Shaft Bonding

[Leonora Davy]

The boat has just been surveyed and the surveyor says that the propeller is electrically isolated from the hull and this is the reason why the anodes aren't deteriorating - in other words, the anodes aren't working and there is no protection from them.

He has suggested a wiper brush or similar on the shaft connected to the boat ground to resolve this.

 

I've rattled my brain and can't work out why this would be beneficial. If the connection is made, a circulatory current would then flow which would cause the anodes to sacrifice themselves to protect the steel. However, an electrically isolated propeller would surely create a much reduced electrolytic reaction as there is no circulatory circuit path through the boat ???

 

Maybe I cannot see the wood for the trees here ??

 

Can you straighten my thinking out ?

[ChrisPy]

I believe the anodes are there to corrode preferentially to the hull, in the proximity of more noble metals like the bronze propellor (that may or may not be electrically connected to the hull), thereby protecting the hull steel. I do not believe the anodes are there primarily to protect the shaft.

 

It may be that in your home waters there exist only minimal conditions favouring corrosion.

[Cafnod]

The boat has just been surveyed and the surveyor says that the propeller is electrically isolated from the hull and this is the reason why the anodes aren't deteriorating - in other words, the anodes aren't working and there is no protection from them.

He has suggested a wiper brush or similar on the shaft connected to the boat ground to resolve this.

 

I've rattled my brain and can't work out why this would be beneficial. If the connection is made, a circulatory current would then flow which would cause the anodes to sacrifice themselves to protect the steel. However, an electrically isolated propeller would surely create a much reduced electrolytic reaction as there is no circulatory circuit path through the boat ???

 

Maybe I cannot see the wood for the trees here ??

 

Can you straighten my thinking out ?

 

Hi Leonora,

I am not sure I can see why either, perhaps someone like Mark, who has a lot of big boat experience will have a better answer but is the surveyor saying that as the prop is isolated then it is acting as an anode and coroding?

If so that would be a good reason to electrically connect it, all of you metal work would be bonded together.

I cannot think of any other reason.

Although in all honesty I cannot see how the prop could be acting as an anode if it is not connected to the metalwork of the boat.

Further to that I cannot work out how the prop cannot be connected, it is bolted to the shaft that runs through bearings to a drive flange.

[alan_fincher]

Further to that I cannot work out how the prop cannot be connected, it is bolted to the shaft that runs through bearings to a drive flange.

My thoughts too.....

 

As Cafnod says, it the metal prop is directly on the taper of the metal shaft, which rotates in metal bearings that are bolted or welded to the metal hull, it's hard to see where the electrical isolation is. The most isolating thing I can think of is the thin layer of grease twixt shaft and bearings, (although I suppose it possibly has one of those water lubricated greaseless types, particular as I think I remember it has a Vetus engine ??).

 

Did the surveyor explain exactly what's causing the isolation, and where ?

 

I've never heard of anything remotely like this before, I'll freely admit.....

Edited July 1, 2006 by alan_fincher

[Guest]

I am pondering over this one. I can see that a Vetus or similar shaft with a neoprene cutlass won't directly connect electrically to the boat, but surely it will connect via the drive flanges and engine? I was looking at R&D coupling website recently and noticed that an earth strap can be added to these to maintain conductivity, so maybe with all the rubber/nylon mounts/couplings these days, there is a call for such action? I would be interested to know more details of the drive system in question.

[alan_fincher]

I am pondering over this one. I can see that a Vetus or similar shaft with a neoprene cutlass won't directly connect electrically to the boat, but surely it will connect via the drive flanges and engine?

Ah, I think there maybe we have it - I was starting to go down this path in my mind....

 

So you are confirming if it has the Vetus cutlass (?) stern gear there will not be any connection there between shaft and hull.

 

I think even the most basic flexible coupling tends not to have any direct electrical path through it. I'm pretty sure on mine that bolts go in each direction from a central "flexible bit' into the metal couplings, but nothing through the whole lot, (or it wouldn't be very flexible, would it ?).

 

If there is a case here for connecting the shaft electrically to the rest of the boat, (is there ?), then I'd have thought some kind of braided strap across the coupling, with one end under a bolt at the engine end, and one end under a bolt on the coupling to the shaft, sounds more practical than trying to have something set up permanently rubbing on the rotating shaft.

