Talpidae: problems,

[smileypete]

Surveyor has given me a few good pointers and a few things to check. Problems are most likely external but there are things onboard I can improve upon and which may have exacerbated.

 

Hi Moley,

 

What pointers/things to check were they?

 

cheers,

Pete.

[Moley]

If I've calculated right, assuming 8 hours daylight per day, your 1.5W solar panel is producing enough current to rust about one gram of steel per day, all by itself.

 

(1.5W at 12V is .125A, .125A for 8 hours is 0.125x60x60x8 = 3600 coulombs per day. Apply Faraday's law, with atomic weight of iron = 56 and valency = 2 (should that be 3?) gives (3600/96485)*(56/2) ~= 1g)

I suppose I asked for that but if switches are open then where the chuffin' heck can 3600 coulombs go, other than into the battery, and why am I now thinking of cheese and biscuits?

 

 

 

Hi Moley, What pointers/things to check were they?

Sorry Pete, just a few things I needed to check about my own lectrix before moving elsewhere.

 

Surveyor is of the opinion that engine systems should be totally isolated from domestic systems and doesn't think there should be any electrical contact with steelwork. That includes earth straps but I fail to see how you can avoid earthing an engine if you've got a steel exhaust system, morse and engine-stop cables.

 

I have checked and double-checked that there is no cross-over between my systems, all engine functions are fed from starter battery +ve, all domestic functions from domestic bank via +ve bus bar. However, horn and nav-light returns go to -ve bus bar and to domestic bank -ve, but all -ve terminals are bridged together.

 

Surveyor suggested double pole isolation (i.e. isolate the negatives as well), any benefit to this?

 

Now then, I could use some help interpreting this:

 

I've been down to our moorings this evening with my multi-meter. Although it's not that easy to find exposed metal on other boats (and their owners probably wouldn't appreciate me scraping paint), from a cratch stud on Tetty to the metal framing of our pontoons I measured a fairly steady 0.15volts difference.

 

From a brass "Diesel" plate on Bronwyn's stern to pontoons my meter was fluctuating wildly from 0.2 to 0.6, 0.3 to 0.9 volts. A side hatch screw gave similar results.

 

NB Just Heaven is now back and connected to shoreline and again I got a reading fluctuating around 0.6v from a front fender securing ring.

 

Then I tried dangling my black probe in the water and red probe to pontoon. I still got 0.6 to 0.9 volts, so it would seem that there's something in the water, Tetty has some protection but Bronwyn and Just Heaven might as well not be there ?

 

My brain hurts - help!

[alan_fincher]

Surveyor is of the opinion that engine systems should be totally isolated from domestic systems and doesn't think there should be any electrical contact with steelwork. That includes earth straps........................

Totally unrealistic on any engine not having the negative of both it's starter motor and it's alternator electrically isolated from their cases.

 

Which just about any automotive derived engine will not have.

 

I am completely sold by the explanations from Gibbo (and others) that you must deliberately earth the 12 volts negative to the hull if your engine is like ours.

 

The consequences of it not being, but there being an inadvertent leakage of positive to the hull, are too serious to ignore.

 

I have recently added that missing connection on Chalice, believe your surveyor is wrong on this point, and certainly have no intention of removing it.

 

I'll be interested what's said about voltages you have measured around the moorings. I've no idea what I'd expect if I tried it at ours, or whether such results are even valid. I'm mindful if you connect yourself across a DC voltmeter of sufficient impedance you sometimes record a reading.

 

EDITED: for spelling, and because I said "neutral" when I meant "negative". (Before anybody tells me off, I do know the difference).

Edited February 24, 2009 by alan_fincher

[Guest]

I suppose I asked for that but if switches are open then where the chuffin' heck can 3600 coulombs go, other than into the battery, and why am I now thinking of cheese and biscuits?

 

 

 

 

Sorry Pete, just a few things I needed to check about my own lectrix before moving elsewhere.

 

Surveyor is of the opinion that engine systems should be totally isolated from domestic systems and doesn't think there should be any electrical contact with steelwork. That includes earth straps but I fail to see how you can avoid earthing an engine if you've got a steel exhaust system, morse and engine-stop cables.

