Anodes

[DavidP]

Anodes ,how long should they last, especially the ones that look like an upside down sharks fin, above the prop.
Mine have been on since late september last year, looked at them on the weekend and one was well eaten ,but thee other one is just a blob now ,did i have crap ones fitted or should they have only lasted about 7 months ,the boat has been in the water all the time and used on a regular basis over the winter.

Cheers Dave.

 

[Piers]

Hi David,

As we heard at the MBM Cruise Club weekend away earlier this year, anodes are not a science. This came frm the main anode maker in the UK, M G Duff, who said we could call them whenever we had a question to ask regarding anodes.

Try Chris Harris on 01243 533 336

The talk was most informative and dispelled many wives tales....

Piers du Pré
MBM Cruising Club enthusiast
www.dupre.co.uk/fsPlaydeau.htm

 

[Chris771]

As Piers said, anodes are not really a black art. If you have two anodes doing similar jobs and one is wasted and the other is not, then you have two options. Firstly the unwasted anode is not properly bonded to whatever it is protecting and is not working. This may be easily established with boat out of water using a DVM on Ohms, should be as low as possible (one ohm or less with reasonably thick wire bonding). If this not the case and this anode is slightly wasted , then the other anode must have become excessively wasted due to a large amount of galvanic action in close proximity to it. It will not be a crap anode, anodes tend to be fairly pure zinc. The reason the anode is wasting (or oxidising) rapidly is that there is a galvanic circuit available to it. The anode bracket must be physically connected to something (normally the object it is protecting by a bonding wire) then there must be a path through conductive seawater. With the electrochemical action the zinc becomes eroded and a thin layer of zinc will be deposited on the object it is protecting (the cathode, or more noble metal). Whilst you will not see "shiny" zinc, if the rate of sacrificial erosion is as high as you stated, then you should be able to see some kind of salty greyish deposit on the object it is "protecting".

As Piers said, it is not rocket science. Anodes tend to work in a very local area, usually within a metre or so on a boat. Also they work in "line of sight" so the object it is protecting will be visible to the anode (before anyone disputes this, I know they will work further and inefficiently around corners, but for this instance it sounds very local).

If you locate some metallic object with a deposit on, then you may continue the detective work with the DVM. Is there an electric potential existing between this object and the shaft and or anode bonding? Check the bonding between this other object and the shaft and anode bonding points, this may require that you run additional earth bonding wires between objects inside the hull.

Other causes can be a bad battery earth connection to one engine, with engine mass floating at an intermediate potential, creating a quasi-impressed current system, or, if you have mains marina power connected, (something like, for example a mains powered engine block heater could have earth leakage). Any other electrical appliances connected can also cause problems if there is earth leakage (for eaxmple mains powered calorifier, slightly hard freshwater in the engine primary system which is slightly conductive) will be enough to set up a galvanic circuit which will cause accelerated depletion of the anode.

If the boat is in the water, try disconnecting the bonding lead from the depleted anode, place a DVM between that and the object which is being protected, on DC Volts, lowest range. The maximum reading should be not more than about 1 volt. or less than about 0.8V if it is outside these ranges try physically isolating things (disconnect Marina power, disconnect battery, disconnect shaft coupling, or if composite materials exist in a flexible coupling there should be an earth bonding wire across the coupling) if you see any change in voltage, you have located the reason.

Even things like an aluminium propellor which is missing a large amount of paint will greatly accelerate anode erosion. Just be thankful it is the anode eroding and not something more expensive. The reason for painting is to provide an insulating and protective membrane over the object you are protecting to reduce galvanic action.

Other apparently silly things can cause problems like for example using anti-seize compounds containing copper, which introduces a third more noble metal into the equation (rather than an electrically inert grease like Aqualube), then you can end up with aluminium propellors protecting the copper in the grease! With bronze propellors de-zincification can occur causing bright erosion and pitting. The key is to use as few dissimilar metals as possible under water. Earth bond everything immersed, inside the hull, and bond all of these items to the anode bracket and keep any paint in good condition.

I have tried not to get too technical, and hope the above will give some pointers of what to start looking at.

Chris



If I am this smart why did I order my new boat with a V*lv* ???????? :-( !!