Sacrificial anodes

[Shiver Metimbers]

My Sacrificial Anodes are not lasting more than 6 months. I renewed the shaft anode and both the iron keel anodes in April. I lifted the boat out on Sunday and they have completely dissolved. I thought maybe the shore supply to my boat could be the problem so I have refrained from using it. Can anyone give me any advice? Many thanks.

 

[jakeroyd]

Is your boat in a Marina and connected to shore power for long periods .
If so it is possible the Marina has stray currents in your area which can erode your anode very quickly.

 

[laika]

I thought maybe the shore supply to my boat could be the problem so I have refrained from using it. Can anyone give me any advice? Many thanks.

Refrained from using it or disconnected it? If the former, do the latter: You want to break the connection between your ground and the shore power ground. Year before last I was plugged in whenever I wasn't on the boat. Anodes completely gone after less than a year. Last year I would unplug whenever leaving the boat. Anodes working but still mostly intact after a year.

Alternatively look at galvanic isolators

 

[VicMallows]

'Connected' is the important point .... not whether you are actually using any power. Even if you NEVER connect to shore power it does seem that boats in marinas tend to get through anodes much faster than those on moorings. Whether this is due to steel pilings etc in the vicinity or other boats (maybe connected to shore power) remains an open question. Perhaps even the water (aeration; temperature; pollutants ...) is also a factor?

 

[VicS]

As Laika suggests : Consider fitting a galvanic islolator if you leave the shorepower connected for prolonged periods.

However before you rush out and buy one check that your shore power earth is connected the to anodes, DC negative etc. If it is not then you have to look elsewhere for the cause of the trouble.

 

[dt4134]

Maybe it is the shore supply or maybe other boats nearby have issues or maybe there are problems on your own boat.

Trace the wiring to see what is connected to your shaft and keel and what it is in turn connected to. The first thing to check is that it is not your own battery that is dissolving your anodes. You also want to make sure that the 240V side of your battery charger is isolated from the 12V side. Your battery charger should be designed to make sure it is, but faults happen.

After that you want to make sure you're not providing an earth path for dodgy set ups on nearby boats. You could of course completely isolate the keel & shaft from any wiring but you probably don't want to do that (but bear in mind that your engine block is probably connected to one side of your battery anyway) .

Your 12V system should also be connected to earth to protect you if you somehow get 240V shorted to the 12V system.

Best way to connect them is with a galvanic isolator. A few companies sell them. I looked at Sterling & Safeshore and opted for the latter.

You can either fit it on the incoming shorepower earth or between the 240V earth and the appropriate side of the 12V system. I opted for the latter but I suspect the former is more common.

 

[Lundylad]

Appologies in advance for what may seem a silly, how long is a piece of string type of question.
But, how many anodes does a boat need?

I have a bilge keel Jaguar 27 with saildrive which I purchased last year.
It had two anodes on each keel bolted back to back through each keel, one on the prop and one on the engine exhaust elbow.

It obviously needs the ones conected to the engine and prop but can I replace the keel ones and fit one to the hull and bond to the keels internally.

Why am I thinking this?

Ease of maintainence, ability to achieve a good lasting bond of the anodes to the keels and cost each year although this is not the main consideration.

Reasons not to do it?

If it isn't broken, don't fix it, do I want another area of the hull that may allow water in (although if done correctly this should not be an issue) and the possibility of internal bonding faults.

The boat will be lifted out soon and is on a swinging estuary mooring so will be interesting to see the state of the anodes when it comes out.

Any thoughts would be welcomed.

 

[Shiver Metimbers]

I have disconnected the earth wire on my shore supply cable and only use it for an hour at the most each week. I have a car type battery charger hard wired into the auxillery batteries which only charges when the shore cable is connected. I have 3 anodes, two on each side of the trailing edge of the keel and one on the shaft. There is a large anode that is redundant on the underside of the hull, redundant as the internal bonding is broken down and don't work.

 

[VicS]

Appologies in advance for what may seem a silly, how long is a piece of string type of question.
But, how many anodes does a boat need?

I have a bilge keel Jaguar 27 with saildrive which I purchased last year.
It had two anodes on each keel bolted back to back through each keel, one on the prop and one on the engine exhaust elbow.

It obviously needs the ones conected to the engine and prop but can I replace the keel ones and fit one to the hull and bond to the keels internally.

Why am I thinking this?

Ease of maintainence, ability to achieve a good lasting bond of the anodes to the keels and cost each year although this is not the main consideration.

Reasons not to do it?

If it isn't broken, don't fix it, do I want another area of the hull that may allow water in (although if done correctly this should not be an issue) and the possibility of internal bonding faults.

The boat will be lifted out soon and is on a swinging estuary mooring so will be interesting to see the state of the anodes when it comes out.

Any thoughts would be welcomed.

It would have been better to have started a new thread of your own rather than h-jacking Shiver me Timber's thread because answers to your question will become all mixed up with any further responses there may be to that.