 

I have checked and double-checked that there is no cross-over between my systems, all engine functions are fed from starter battery +ve, all domestic functions from domestic bank via +ve bus bar. However, horn and nav-light returns go to -ve bus bar and to domestic bank -ve, but all -ve terminals are bridged together.

 

Surveyor suggested double pole isolation (i.e. isolate the negatives as well), any benefit to this?

 

Now then, I could use some help interpreting this:

 

I've been down to our moorings this evening with my multi-meter. Although it's not that easy to find exposed metal on other boats (and their owners probably wouldn't appreciate me scraping paint), from a cratch stud on Tetty to the metal framing of our pontoons I measured a fairly steady 0.15volts difference.

 

From a brass "Diesel" plate on Bronwyn's stern to pontoons my meter was fluctuating wildly from 0.2 to 0.6, 0.3 to 0.9 volts. A side hatch screw gave similar results.

 

NB Just Heaven is now back and connected to shoreline and again I got a reading fluctuating around 0.6v from a front fender securing ring.

 

Then I tried dangling my black probe in the water and red probe to pontoon. I still got 0.6 to 0.9 volts, so it would seem that there's something in the water, Tetty has some protection but Bronwyn and Just Heaven might as well not be there ?

 

My brain hurts - help!

Definitely one for Gibbo. I am puzzled about earthing the battery negative to hull though; I have always believed this should be done.

[Ray]

I've got my tin hat on already but . . . .

 

Is there any chance that the return from the bilge pump, being taken back to the earth stud rather than to the battery, is setting up a parallel return route from that stud, through the boat via the engine or any other part of the boat and back to the battery that way?

 

I assume such an option would be avoided if the bilge pump went direct back to the battery.

[Moley]

I've got my tin hat on already but . . . .

 

Is there any chance that the return from the bilge pump, being taken back to the earth stud rather than to the battery, is setting up a parallel return route from that stud, through the boat via the engine or any other part of the boat and back to the battery that way?

 

I assume such an option would be avoided if the bilge pump went direct back to the battery.

I'm buggered if I know but the bilge pump will be re-wired to go back to the battery, or I might even put the starter battery that I've just replaced back in as a bilge pump battery and isolate everything else completely.

 

This is probably academic as the bilge pump can't be in service for more than 5 minutes a month.

 

And I've got it! Coulombs is very similar to brie

[Ray]

Wait for ChrisW or Gibbo to comment. I'm not as knowledgable as them and may be barking up the wrong tree. I was simply asking the question.

[MoominPapa]

I suppose I asked for that but if switches are open then where the chuffin' heck can 3600 coulombs go, other than into the battery, and why am I now thinking of cheese and biscuits?

It will be going into the battery, but it's an indication of the ratio of damage done to electrickery expended. If the solar panel can oxidise 1g of steel, the batteries can do quite a bit more without becoming completely discharged.

 

Here's a possibility: you said that there's no direct connection to the hull from battery negative, but there is a connection from battery negative to the engine. For the sake of argument, lets assume that there is not a good connection between the engine and the hull via exhaust, etc. Now, your engine is connected to battery negative, and therefore so is your prop-shaft and propellor. The float switch is connected (all the time) to battery positive, and is likely lying in a pool of mucky water in the bottom of the hull. There's a reasonable chance that the connections are not completely waterproof, or the switch isn't, so that mucky (and therefore conductive) water will be in contact with the wire from battery positive. Not a low resistance connection, but some resistance, enough to let some current flow. The mucky water is also in contact with the hull.

 

So now you have your prop in the canal connected to battery negative, and your hull in the canal connected via the float switch to battery positive. There's a circuit battery positive -> float switch -> mucky water in bilge -> hull -> canal water -> prop -> shaft -> gearbox -> engine -> battery negatve. This will corrode the prop and hull as the current flows between them and the canal water,

 

Now, if your engine is connected to the hull, then that circuit gets simplfied: battery positive -> float switch -> mucky water -> hull -> engine ->battery negative. That's better, it won't be corroding the outside of the hull and the prop anymore. So you are relying on the exhaust and control cables to stop current leaking from the float switch going through the prop and external hull surfaces. Are you _sure_ they are low resistance? They aren't designed to be. Exhaust joints are full of rust and asbestos. control cables are greased.