 

"Leonora".

 

Can you confirm if it has the Vetus style stern tube, and do you know if that's why the surveyor said it was electrically isolated ?

[Guest]

Ah, I think there maybe we have it - I was starting to go down this path in my mind....

 

So you are confirming if it has the Vetus cutlass (?) stern gear there will not be any connection there between shaft and hull.

 

I think even the most basic flexible coupling tends not to have any direct electrical path through it. I'm pretty sure on mine that bolts go in each direction from a central "flexible bit' into the metal couplings, but nothing through the whole lot, (or it wouldn't be very flexible, would it ?).

 

If there is a case here for connecting the shaft electrically to the rest of the boat, (is there ?), then I'd have thought some kind of braided strap across the coupling, with one end under a bolt at the engine end, and one end under a bolt on the coupling to the shaft, sounds more practical than trying to have something set up permanently rubbing on the rotating shaft.

 

"Leonora".

 

Can you confirm if it has the Vetus style stern tube, and do you know if that's why the surveyor said it was electrically isolated ?

Of course.....the bolts don't pass thru a flex coupling. I blame the sun on my bald patch for my memory loss today. The strap you describe is roughly what R&D sell for their coupling. Leonora, we need more info.!

 

Is it like this?

 

Edited July 1, 2006 by Guest

[Cafnod]

Hi Stan

I have been wondering about the earthing of the 12vdc potential on boats generally lately.

Looking at my own engine, which is rubber mounted I do not see how the earthing occurs. Sure there is a big earth strap from the starter battery to a stud on the engine but there is no earth strap between the engine and the hull, I would have though for the 12vdc to be fully bonded to the hull there would have to be an earth strap running from the engine to the engine bearers. Thus giving coninuity of the earth with low resistance, or am I missing something?

BTW I see you have edited your feet out of the picture this time lol.

Edited July 1, 2006 by Cafnod

[Guest]

Hi Stan

I have been wondering about the earthing of the 12vdc potential on boats generally lately.

Looking at my own engine, which is rubber mounted I do not see how the earthing occurs. Sure there is a big earth strap from the starter battery to a stud on the engine but there is no earth strap between the engine and the hull, I would have though for the 12vdc to be fully bonded to the hull there would have to be an earth strap running from the engine to the engine bearers. Thus giving coninuity of the earth with low resistance, or am I missing something?

BTW I see you have edited your feet out of the picture this time lol.

Richard

You are correct. Mine has rubber feet, so in this case no earth. So the engine/propshaft and prop in this case will be isolated IMHO. I had forgotten the lack of an earth strap as a vehicle would normally have.

 

I can say that on dry docking the other week that our anodes will definitely need renewing next time out-like my feet in the picture they have vanished.

 

Vetus propshaft-the cutlass is neoprene on mine:

 

[Leonora Davy]

The engine is a Vetus. There is a flex coupling and a Vetus water-lubricated sealed gland.

I am quite sure that the surveyor is right in saying that the propeller is electrically isolated from the rest of the boat because the coupling will act as an insulator.

 

The real question here is:

 

Is it better to have an isolated propeller or a grounded propeller, from an electrolysis point of view ?

[ChrisPy]

I am not sure I can see why either, perhaps someone like Mark, who has a lot of big boat experience will have a better answer but is the surveyor saying that as the prop is isolated then it is acting as an anode and coroding?

if the prop is bronze it won't corrode. It is more noble than the hull and the shaft. If it is anything it is a cathode, not an anode.

[Guest]

The engine is a Vetus. There is a flex coupling and a Vetus water-lubricated sealed gland.

I am quite sure that the surveyor is right in saying that the propeller is electrically isolated from the rest of the boat because the coupling will act as an insulator.

 

The real question here is:

 

Is it better to have an isolated propeller or a grounded propeller, from an electrolysis point of view ?

 

You have same setup as me. Don't know to be honest, but I can say there is no corrosion on propshaft or prop after two years afloat. Also anodes appear to be working (at an alarming rate).

