Wiring anodes

[roberth]

On taking the boat out of the water last month, found very little erosion of zinc hull anode. On investigation, found only the engine block was wired up to the anode, and the engine's own internal anode was well worn and will be replaced.

Other prone areas - prop shaft and prop - show little if any signs of dezincification. However, boat was only in the water for six months last year, and will be in a lot longer this season. I would like to protect the prop and shaft at least (possibly rudder stock too?)

Question is, will fixing one of those MgDuff "springy things" with carbon brushes on to connect the internal part of the shaft to the anode wiring be sufficient? And does it matter to which anode bolt this is connected. Should I also, or preferably, fix a shaft anode near the prop?

Hope someone out there can help.

Thanks in advance

Robert

PS I read the recent Sailing Today piece on this but still feel I need further advice.

 

[Spacewaist]

Re: and on a related pont

There is not exactly unanimity about whether wiring things together is a good idea. Some boat builders think it is a god idea; others dont.

Those who do claim they are trying to keep the potential of all metal fittings equal. Those that dont say that it is either of absolutely no value or worse can concentrate the erosion on e.g. one through hull fitting with alll the disastrous consequences that entails.

THe most recent advice I have had in building my current boat is to keep all the through hull fittings as far apart as possible. If they are each more than about 100cms apart there is lottle liklihoof of differential corrosion.

Views welcome.

 

[boatmike]

Re: and on a related pont

If your skin fittings are bronze they are not at risk. If however they are brass and not immediately adjacent to an anode, wiring them to anything else may well cause them to act as an anode themselves and speed the dezincification process. Safest thing is to ensure they are all genuine gunmetal bronze wherever they are in which case they will contain no zinc in the first place.
The same risk applies to prop shafts in fact but here you have a big lump of what could well be Aluminium-Silicon Bronze or Manganese Bronze called a propeller to protect and in extreme cases the prop shaft itself. Here the best arrangement is to have a slip ring connection on the shaft connected directly to as big an anode as you can fit as near as possible to the prop which will ensure the anode depletes before the prop does. Many people use an earthing strip bridge across the coupling earthing the shaft to the engine which is connected to the anode. This is fine if you are sure there is direct connectivity between the shaft to the engine through the gearbox. In some cases there is. In others not. You need to check continuity both in gear and out before relying on this alternative. In the case of saildrives, NEVER connect the gearbox to the engine. Fit the anode directly on the saildrive. The best arrangement on a conventional drive is to fit the anode directly to the shaft also if you have room, avoiding all the problems of connection.