shaft anode do I need one?

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paulsmith

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looking round the boatyard some boats have shaft anodes some don't. Mine a westerly griffon hasn't.I have just bought the boat so I dont know if there is good reason for this ommision.it has the usual mix of iron keels stainless shaft, bronze prop etc and a big anode on the hull more or less in the middle of all the different bits. so do I put one on or not.I don't want to spend money if I don't need to. any thoughts?

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the only anode on my boat is on the shaft, there is no sign of electrolitic action.
i re-engined 2 yrs ago & the anode now "wastes" quicker. the previous Perkins 4108 has a insulated return system that the new Nanni doesnt have

<hr width=100% size=1>Mines a Crocodile sandwich & make it Snappy
 
Anodes and cathodic protection can be a difficult subject.

Assuming no complexities (steel/aluminium hull, freshwater sailing etc) then what you are trying to ensure is that differing metals which are underwater AND which are connected together electrically are protected by a sacrificial piece of metal - the anode that you fit.

Whether it's a shaft anode or a hull anode is irrelevant in a sense.

The reason some boats need shaft anodes might be that the coupling between engine and gearbox is an insulator, or the gearbox might have special electrical insulation etc. Even if the insulation exists, some boats fit a pair of 'brushes' that rub against the shaft near the stuffing box to connect their hull anode to the prop and shaft, or fit a braid round the engine coupling etc.

The bonus of a shaft anode (assuming that it doesn't come loose and make annoying rumbling noises when you are motoring) is that the electrical connection is short and direct.

Assuming that you anode is being eroded, and that the prop, and any other underwater items you want protecting have their electrical connections clean and of low resistance, I cannot think of any reason to change to a shaft anode. You need to check that the anode has a low resistance electrical connection to the prop shaft.


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That's an excellent and concise reply and I agree entirely. Many boats have anodes all over the place that perform little function so I would not fit a shaft anode "because everyone else has one" unless you have a particular problem. If the vessel has operated for some time without one, examine your propeller closely. It's probably made out of manganese bronze. (See thread re "marine brass?") If you see areas of deep coppery red on it that are not removed by light sanding this is evidence of "de-zincing" which is what anodes are there to prevent. Even then though a certain amount of de-zincing is fairly common and its only when accompanied by pitting should you be particularly concerned.
If you have room between the outer bearing and the shaft to fit one you would probably find a rope cutter more useful than an anode!

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Simple - if there is evidence of any metal being 'eaten away' on the shaft, prop, or other metallic items connected to the shaft, you need an anode. On the other hand, if all looks OK, you don't.

<hr width=100% size=1>Piers du Pré
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