Bonding hanging anode to propshaft

[[3889]]

I have a Mg hanging anode whilst in fresh water and currently resort to bonding the cable to the shaft where it exits the gear box by clamping mole grips onto several turns of wire around the shaft. Is there a more elegant solution? I can see no obvious bonding studs on the stern gear or engine; would bonding onto the engine block be a better idea or would this excessively increase resistance to the shaft ?

 

[Neil]

Am I missing something? you can't use a shaft anode?

 

[[3889]]

Am I missing something? you can't use a shaft anode?

The shaft anode is zinc for salt water. The boat moves between salt and fresh and an Mg anode will disappear in weeks in salt water, so no.

 

[Tranona]

You should be able to bond it through the gearbox using a bell housing bolt. If you have a flexible coupling you will need to bridge that. Alternatively if you have room MG Duff make a device which has a contact running on the shaft behind the coupling. No reason why your hanging anode should not be permanently connect but only put overboard when you need it.

 

[Robin]

We have a lead inside the boat from the backstay chainplate bolts to the gearbox which is connected via the shaft to the prop. our hanging zinc is clamped onto the external backstay fitting, the outside part of where the internal cable is connected., to go out we lift the hanging zinc and sit in the kids sandcastle bucket that sits on the aft deck and which under waydoubles as a pee bucket to reduce the number of 'pee flushes' into the obligatory holding tank over here.

OTher than that I think you can buy a sort of brush gizmo that is designed to press on the shaft to make the connection..

 

[Andrew G]

I am no expert (and I have an aluminium boat) but I have read that:
- one MUST NOT mix Zinc and Magnesium anodes on the one vessel as it will set up corrosion on the vessel and/or its parts (and erode the magnesium quickly). Think about it, if an anode is eroding quickly then there is a current and if there is a current something other than the anode may be eroding too;
- it is better to use zinc in salt and brackish water (and IIRC in fresh water zinc develops a “skin” and becomes less effective).
May I urge you to Google something along the lines of “mixed zinc and magnesium anodes”. I’ll keep looking for my source but it will be a couple of days. Cheers, Andrew

 

[William_H]

I think reliability of connection is more important than actual low resistance. So as said provided there is no rubber or other insulating shaft coupling then a connection to the gearbox should be adequate. Use robust wire and terminals that will survive a period of corrosion and abuse even though light wire will do the job. I liek the i9dea of usinng chain plate bolts with termianl inside and another on the outside of the bolt. good luck olewill

 

[Robin]

I think reliability of connection is more important than actual low resistance. So as said provided there is no rubber or other insulating shaft coupling then a connection to the gearbox should be adequate. Use robust wire and terminals that will survive a period of corrosion and abuse even though light wire will do the job. I liek the i9dea of usinng chain plate bolts with termianl inside and another on the outside of the bolt. good luck olewill

In our instance we simply clamp onto the chainplate outside, the cable from the gearbox goes to a ring terminal attached to the inside of a chainplate bolt, there was enough thread to simply add another nut to hold the ring terminal in place. WE have a solid mechanical connection between gearbox flange, shaft connection and shaft, no flexible connection involved but if there were a simple bridging wire looped across a flexible coupling would do the job nicely. We do have a prop nut anode as well , but use the hanging 'fish one to put a biyt more zinc weight into the equation. we had nowhere convenient to attach a more normal pear shaped hull zinc that we could access inside for wiring connections ( Beneteau with all interior mouldings)

 

[[3889]]

Alternatively if you have room MG Duff make a device which has a contact running on the shaft behind the coupling.

OTher than that I think you can buy a sort of brush gizmo that is designed to press on the shaft to make the connection..

That's what I'm looking for:

http://www.mgduff.co.uk/mgduff-product-catalog/Electro+Eliminators/Electro+Eliminators/EE1.html

Thanks.

 

[[3889]]

I am no expert (and I have an aluminium boat) but I have read that:
- one MUST NOT mix Zinc and Magnesium anodes on the one vessel as it will set up corrosion on the vessel and/or its parts (and erode the magnesium quickly). Think about it, if an anode is eroding quickly then there is a current and if there is a current something other than the anode may be eroding too;
- it is better to use zinc in salt and brackish water (and IIRC in fresh water zinc develops a “skin” and becomes less effective).
May I urge you to Google something along the lines of “mixed zinc and magnesium anodes”. I’ll keep looking for my source but it will be a couple of days. Cheers, Andrew

My research, from a starting point of total ignorance, suggests that an active zinc anode becomes completely inactive in fresh water as it develops an oxide coating. A magnesium anode will erode far too quickly in salt water so, given that the boat moves between both environments, the solution is to use both, deploying the Mg only in fresh. The zinc will contribute to the corrosion of the Mg but as it is only £20 for a 1 kg Mg lump, overall corrosion in fresh water is low and it can be easily replaced afloat, this is a minor issue.

 

[vyv_cox]

MG Duff's advice is exactly what has been done. Hang a magnesium anode over the side in fresh water and have a zinc anode on the shaft for when on the sea. Although I berthed in fresh water for seven years and never used a magnesium anode. The white hydroxide/carbonate coating on the zinc was never complete or thick enough to prevent it from working and annual filing and/or wire brushing was sufficient recovery treatment.

Personally I like the mole wrench method. Cheap and cheerful and, so long as you don't forget it before starting the engine, probably gives the best possible electrical connection at zero cost.