 

If there was a nice fat wire connected to the battery negative and the hull, one that you could be sure was low resistance, then that would be sure to divert leakage from the float switch away from the prop and external hull: battery positive -> float switch -> mucky water -> hull -> big fat wire -> battery negative.

 

That's why bonding the hull is a good idea: If you can be sure that there's no leakage paths anywhere, it doesn't matter and it's even better to have everything isolated from the hull, In the real world, bonding the hull makes the consequences of inevitable leakage much less: you know where the current is going, and you know that everything conductive hanging out in the canal is at the same voltage, and therefore is not passing current through the canal and corroding itself.

 

MP.

 

Edited to say: and the really nasty thing about the leakage circuit I postulated is that it's there all the time, even when the battery isolator is off. The battery is being topped up by the solar cell all day, and the current is leaking via the float switch 24/7.

Edited February 24, 2009 by MoominPapa

[alan_fincher]

Except that if he has a typical set-up, there will be a standard type of flexible coupling in the prop-shaft between gearbox and stern gear.

 

These are usually based on a nylon type inner, with no bolts passing between the two couplings on either side.

 

So usually electrically insulating, I'd have thought ?

[MoominPapa]

Except that if he has a typical set-up, there will be a standard type of flexible coupling in the prop-shaft between gearbox and stern gear.

 

These are usually based on a nylon type inner, with no bolts passing between the two couplings on either side.

 

So usually electrically insulating, I'd have thought ?

Yes, that's one of a dozen reasons why the circuit a described might not exist, but just as exhausts aren't designed to be conductors, so prop-shaft couplings aren't designed to be insulators.

 

It would be good to try putting the Moley-multimeter between the prop-shaft and hull on a voltage range, and also temporarily in series with a battery terminal on a lowish current range (with the solar panel disconnected). Both of those tests would reveal the problem.

 

MP.

 

Another thought: if there's an insulating flexible coupling, theres another possibility which doesn't require the hull to isolated from the engine: there may be a leakage path from the float switch, though the bilge-muck to the prop shaft. That make the prop shaft positive and the hull negative. battery postive -> float switch -> bilge water -> prop shaft -> hull -> engine -> battery negative. This seems less likely since the leakage path would be through dampnes on the outside if the stern-tube and stuffing box. I suppose it might be possible.

 

MP.

Edited February 24, 2009 by MoominPapa

[alan_fincher]

Yes, that's one of a dozen reasons why the circuit a described might not exist, but just as exhausts aren't designed to be conductors, so prop-shaft couplings aren't designed to be insulators.

 

It would be good to try putting the Moley-multimeter between the prop-shaft and hull on a voltage range, and also temporarily in series with a battery terminal on a lowish current range (with the solar panel disconnected). Both of those tests would reveal the problem.

 

MP.

 

Another thought: if there's an insulating flexible coupling, theres another possibility which doesn't require the hull to isolated from the engine: there may be a leakage path from the float switch, though the bilge-muck to the prop shaft. That make the prop shaft positive and the hull negative. battery postive -> float switch -> bilge water -> prop shaft -> hull -> engine -> battery negative. This seems less likely since the leakage path would be through dampnes on the outside if the stern-tube and stuffing box. I suppose it might be possible.

 

MP.

Here's a picture that sort of shows both the flexible coupling and the exhaust that was refitted.

 

Not that I can look at either and estimate their electrical resistances, mind!

 

[Guest]

Yes, that's one of a dozen reasons why the circuit a described might not exist, but just as exhausts aren't designed to be conductors, so prop-shaft couplings aren't designed to be insulators.

 

It would be good to try putting the Moley-multimeter between the prop-shaft and hull on a voltage range, and also temporarily in series with a battery terminal on a lowish current range (with the solar panel disconnected). Both of those tests would reveal the problem.

 

MP.

