Dumb Electrical Question

[chas]

This is probably a silly question. Should there be continuity between the negative side of the bar I connect electrical appliances to and my anode?

Thanks in anticipation.

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[snowleopard]

your engine will be connected to the negative side of the electrics as this is conventionally the 'earth'. that means the shaft and its anode will be at that voltage. any other anodes must also be at the same voltage or you will set up a cell and one of them will disappear very quickly. instead of connecting to the negative bus bar you could link the anode to the engine.

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[bedouin]

Depends on the details of your installation.

The important factors are that all the external fittings the anode is protecting are connected to the anode (normally prop shaft and rudder stock).

If your prop shaft is electrically connected to your engine, and the engine is connected to battery negative (almost always the case) then you will have connectivity between your negative busbar and the anode. However I think in may cases a non-conductive flexible coupling is used between the engine and shaft, effectively insulating the engine from the shaft and hence the anode.

If you're worried test the resistance between the shaft and the anode and between the shaft and the engine.

BTW be careful of "continuity" - it is not a well defined term and can represent a resistance of several ohms. For your anode to work it needs a very low resistance connection to the shaft etc.

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[anabel]

In the majority of cases the answer is yes. Normally the engine is connected to the negative pole of the battery. (Check it out to make sure). This means the shaft & prop are at the same negative potential. If the engine is 'isolated' from the shaft then a continuety strap is connected to bridge the non metallic coupling in order to have shaft & engine etc at the same potential. The anode must be connected to this negative potential. This will be either an anode bolted to the hull with the bolt being wired to this negative potential, or a shaft anode which is clamped on to the shaft. (don't let these anodes get covered with antifouling paint). Your metallic skin fittings should also be connected to the anode. The resistance between the shaft or prop and the anode should be less than 1 ohm. It is also considered good practice if the anode can 'see' whatever it is supposed to be protecting.

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[roger]

Am I right in assuming you are talking about low voltage (12/24 V) DC equipment. If you are talking about High voltage AC ita a different ball game.

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[chas]

Thanks all - that makes it clearer. I have just replaced the anode and when checking connections noticed that I had continuity between it and the negative bar. I could not rememeber whether this had been the case before.

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[Chris_Robb]

Are some engines totally insulated from the negative? I have a Perkins 4236 - the starter motor has both positive and negative leads. In this case the negative on the buzzbar will not be connected. The only connection to the anode is the prop shaft. Do I need to do anything else?

I suppose the proof of the pudding is that after 26 years there is no corrrosion activity at all, except deep pitting in the stainless shaft through the bronze sterntube and bronze cased cutlass bearing. This has necessitated a new propshaft this year.

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[bedouin]

I don't know if any engines are themselves totally isolated from the negative. In theory it is possible, but you could tell is it would require two wires to each sensor (temp, oil pressure).

However connectivity between the battery negative and anode is not important.

What matters is that whenever you have two dissimilar metals electrically connected together and are in contact with the sea-water, then they also have a low-resistance connection to the anode, which should be physically close to the metal being protected.

On most boats it is only the sterngear that needs this protection - so the only important connection is between the anode and the shaft.



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[tr7v8]

Yes some Perkins were true marine engines with a totally isolated earth return. And yes you will have 2 wires to every connection.

<hr width=100% size=1>Jim

Draco 2500

 

[Chris_Robb]

Yes - there are 2 wires to everything. I believe it was an MOD requirement - for which the 4236 was originally marinised.



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