: anodes are just lumps of metal usually zinc that are screwd to the outside of the hull. You will see them when the boat is standing, out of the water, big chunks of metal that seem to do nothing and look a bit eaten away at (unless they are new).

Good thing about anodes is that you can't go out and get some really hotshot bulshit anodes that are twice as good as some others, which saves a packet of loot and lots of arguing about go-faster anodes. They just sit there and get eaten away, then you get some more.

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[craigbalsillie]

Re: Anodes - newby question

Thanks Brendan and TCM..

I take it from what you said then they just attach directly onto the hull and
they dont need any sort of electric wiring (i.e. an earth) ...
or do they??

I bielieve I read a post about a "through hull" anode ...

Sounds like a hotshot bulshit go faster thing to me.. (or is it??)

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[Sinbad1]

Re: Anodes - newby question

No, its slightly more complicated than that. You can't just have a lump of zinc in proximaty of under water metal parts and hope it corrodes in place of the boats metal bits. You need to ENSURE that the parts you don't want to dissolve are electrically connected to the anode. That way the electrical current that is produced between dissimilar metals is fed through to the zinc so that the zinc dissolves.

You do this by having a wire from the prop to the anode. Obviously you can't just wire the prop direct, so you work the other way and wire from the anode bolts inside the hull to the engine. This will only work to protect the prop if the engine/gearbox/shaft and prop are all solidly connected by way of metal components. An alternative is to have a shaft 'brush' which runs on the shaft and connects direct to the anode bolt. This allows you to have a rubber/plastic sacrificial join between the shaft and gearbox.

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[craigbalsillie]

Re: Anodes - newby question --thanks for that - NM

.

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[Gunfleet]

Re: Anodes - newby question

You don't have to have anodes bolted through the hull. I don't. I have an anode on a steel wire hung over the transom. It's not a home brewed lash up but a pukka MG Duff job. It still needs connecting to the metal circuit somewhere, though, of course.

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[DeeGee]

Re: Anodes - newby question

Recapping and adding a teeny bit...

Any two dissimilar metals electrically connected together with an electrolyte (salt water) surrounding them, will create a battery (galvanic cell). This includes the MIXTURE of metals (no, not a molecular compound) comprising your propellor. The 'polarity' (plus and minus) of the battery is determined by where the metals sit in a table (galvanic table). In this case the anode is the positive terminal and the cathode the negative. The anode will always be eaten away.

Your propellor will form millions of micro-batteries with the sea water and the different metals in the prop, and those which are lower down in the galvanic scale (particularly zinc) will vanish, leaving a horrible brittle thing which will eventually disintegrate, and will never give a nice bell-like ring (always worth testing when boat hauled out).

How do you stop this happening? By providing a lump of metal which is so far down the galvanic scale that it will always be the anode, for any other metal around. You must make sure that the sacrificial anode is electrically connected to the prop, with no/little resistance, see below. So the sacrificial anode is a bit like a sin-eater, if you know the reference, and the sin-eater stops anyone else in the village going to hell.

[BELOW] Two interesting cases: first a conventional prop-shaft connected back to the engine through some cush-drive, which electrically disconnects the prop and shaft from anything else. This is referred to in some of the posts and requires either electrical brushes on the shaft (commercial product on the market) or an elctrical jumper (not quite so good, as it may lead to some resistance via the oil in the gearbox) across the cush-drive. The brushes or jumper are both intended to provide an electrical connection back to the sacrificial anode via it's through-hull bolts. Second case is the original question: a saildrive, where the prop is isolated from both the leg and the motor. This is typical of e.g. volvo. In this case, you cannot use a through-hull anode, as there is no way to get the electrical connection back to it. YOu have to have local anodes on the propshaft, or on the boss. My understanding is that these do tend to go quite quickly as they are relatively small. The rate of disappearance is proportional to the ratio of areas, prop surface area to the anode surface area. So, a big prop will waste the anode faster than a small prop. My friend replaced his original two-bladed prop with a three-bladed, and the three blades were big clover-shaped thingys, and his anode was wasting at about one per year.

For more info, use your web search on 'galvanic corrosion'.


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