 

Out of curiosity did your surveyor mention the inclusion of a flexible coupling on a Vetus flexible shaft? That is another can of worms.

[Leonora Davy]

There is some slight pitting on the hull underwater. The boat is fitted with a shoreline and galvanic isolator.

As the anodes are still new after a year in the water, I can only presume that the surveyor thinks that the insulated prop and shaft are the possible cause. He has also advised the checking of the galvanic isolator or a fitting of an isolation transformer.

[ChrisPy]

He has also advised the checking of the galvanic isolator or a fitting of an isolation transformer.

I cannot see how either of those issues could be related to the lack of anode corrosion. But maybe his comments were not related to this issue.

[Cafnod]

Right while I have Stan and Chris on one thread I will aks another question.

I have not seen ANY boat with an earth running from the engine mount to the engine bearers, thus earthing the 12vdc potential to the hull.

Now the 240vac potential has to be bonded to both the 12vdc supply earth, (I believe Chris recently quoted an RCD requirement for this on another thread) AND to the hull.

So therefore is it the case that there can be a potential problem when one potential is bonded to the hull and the other is not?

[Guest]

Right while I have Stan and Chris on one thread I will aks another question.

I have not seen ANY boat with an earth running from the engine mount to the engine bearers, thus earthing the 12vdc potential to the hull.

Now the 240vac potential has to be bonded to both the 12vdc supply earth, (I believe Chris recently quoted an RCD requirement for this on another thread) AND to the hull.

So therefore is it the case that there can be a potential problem when one potential is bonded to the hull and the other is not?

 

This is a problem that I have never seemed to be able to get a firm answer to. The people our sailaway came from do not earth the DC side of the electrics to the hull. Therefore I currently have no earth connection from the DC side to the hull. I have earthed the earth terminal of the inverter to the hull which is via an RCD. When the genny is used that will earth through the same terminal. If I understand what you are saying Richard then the dc side will be bonded to the hull by default. I cant think of a reason why the 12v side needs to be earthed on a steel boat, but of course I am always happy to learn. I do know that in some cases DC earrthing is deliberately avoided by using insulated return alternators etc.

[NB Willawaw]

Cafnod,

 

Ships use impressed current systems to control electrolysis, so the problem is slightly different to that in smaller boats, but thanks for the vote of confidence.

 

All:

 

The BMEA code of practice and American equivalent ABYC ask for bonding - it is generally called for on RCD boats.

 

However, the decision as to whether to bond or not is very controversial.

Some builders do not bond their steel boats because they believe it to be counter-productive. They use localised zinc anodes.

Some very informed people have said that one should not bond just for the sake of corrosion protection, but its debatable.

 

Just to quote one informed but unofficial source:

 

QUOTE

Through-hull fittings that are electrically isolated, in little danger of stray current corrosion and remote from protective zinc anodes, need not be bonded (fittings that are far away from your zinc anodes are not protected anyway). Keeping these fittings unbonded is desirable since a large system is more likely to pick up stray currents flowing through the water.

UNQUOTE

 

In the case of the Leonora Davy, the shaft will be electrically isolated from the engine by the coupling. The type of stern gland is not that relevant. Even if the coupling were bypassed by a strap, the engine might not be earthed through the negative supply.

However, the shaft is probably stainless steel and the propeller is probably a Bronze alloy, which is more noble. If it isn't bonded through the hull to the zinc anodes, then the two metals of the prop/shaft could react. However, if it is a Vetus prop and tailshaft, there is a zinc fitted in the assembly by Vetus. In other words, it has its own mini-localised cathodic protection system.

 

The BMEA code of practice also recommends that the DC negative terminals of the batteries are connected to a DC earth point on the hull in most cases.

 

P.S Leonora - check that the zinc anodes you have are making good electrical contact with the hull. Presumerably, you have two forward and two aft - you don't say whether they are bolted or welded on. The bolted type don't always make good contact.

The next thing to check is that the galvanic isolator has been installed correctly and that the diodes inside aren't faulty.

Edited July 2, 2006 by NB Willawaw

[Cafnod]

Thanks for the replies, Mark, Stan

What I am trying to ascertain is if the 240ac is bonded to the hull, as everyone tells me it must be, and the 12vdc IS NOT then is there a problem as I thought the two potentials earths had to be connected together, but not to the same stud?