However

If your keels need protection from anodes then you do need to position them so that they can "see" both faces of both keels. A single hull mounted anode will not do that.

I don't know what your keels are made of, cast iron presumably. Steel plate would almost certainly need the protection of anodes or a good protective coating. OTOH it is not usual for cast iron keels to need cathodic protection.

Dont overlook the anodes on your sail drive. Make sure you renew them before they become totally depleted.
Anodes on props often don't last very long.

If your engine is directly cooled and has an anode in the water circuit, or even in the seawater side of a heat exchanger if indirectly cooled, be sure to replace it as necessary to maintain protection there.

 

[vyv_cox]

But, how many anodes does a boat need?

I have a bilge keel Jaguar 27 with saildrive which I purchased last year.
It had two anodes on each keel bolted back to back through each keel, one on the prop and one on the engine exhaust elbow.

It obviously needs the ones conected to the engine and prop but can I replace the keel ones and fit one to the hull and bond to the keels internally.

Anodes on the keels could be considered to be unusual and unnecessary. Keels are pretty much protected by painting, give or take a little occasional corrosion, which doesn't affect anything to any significant extent.

I have never come across an anode on an exhaust elbow. What is it supposed to be doing? Raw water cooled engines need an internal anode in most cases, fresh water cooled engines may need one in the heat exchanger but this is material dependent.

The one on the propeller is the important one. Specific propeller ones, for one make, e.g. Bruntons, tend to be small and corrode quickly, so it is well worth fitting a shaft anode as well, if possible. This will delay the replacement of the prop anode, in my case I get two years instead of one and the shaft anode lasts for three. In your case with a saildrive, it should have its own anode.

So the answer for you seems to be two - on the prop and saildrive, unless the engine or heat exchanger need one. In my case also two, one on the prop and one on the shaft.

 

[Tranona]

The saildrive only needs its own anode if it has a standard aluminium prop. If it has a bronze folding prop that will also (probably) have its own anode. Agree no need for anodes on a cast iron keel.

 

[Lundylad]

VicS, Sorry about the thread drift, point taken about it detracting from the original question.
Lots to digest and very interesting responses.

Viv, when I bought the boat the exhaust elbow was cracked and the exhaust hose was de-laminating, both of which were replaced by the seller and I am sure that there is an anode fitted between the engine and the elbow, but will check.

 

[vyv_cox]

The saildrive only needs its own anode if it has a standard aluminium prop. .

Is that so? I have never owned a saildrive but I assumed that the aluminium needed the protection of a zinc anode.

 

[Tranona]

Yes. There is a ring anode on the housing. The standard prop is rubber bushed and therefore isolated from both the shaft and the housing. Bronze props also sometimes have rubber bushes, but with the mix of metals (pins, gears etc) some seem to need anodes. My Flexofold 2 blade does not, although the latest ones do, mainly it seems because buyers expect an anode!

 

[vyv_cox]

I knew about the propellers but assumed the leg itself always needed an anode.

 

[Tranona]

I knew about the propellers but assumed the leg itself always needed an anode.

See why you are questioning my original post which could read that the anode is unnecessary if you have an aluminium prop, whereas I intended to say it is only the saildrive that does, not an aluminium prop.

Poor punctuation on my part, must read "Eats shoots and leaves" again!

 

[Shiver Metimbers]

Thanks for the thread Hi-jack guys.

 

[dt4134]

I have disconnected the earth wire on my shore supply cable and only use it for an hour at the most each week.

I presume (hope) you mean you just disconnect the entire shorepower most of the time rather than connecting it with the earth disconnected.

I have a car type battery charger hard wired into the auxillery batteries which only charges when the shore cable is connected.

I don't expect the designer of a car battery charger would concern himself too much with anode wastage. Worth reading the manual to make sure the 12V output is isolated from the 240V. You could also measure the voltage differences to earth with a multimeter. Be very careful, of course, as you don't want to short anything (especially if your earth wire is missing).

I wouldn't like to say that you could rule out the shorepower but it would have to be bad for one hour a week to zap your anodes so quickly.

If it were me I'd carefully trace the wiring to see what the shaft and keel are connected to. You should be able to determine whether your boat is negative earth or positive earth by checking which polarity is connected to the engine block. You can measure the voltage between the engine block and the +ve & -ve terminals using a multimeter.

 

[vyv_cox]

Thanks for the thread Hi-jack guys.

Answers given to Lundylad are also answering your question. Keel anodes really don't do a lot for you considering what they cost. Make sure the paint is in good condition instead and do away with them.

What anodes do you have on the stern gear? What type of prop, how big, etc. Is the shaft insulated from the engine, are there any stray DC currents on the engine itself? Put your meter between engine and a good earth, for example a seacock, then between engine and the shaft.

 

[oldsaltoz]

Just to add a little more confusion, as no one has mentioned it.

many marinas run an impressed cathodic protection system to prevent the piles corroding, however if they do not look after the cathode and anodes you end up with a current running through the water, this will have the effect of eating your anodes.


Good luck and fair winds.