Anodes – An Interesting Question?

[JanetAnne]

If a boat is wired positive earth which side would you connect to the anodes and why?

 

This has caused much discussion and debate today but without a definitive answer. If we were to connect the negative side to the anodes then we would be technically connecting the live side of this boats electrics to the negative side of all the boats around it whereas connecting the earthed side, which in this case is positive) we are now connecting our positive earthed side to everyone elses negative earth also creating a circuit.

 

Anyway, it was suggested that I throw the problem amongst to the wealth of knowledge held by the forum members.

[Guest Jayfire]

interesting you say? 

[grendel]

negative, because thats the anode of the battery. 

the electrons flow from the anode to the cathode, when electroplating you ensure the item to be plated is on the cathode as the electrons flow to it and deposit the plating substance onto it.

so you want your anodes to be connected to the negative side of the battery so the current flows away from the anode.

it doesnt matter how the circuits are wired up and there is a flow and return, even if you have positive earth, you are just placing your devices in the path of flow from negative to positive.

 

[JanetAnne]

13 minutes ago, Jayfire said:

interesting you say? 

Would a cold beer served afloat for the best answer help?

[Smoggy]

Is the shaft bonded to the engine or is it isolated?

[Guest Jayfire]

6 minutes ago, JanetAnne said:

Would a cold beer served afloat for the best answer help?

I'm taking a degree in solution's to interesting anodes questions as we speak.... get me beer ready pal 

[grendel]

if you connected the other way around, the anodes would actually be cathodes and be attracting all of the anode material being eroded off everyone elses boat

[grendel]

actually thinking about it, being positive earth may mean you dont need anodes at all as anything connected to earth wont be eaten away.

[JanetAnne]

1 hour ago, Smoggy said:

 

 

1 hour ago, Smoggy said:

Is the shaft bonded to the engine or is it isolated?

In theory it's isolated but it's on a steel cased spider so ultimately it's not insulated.

40 minutes ago, grendel said:

if you connected the other way around, the anodes would actually be cathodes and be attracting all of the anode material being eroded off everyone elses boat

 

38 minutes ago, grendel said:

actually thinking about it, being positive earth may mean you dont need anodes at all as anything connected to earth wont be eaten away.

If the shaft is earthed as positive and I connect the negative to the anodes as suggested would I not just have a dead short with 6ft of water between them?

I also have a number of brass skin fittings either below or on the waterline to consider. They are all isolated currently but need to be earthed in some way as well.

 

[grendel]

6 hours ago, JanetAnne said:

If the shaft is earthed as positive and I connect the negative to the anodes as suggested would I not just have a dead short with 6ft of water between them?

effectively yes, and your shaft will attract all sorts of muck and build up. the main question is do you have shore power as its generally the earthing for this that creates big issues with boats, no shore power, means less issues.

the biggest issue is i cant find anything about positive earth with respect to boats, I know some old cars used it. surely that means replacing components such as starters and alternators will also be an issue (I can just about remember having to set the polarity on an alternator before I fitted it many years back, but now you just fit them and they work.

[Vaughan]

Positive earth is an American thing and boats wired like that are normally wired "insulated return"  - that is that the engine is not used as an earth and all fittings have their own return.  You can tell if this is the case, as all the senders on the engine for temp or oil pressure will have two wires coming out of them.  There is also no need or regulation to have hull fittings bonded unless you have a steel hull.

But there is in the States!

I shall have a good read of one of my books on this and get back again, as it is a complex subject that I have never more than half understood!  I do know though, that one of the main reasons for having negative earth wiring is because of electrolytic action on hull fittings.

[JanetAnne]

They are both single wire senders Vaughan and yes, the engine block and engine bed are used as an earth.

As an aside, not really relevant but you'll appreciate the situation. Throughout the domestic side of the boat, lighting, pumps etc, where the usual twin cabling is used, all the red wires are negative and the black wires positive! While it's technically correct as the red side is still the  'live' side, it does cause a bit of thinking about when connecting bits and bobs.

 

[Cockatoo]

Just curiosity but what engine is it?

[JanetAnne]

1 hour ago, Cockatoo said:

Just curiosity but what engine is it?

BMC 1.5 on a shaft with a Velvet Drive

[annv]

Hi Why not change over to Neg earth straight forward you will only  have to change the polarity of the alternator unless you have unusual electronics. John

[JanetAnne]

That is ultimately what I will do at the end of the season.

[trambo]

I found this thread to be a Broads Forum equivalent  of playing Mornington Crescent. Not understood a word! 

Fred

 

 

 

 

[Smoggy]

If you change it over I'd suggest either go through changing the red/black cables or label everything or at some point it will cause a problem, possibly with something expensive going bang.

[Vaughan]

2 hours ago, annv said:

Hi Why not change over to Neg earth straight forward you will only  have to change the polarity of the alternator unless you have unusual electronics. John

Well, I have just spent a happy couple of hours looking up things I knew I knew from 40 years ago but had forgotten why I knew them!   Meantime I think John's post sums it up in one sentence!  

