anode - need I panic?

[billywilliams]

Mine is a 6metre GRP angling boat with 50hp diesel on a shaft. Whilst snorkelling and scrubbing the bottom this afternoon I checked the shaft boss anode and found it missing. I suspect it has fallen off rather than eroded as there was no trace whatsoever. There are two 3" diameter button type anodes on the steel rudder which are about 6" from the end of the shaft. These are well pitted and seem to be doing their job. Is the shaft boss anode critical? Need I panic and haul out to replace it or are the other two sufficient?

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[Badger]

You need a new shaft anode now.

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[omega2]

If your shaft makes a contact (electrically speaking) with your engine/gearbox, and they in turn are connected to an anode then worry not, however if you do not have a proper anodic system on your boat then replace the shaft anode, if your shaft is isolated from your gearbox by a R&D type coupling, make the connection by bridging the joint with wire or solid from the front to the rear bolts. I hope this makes sense.

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[DavidP]

hi
Try this site for Anodes http://www.zincsmart.com/

Dave.

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[MainlySteam]

Sorry to disagree with some of the other posts but if your prop and shaft are made from reliable ss and bronze AND the shaft is insulated from the engine, then you have no need to panic, and you probably do not need an anode on the shaft in any event.

If the shaft is insulated from the engine by a plastic coupling, then the best advice you can have is do NOT bridge the coupling. If you bridge the coupling you are then probably putting the shaft and prop into the galvanic cell made up of the other connected metals in your boat, whereas good shaft and prop materials will live happily together. If the coupling is already bridged (or is not insulating) then leave it as it is if there is no history of galvanic corrosion on the boat, but replace the shaft anode to be conservative.

If the shaft IS electrically connected to the engine, then no one can give you any complete, reliable advice at all about the need for the shaft anode without having complete details, probably requiring a visit to the vessel, as to how the electrical system and the bonding is currently arranged on your boat.

If the vessel has had the anode on the shaft for some years, and there has been no history of galvanic corrosion problems, then the conservative advice would be to replace it and not worry any more, even if the shaft is insulated from the engine and the anode probably not required. This will also be cheaper than having someone visit the boat to give advice, and cheaper than experimenting and finding the experiment was wrong.

John

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[jfkal]

Yes you need a new anode. But why haulout? It is easy to replace a shaft anode under water.
It is always amusing to read the clashes of the bonding and not bonding clans.
R&D couplings are great to isolate shaft from engine. However thet come with a bonding strip to bridge the gap )))))).
I think it is straight forward. If there is a lump of similar metal under water (ss shaft and ss prop. LEAVE EM ALONE (No zinc no bonding). If a lump of dissimilar metal are under water *ss shaft and bronze prop) clamp on a zinc. I have not seen any scientifc reason why on earth you should connect the shaft to the engine (electrically). Lightning protection ????? naw, I prefer the keel )).

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[MainlySteam]

Sorry (again). In case of any misunderstanding, my advice is not based on any belonging to either a bonding or not bonding clan. Some parts may be best bonded and other parts not - there is no one way or the other. My advice is based on professional practice as I experience it to be in the quality end of the market (among other things I project manage the design and construction of high quality, high performance vessels - mostly in aluminium).

Bronze and passive ss in propeller/shaft combinations are rarely a problem as a result of being dissimilar metals. Unless there is poor quality in either material you do not require an anode as long as the shaft is not electrically connected to any other of the vessels metal parts to create a cell. If there is poor quality in either material, you will probably have problems of some sort no matter what (eg anerobic conditions induced crevice corrosion in the shaft).

If the coupling is not insulated, or it is jumpered, then you may require an anode on the shaft, but that decision depends on how the rest of the boat is wired, the location of any other anodes, and the nature of the other metals that may be in the cell. One cannot solve that problem for a specific boat by a forum discussion.

As always, the best advice is, if you have any anode/bonding/no bonding arrangement in an existing boat and you have no galvanic cell problems, is to keep to it. In the original posters case that would mean replacing the anode, however, I doubt whether there is any panic to do so if his shaft is not electrically connected to the rest of the boats metal parts. On my own vessel, there is no anode on the shaft which is electrically isolated from the rest of the vessel and no galvanic cell problems, in that case I would not recommend ever putting an anode on it.

John

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[billywilliams]

thanks for all the views & advice. The boat was constructed by Beneteau and had a shaft anode fitted from new. This wastes considerably over a couple of seasons as do the two on the steel rudder. I will certainly replace it - it was the urgency of the matter I was considering, given the current weather & forecast!! billy Williams

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