galvanic corrosion. back to basics please

[steveeasy]

Hi All
Just replaced my prop and propshaft which both were bronze. I fitted a new bronze prop to a SS shaft. these are connected to a flexible coupling. I have received conflicting advice. I have fitted an anode on the propshaft. I was told I did not need to do anything else as the flexible coupling isolated the engine and batteries. A day later I was told by another person to fit a wire across the flexible coupling to connect the engine to the propshaft. So with little knowledge of Galvanic corrosion, why would you want to connect the engine and therefore batteries to the effects of this. surely they are isolated by the couplings design.

Many thanks
Steveeasy

 

[Graham376]

Engine is presumably wired to hull anode so a bridging wire across the flexible coupling is a normal fit, shaft anodes have been known to fall off.

 

[VicS]

Hi All
Just replaced my prop and propshaft which both were bronze. I fitted a new bronze prop to a SS shaft. these are connected to a flexible coupling. I have received conflicting advice. I have fitted an anode on the propshaft. I was told I did not need to do anything else as the flexible coupling isolated the engine and batteries. A day later I was told by another person to fit a wire across the flexible coupling to connect the engine to the propshaft. So with little knowledge of Galvanic corrosion, why would you want to connect the engine and therefore batteries to the effects of this. surely they are isolated by the couplings design.

Many thanks
Steveeasy

Your shaft anode should provide the protection needed. It will need replacing from time to time and they can work loose and be lost

A hull anode will provide back up in the event of the shaft anode being lost or completly consumed at a time when it would be difficult or inconvenient to fit a new one.

If a hull anode is fitted it must be as close as reasonably possible to the stern gear and there must be a good, low resistance, electrical connection between the two. This may be via system of brushes making contact with the shaft immediately inboard of the stern gland, or as is common practice, by bonding the anode to the engine block or gearbox housing and bridging the flexible shaft connection

 

[Catalina36]

I've previously lost a shaft anode, presumably it just fell off, so nowadays I always fit two, flat ends back to back and with the bolts oriented at 90 degrees to one another. More expensive, but against the overal cost of an annual haul-out and A/F, small beer.

 

[vyv_cox]

Fit shaft anodes by bolting them together on the shaft, then bang the two halves together using a hammer in each hand. Tighten the bolts and repeat until further tightening is not possible. Anodes fitted this way will never fall off.

 

[steveeasy]

Thanks for the responses. So to clarify, as long as I have an anode on my prop and the flexible coupling isolates the engine and electrics, I do not need anything else. If however the flexible coupling does not isolate the engine and batteries then I need to connect the engine to a Hull fitting Anode and can fit a bridging wire across the coupling.

I am confused though, as if the engine is isolated by the design of flexible coupling why would anyone want to connect it up. Just trying to understand. thanks
Steveeasy

 

[vyv_cox]

Thanks for the responses. So to clarify, as long as I have an anode on my prop and the flexible coupling isolates the engine and electrics, I do not need anything else. If however the flexible coupling does not isolate the engine and batteries then I need to connect the engine to a Hull fitting Anode and can fit a bridging wire across the coupling.

I am confused though, as if the engine is isolated by the design of flexible coupling why would anyone want to connect it up. Just trying to understand. thanks
Steveeasy

Yes, I can see that you are confused. The purpose of an anode is to protect two items with differing metallurgies that are connected together and immersed in water. Without an anode either the prop or the shaft would corrode preferentially while 'protecting' its opposite number. Adding an anode makes it corrode in preference to either of them. The presence of the engine, either with or without an insulating coupling, makes absolutely no difference.

Where the coupling conductivity makes a difference is where a shaft anode cannot be fitted, with a shaft log for example. In this case the anode is placed on the hull where it can 'see' the propeller and an electrical connection is arranged through the shaft, coupling, gearbox and engine and thence by wire from the engine to the internal connection of the anode. If the coupling is insulating it may be necessary to bridge it with copper to ensure completion of the circuit. Alternatively, a brass brush can be fitted to rub on the shaft and this is connected directly to the anode.

 

[VicS]

Yes, I can see that you are confused.
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Alternatively, a brass brush can be fitted to rub on the shaft and this is connected directly to the anode.

Such as an "electro eliminator" ....... Electro Eliminators For Prop Shaft Bonding

 

[Catalina36]

Fit shaft anodes by bolting them together on the shaft, then bang the two halves tofether using a hammer in each hand. Tighten the bolts and repeat until further tightening is not possible. Anodes fitted this way will never fall off.

Cheers Vyv, very useful tip.