There still remains the OP's original question about how this would affect cathodic protection and I would offer some "bullet points" without going into too much detail :-

1/. There is a big difference between galvanic corrosion and electrolytic corrosion.

2/. Galvanic corrosion is the action of two different metals - or impurities in the same metal - which causes an electric current in salt or brackish water.

3/. Sacrificial anodes are there to prevent fittings such as the prop and rudder from being eroded, so they are bonded to these areas or fitted straight on to them.

4/. Galvanic corrosion is a separate matter from the electric circuits in the boat, whether DC or AC.

5/. Electrolytic corrosion is caused by a current leakage to ground in the boat's wiring  (AC or DC) and if there is no earth leakage there will be no corrosion.

6/. Shore power on 220V must be earthed back to the shore by the third wire in the cable.

7/. Shore power points must be provided with this earth but the polarity of the supply is not an issue if the boat is fitted with a bi-pole cut-out.

8/. 220V circuits in a boat must never be earthed or connected, either to the DC earth return circuit, nor to the galvanic bonding circuit of the prop shaft, engine or rudder. This particularly applies to boats with 220V generators.

9/. A fault in someone else's shore power in a marina will not cause specific damage to your own boat but it is a big danger to the occupants of the boat with an earth leakage! Stray currents in the water will erode your anodes, but that is what they are there for.

 

I have not found any mention of any difference between positive or negative earth, concerning cathodic protection against galvanic corrosion.  What matters is that the boat's circuits AC or DC should not have any voltage leak to ground. 

 

[Vaughan]

3 hours ago, trambo said:

I found this thread to be a Broads Forum equivalent  of playing Mornington Crescent. Not understood a word! 

So how do you feel now??   

[Vaughan]

49 minutes ago, Vaughan said:

This particularly applies to boats with 220V generators.

Perhaps I should mention that this is, effectively, what you are doing when you fit a 220V inverter to a DC circuit to run a microwave.  The earthing of such a circuit must be carefully thought out.

 

[trambo]

53 minutes ago, Vaughan said:

So how do you feel now??   

A lot to take in but much better thank you. 

All this applies to fibreglass boats as well as steel ones? If it does then the only metal is the propeller and propeller shaft, correct? Does that need protecting as an essential or a precautionary part of the boats maintenance ?

Now when the three syndicate boats at Horning, Evening Shadow, Silver Cloud and Thunder are all moored in a line and are connected to shore power are we potentially doing damage to each other or is Rico's steel tug "Hero" taking the hit?

Sorry if I'm sounding more of a dumbo than normal but I really am thick when it comes to electrics.

Fred

P.S Sorry to any elephants reading who are offended by my using the term "dumbo_

[Vaughan]

A steel hull is a very different matter as the galvanic corrosion is taking place between the hull and the hull fittings. It shouldn't hurt the hull owing to the direction of the electric current but it will eat away at skin fittings. This is why steel hulls must have all their skin fittings bonded.

A GRP hull does not need this bonding and there should be no issues unless you are using an old fashioned copper based antifouling.

The prop shaft and rudder must still have sacrificial anodes  and I like to see these inspected ever year, especially in a marina.

Shore power on a boat (on water) is a much bigger potential danger than some seem to realise. A quick check by an electrician with a multi-meter will soon tell you if you have problems.

[Vaughan]

Who was it who said "I've started - so I'll finish"?

 

On 14/07/2020 at 07:57, Vaughan said:

one of the main reasons for having negative earth wiring is because of electrolytic action on hull fittings.

Reading this back, I have realised that I didn't explain what I had said earlier.

Galvanic corrosion is caused by an electric current between two metals and this always goes in the same direction, from the prop shaft or rudder, to other places, such as skin fittings or a steel hull.  The anodes on the prop and rudder are allowed to erode, and thus protect the important bits.

A DC circuit in the boat should not have any voltage leaks.     But if it does, then the electrolytic corrosion (not the same thing) from a leak on negative earth will be going in the same direction as the galvanic corrosion and the anodes will still protect.

I do hope that makes sense. I said it was a bit complex!

While I am at it I would point out that battery chargers are the usual source of a low voltage leak in a 220V shore power circuit.  If they are not installed with good ventilation (which often they are not) they will overheat and a part of the insulation on the transformer windings will break down.  If the circuit is not earthed back to the bank as it should be, then a current leak of perhaps 30V or more (AC) will go to ground through the 12V circuit to which the charger is connected.  This means when you are working on the engine you will feel a little "tingle" when you touch it.  Sometimes even when you touch the steering wheel.  Fine if it's only 30 volts but a full voltage might kill you! 

This sort of leak is what will then find its way into the water outside.

The only way you would know if you had a leak like this is to have the circuit tested regularly by an electrician.

[FairTmiddlin]

10 minutes ago, Vaughan said:

Who was it who said "I've started - so I'll finish"?

And that would be the eloquent Magnus Magnusson