It then seems that the large metal area of hull is isolated from the 12vdc and I am wondering if this could lead to probs with the 240ac?

Sorry if this is a bit inarticulate, long week.

[NB Willawaw]

BMEA Code of Practice which would need to be complied with for RCD:

 

2.4.1 The system type shall be either a fully insulated 2-wire DC system or a 2-wire DC system with a negative ground. The hull shall not be used as a current-carrying conductor.

 

2.4.1 iii It is recommended that there should be one single earth point (common ground point) where the battery negative and any equipotential bonding or protective conductors are all connected together.

 

2.4.2i If an AC system is installed on the craft, the DC negative shall be connected to the AC protective conductor at the crafts ground (earth) point unless a whole craft RCD or isolation transformer is fitted.

 

For whats it worth, I always use an Isolation Transformer for shore supplies, but still connect DC negative and AC PE cable to earth studs - in practice, two separate studs, but quite close together on the same steel bulkhead or stringer.

 

I like belts and braces.

 

 

Thanks for the replies, Mark, Stan

What I am trying to ascertain is if the 240ac is bonded to the hull, as everyone tells me it must be, and the 12vdc IS NOT then is there a problem as I thought the two potentials earths had to be connected together, but not to the same stud?

It then seems that the large metal area of hull is isolated from the 12vdc and I am wondering if this could lead to probs with the 240ac?

Sorry if this is a bit inarticulate, long week.

[blackrose]

I know you can install brush systems which connect your prop shaft to the hull, but wouldn't it be possible to simply lay a peice of wire across the shaft in the engine room and connect it to your hull or the engine, making sure its removal is added to your pre-departure checklist along with taking down the TV ariel, attaching your tiller handle (I once moved off without it ), etc?

 

Since for the majority of the time the boat is moored up, this is when most of the galvanic action/electrolysis is taking place, not during the few hours each month that you are moving the boat.

 

Just an idea?

Edited July 4, 2006 by blackrose

[Alastair]

Just for the sake of not confusing people, can we stop calling it 'earthing' the 12V DC?

 

AFAIK, there is no such thing, just the 12V -ve terminal of the battery normally being connected to the engine body (or body of the starter motor).

 

As for the engine being insulated from the boat, what about the fuel pipes? If they are the 'armoured' type, then surely they can carry current?

[blackrose]

Right while I have Stan and Chris on one thread I will aks another question.

I have not seen ANY boat with an earth running from the engine mount to the engine bearers, thus earthing the 12vdc potential to the hull.

 

 

My 12v engine earth cable is bonded to my hull (I thought this was normal practice?)

Edited July 4, 2006 by blackrose

[ChrisPy]

My 12v engine earth cable is bonded to my hull (I thought this was normal practice?)

It should be normal practice, according to all the references I have seen.

 

We should remember that most marine engines are installed in plastic bath tubs, so they are not normally bonded to anything.

 

 

 

Just for the sake of not confusing people, can we stop calling it 'earthing' the 12V DC?

 

AFAIK, there is no such thing, just the 12V -ve terminal of the battery normally being connected to the engine body (or body of the starter motor).

 

As for the engine being insulated from the boat, what about the fuel pipes? If they are the 'armoured' type, then surely they can carry current?

the 12V negative should be normalised to zero potential by connecting it to the steel hull. 'Earthing, grounding, bonding to a zero potential', what's in a name?

 

12V negative must be at the same potential as the AC earth, by connecting both to adjacent earth studs.

[Alastair]

It's not really the same tho', is it? The AC earth should never actually be carrying current, unless there is a fault. Whereas it is normal for there to be current flowing through the 12V terminal. It used to be quite common for some 12V circuits to use the boat as part of the circuit (yes I know it is very very bad practise).

[ChrisPy]

It used to be quite common for some 12V circuits to use the boat as part of the circuit (yes I know it is very very bad practise).

yes it is.

but please don't tell anyone my thruster negative return conductor has a cross-sectional area of about 50,000sq.mm. which is not multistrand copper.

of course it will only be used for about 0.01% of the time.