Anodes & Galvanic Isolators

[discovery2]

How frequently do you check/replace the anode on the end of the propeller shaft?

I have an Antares Series 9 motor cruiser, and have replaced the propeller anode three times during the last 12 months. I can understand that an annual replacement would be necessary, but three/four monthly intervals seems to be excessive. (Having said that, the anodes are obviously functioning correctly!)

As the boat is berthed in a marina, and hooked up to shore power, I am considering fitting a galvanic isolator in the hope this wear might be reduced. Do any owners have experience of these isolators, and in particular, would they prolong the life of the anode?

Thanks in advance for any comments

David


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[MainlySteam]

My responses on these matters usually spark howls of disagreement, however they are based on the protection of new vessels we are associated with.

1. Personally, I never check the anode on my propellor shaft - there is no anode to check. The bronze propellor and ss shaft are insulated from the rest of the boat so there is no possibility of galvanic action as long as a competent bronze is used. However, if I did have an anode it would erode in the presence of the nobler metals but would not be protecting anything. Obviously outdrive legs, sail drives and waterjets with aluminium components are a special issue, but I believe that the Antares are shafted.

2. Contrary to your comment (and it is a comment made by many) because an anode erodes that is not necessarily any indication whatsoever that it is "working". For example, if you bolt an anode to a plate of copper and put it in the sea the anode will dissolve quickly, but in fact it is obviously not protecting anything, one has just made a battery through overprotection. Such unnecessary batteries are the last thing one wants to create on a boat.

3. If your underwater metal fittings are connected to your ac ground, which should be the case, then a galvanic isolator or an isolating transformer is mandatory if the boat is left on shore power.

In the case of the possibility of uneccessary protection (1 & 2) I suspect those are probably not issues as from what I have seen Beneteau have a good record in this area.

Therefore my first suspicion would be that 3. is your problem and your propellor shaft and hence its anode, is electrically connected through your shore power cable earth to a noble metal on a nearby boat or on some other thing on shore also connected to the ac earth creating a cell. You should, if you wish, be able to check this by measuring the current between your shaft and the ac earth on land while your shore cable is disconnected - if you know what to expect and how to do that safely you do not need to be told how, if you do not know get a marine electrician in to do it for you.

John

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[DickB]

I just purchased a Galvanic Isolator from Adverc after suffering excessive corrosion last year. Suspected poor grounding somewhere on pontoon!!! One thing it has to have is an RCD trip since the principle of operation involves bridged diodes to prevent a low voltage leak. It cost about 60 pounds.. Seemed to be the cheapest around.

http://www.adverc.co.uk/technical/technical-galv.asp

The jury is out on whether it works or not!!!

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[steve_cronin]

Just fitted a galvanic isolator as part of a comprehensive shorepower/inverter/solar panel system.

A galvanic isolator is a very good idea indeed if you are in a marina connected to shorepower that also supplies other vessels and even if you ar alone on the pontoon over the winter, the pontoon's own earthing arrangements could also make you a part of a galvanic cell.

Make sure that the unit complies with EN ISO 13297 which has some seemingly quite high current carrying and duration requirements but since any danger is likely to occur whilst you're away from the vessel it is vital that the isolator allows a full blown short to earth safely and that if it isn't detected straight away or the rcd doesn't trip that the wiring is up to the job of carrying the earthing current for a long time (24 hours in the spec. as I remember)

A very good source of information is:-

http://www.sterling-power.com/html/products-isolator.htm

Steve Cronin

<hr width=100% size=1>The above is, like any other post here, only a personal opinion

 

[mriley]

I have bought an RCD and galvanic isolator, which I'm waiting to fit (after I've finished the 101 other jobs that need doing in the next fortnight). My question is, should the RCD, or the galvanic isolator, go next in line to the shore power connection. I have also seen advice that the galvanic isolator should be earthed to the same point as the 12 volt negative (ie engine block), and then again pople say not to mix 12 v and 220 v. Which is right - if the latter where should the isolator be earthed??

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