Iwas just considering that my R&D coupling was an insulator but I belive there is a fail-safe strap in it that joins the bolts between shaft and gearbox, thus making it a conductor. Think I will be playing "find the multimeter" at weekend.

[Moley]

Thanks Moomin.

 

The float switch is fairly recent, the electrical connections are sound, insulated and well clear of the mucky water, and I do try to keep the mucky water well below the level ever required to trip the float switch, but I did actually understand most of that and will make sure a decent earth strap is fitted immediately.

 

Also, I just 'previewed post' and saw that Alan has found a photo

 

That old red float switch has since been replaced, the wiring is more tidy, the exhaust lagged and the morse cables not in contact, but even so, thanks Alan

[alan_fincher]

Iwas just considering that my R&D coupling was an insulator but I belive there is a fail-safe strap in it that joins the bolts between shaft and gearbox, thus making it a conductor.

Is there, by Jove!

 

I would never have expected that.

 

The problem with multimeter type investgations is that there are many ways other than (electrical) cables that engine and gearbox may be electrically joined to the shell. (Exhaust, 2 control cables, stop cable, shaft and couplings, fuel lines, fuel return lines, etc, etc)

 

Whilst it's possible to easily break some, others like exhaust are clearly harder to disconnect without damage. I gave up attempts at tracing electrical paths when I was unable to loosen a single nut on my R&D style coupling without taking my knuckles off.

 

I'd say establish the definite bond (that the surveyor says should not be there ), then you don't need to know what other connection paths exist.

[Guest]

Just had a quick look on R&D site, and it looks like the straps may not join the box and gearbox electrically after all. I must have been having a senior moment

[Moley]

If I may back-track, I have tonight measured a voltage difference between the canal water and the metal of the pontoons. It may only average around 0.75 volts but there is a difference.

 

I do moor with a chain at the stern. This goes to an alloy mooring cleat, bolted through wood, but does cross an aluminium insert, so there is a definite possibility of metal-to-metal contact.

 

Could this be a factor?

[Guest]

If I may back-track, I have tonight measured a voltage difference between the canal water and the metal of the pontoons. It may only average around 0.75 volts but there is a difference.

 

I do moor with a chain at the stern. This goes to an alloy mooring cleat, bolted through wood, but does cross an aluminium insert, so there is a definite possibility of metal-to-metal contact.

 

Could this be a factor?

Another one for Gibbo, but I was never happy chaining to the pilings on our old mooring. Wonder if you could insulate the chain from the boat (some lateral thinking person will know a way) and use tyre fenders?

[smileypete]

My brain hurts - help!

Going back a little while, were the silver pits on the hull covered with rust or blacking when you slipped the boat? Were they only obvious after the loose blacking and rust had been pressure washed off?

 

 

I do moor with a chain at the stern. This goes to an alloy mooring cleat, bolted through wood, but does cross an aluminium insert, so there is a definite possibility of metal-to-metal contact.

 

Could this be a factor?

If the hull is electrically connected to the pilings etc I'd expect the anodes to waste away quite quickly.

 

This is because the anode tries to protect the pilings too IYSWIM.

 

cheers,

Pete.

Edited February 24, 2009 by smileypete

[Moley]

Going back a little while, were the silver pits on the hull covered with rust or blacking when you slipped the boat? Were they only obvious after the loose blacking and rust had been pressure washed off?

Rust, and yes.

 

If the hull is electrically connected to the pilings etc I'd expect the anodes to waste away quite quickly.

This is because the anode tries to protect the pilings too IYSWIM.

I'll try again - I think this one shows quite a bit of degradation:

 

[chris w]

If I may back-track, I have tonight measured a voltage difference between the canal water and the metal of the pontoons. It may only average around 0.75 volts but there is a difference.

 

I do moor with a chain at the stern. This goes to an alloy mooring cleat, bolted through wood, but does cross an aluminium insert, so there is a definite possibility of metal-to-metal contact.

 

Could this be a factor?

Yes indeed. If you are connected to the pontoon via a steel chain, then you are also connected to all the other boats that are similarly connected to the pontoon AND you are also connected to all those boats that are connected to shore mains, via their shore mains earth lead, because the pontoon is also at earth potential.

 

So your boat and a few other boats are all connected together in an electrolyte (the canal) and thus form many battery pairs which will definitely cause galvanic corrosion. The theoretical maximum voltage that you should detect for galvanic currents will be around 0.8v maximum which appears to be exactly what you are seeing. (This is why galvanic isolators consist of a bidirectional pair of series connected diodes. Two diodes in series will block up to 1.2v so all galvanic currents will therefore be blocked).

 

My suggestion would be to moor with a rope in future and also insert a galvanic isolator into your shore mains lead. (I'm too lazy to read the whole thread again, so apologies if you already have a galvanic isolator or have no shore mains lead).

 

I think you've solved it.

 

Chris

Edited February 25, 2009 by chris w

[Gibbo]

Sorry Pete, just a few things I needed to check about my own lectrix before moving elsewhere.

 

Surveyor is of the opinion that engine systems should be totally isolated from domestic systems and doesn't think there should be any electrical contact with steelwork. That includes earth straps but I fail to see how you can avoid earthing an engine if you've got a steel exhaust system, morse and engine-stop cables.

 

If one can be absolutely certain of the entire electrical system and that there is no possibility whatsoever of an inadvertent connection between +ve and the hull then there is no need to have the negative side of the DC system bonded to the hull. However in most boats no one can ever be certain of this.

 

If AC is onboard then the DC system must be bonded to the hull. If MUST....... http://www.smartgauge.co.uk/earthing.html

 

I have checked and double-checked that there is no cross-over between my systems, all engine functions are fed from starter battery +ve, all domestic functions from domestic bank via +ve bus bar. However, horn and nav-light returns go to -ve bus bar and to domestic bank -ve, but all -ve terminals are bridged together.

 

So there is crossover between the systems! The negatives are connected together.

 

Surveyor suggested double pole isolation (i.e. isolate the negatives as well), any benefit to this?

 

No. But there are some downsides to it.

 

Now then, I could use some help interpreting this:

 

......................

 

From a brass "Diesel" plate on Bronwyn's stern to pontoons my meter was fluctuating wildly from 0.2 to 0.6, 0.3 to 0.9 volts. A side hatch screw gave similar results.

 

NB Just Heaven is now back and connected to shoreline and again I got a reading fluctuating around 0.6v from a front fender securing ring.

 

Then I tried dangling my black probe in the water and red probe to pontoon. I still got 0.6 to 0.9 volts, so it would seem that there's something in the water, Tetty has some protection but Bronwyn and Just Heaven might as well not be there ?

 

Those sorts of voltages are A. To be expected and B. perfectly ok.

 

That discounts any idea of someone's boat forcing stray currents through your hull.

 

Gibbo

 

I do moor with a chain at the stern. This goes to an alloy mooring cleat, bolted through wood, but does cross an aluminium insert, so there is a definite possibility of metal-to-metal contact.

 

Could this be a factor?

 

Yes. Stop it!

 

Get some thick string.

 

Gibbo

[smileypete]

Another one for Gibbo, but I was never happy chaining to the pilings on our old mooring. Wonder if you could insulate the chain from the boat (some lateral thinking person will know a way) and use tyre fenders?

Hi Catweasel,

 

In this post (clicky), were the pits covered over with rust or not? Had the hull been pressure washed before you looked at it?

 

 

Rust, and yes.

Then I think it's probably just extremely localised galvanic pitting occurring under the rust, likely due to blacking poorly applied and/or applied to a poor surface.

 

If your hull was acting as an anode to the pilings or another hull, I'd expect the Fe ions from the pits to wander off towards the cathode somewhere - and not form an oxide crust over the pits.

 

cheers,

Pete.

[chris w]

sorry, posted in wrong place , I thought this was the "Whips International" forum

 

Chris

Edited February 25, 2009 by chris w

[carlt]

sorry, posted in wrong place , I thought this was the "Whips International" forum

 

Chris

You subscribe too??

 

Edited to say:Sorry

Edited February 25, 2009 by carlt

[chris w]

You subscribe too??

 

Coincidentally I had noticed the Managing Editor was named Carl.......

Edited February 25, 2009 by chris w