Zincs, Cathodes, Anodes and Confusion

[tim3057]

Can I see if any wooden boat owners have any thoughts on the battery to seacock debate in on the PBO forum?

battery to seacock?

I was about to earth every skin fitting to battery negative, and haven't even thought about where a zinc might go - my boat was delivered without one. How about a simple solution! Is there one?

 

[Mirelle]

Yes, definitely.

I have a very definite view, "grounded" (scuse the pun) in the experience of running my own boat for more than twenty years now.

1. Don't fit anodes of any sort.

2. Don't use an earth return system; change to a negative return system by isolating the negative side of the engine electrics from the engine. This is seldom done, but is MOST important.

3. Don't fit an electric bilge pump.

4. Always switch the battery isolating switch off when not on board.

5. Use good quality wiring with waterproof junction boxes, and have it installed by a competent professional.

Result : negligible "galvanic" corrosion even where a bronze fitting abuts an iron one. Which tells me that most "galvanic" corrosion is actually electrolytic stray current corrosion.

 

[Ships_Cat]

Re: Yes, definitely.

You left out the "Amen" at the end Mirelle.

While I am sure that "plastic" is a dirty word on this forum, the same should apply to correctly built plastic boats as well.

I have a steel boat and it has always just had only one anode on it (on one side of the fin keel). I have no evidence that it serves any purpose whatsoever - I have just never gathered up the courage to take it off.

John

 

[Mirelle]

Re: Yes, definitely.

Amen!

 

[tim3057]

Re: Yes, definitely.

'negative engine isolation from the battery negative' aren't there various bits of engine electrics that are bolted to the engine and therefore pick up negative earth? Wouldn't I have to run negative wires to thermosat, warning light sensors etc?

And if there is no current leakage anyway, through careful wiring, then no corrosion as a result? A current leak would try to find it's way back to earth through the engine block.. but if there were no leaks?

Wiring all the skinfittings together - would this not stop stray current trying to join them up via the seawater?

As ever, lots of questions! Keep it simple folks (like me!)

T

 

[Mirelle]

Re: Yes, definitely.

"'negative engine isolation from the battery negative' aren't there various bits of engine electrics that are bolted to the engine and therefore pick up negative earth? Wouldn't I have to run negative wires to thermosat, warning light sensors etc?"

Yes, you would, and you would have to mount the starter and alternator on nylon or Tufnol bushes to isolate them from the engine block. My engine, which is an old fashioned rigid mounted set up, has this done. It's not that hard to do, and I emphasise that it is well worth it.

However, I am told that if you have a modern type flexible coupling between the engine and the shaft this may not be necessary, as the coupling is itself an insulator. I suppose this could be checked easily enough with a multimeter.

If you wire all the skin fittings together you effectively turn them into one big skin fitting and the least galvanically noble of them will become anodic to the rest and disintegrate. The usual idea is to wire them all up to a zinc sacrificial anode.

At this point, matters become more complex. Wiring everything up to an anode MAY be a good idea for GRP hulls. It has been proven to be a BAD idea for wooden hulls.

This is because whilst the metals are nicely protected in such a set up (assuming the anode is renewed as needed) the ions that build up at the cathode are alkalne and as we all know paper is made by treating wood with alkali...i.e. the wood adjacent to the fittings starts to turn into paper pulp, which is Not a Good Thing.

 

[trouville]

Re: Yes, definitely.

Thats quite correct! If you fit an anode always fit a mounting block between the anode and hull.and put teflon around the through hull fixing bolt and teflon washer under the nut.

It always worries me when i get a reading on my meter between my rivits and seacock,engin earth plate etc!!But it depends on the voltage

 

[Kristal]

The willies

Hmm. This thread is getting me more and more concerned. I have a rewire planned for some point in this year, and the current loom is rather confusing as all the wires are black - I haven't yet had a proper look. Yet I know there is a half-corroded anode on the hull, and I don't think the seacocks are wired together - the prop shaft appears to have a plastic coupling, but the two parts are connected together.

I am going to move the rewire forward, using Mr Calder as my guide, and sort it out once and for all before I start losing sleep. I don't need a trip to Burma to source replacement planks any time in the next two decades, if I can help it...

/<

 

[tim3057]

Re: Yes, definitely.

Ok, I got the bit about the degradation of wood around an anode, so mount it on something disposable to keep it off the hull. Good sense.

But surely if you wire everything together then there won't be any need for any one item to act as an anode, another a a cathode; any current will surely take the easiset route - ie, through the wire?

What I'm also finding strange, on my zinc-free boat, is that none of my bronze seacock fittings, or the prop, or the iron keel, show any sign of corrosion - and I know they have been immersed for most of every day in a Chichester Harbour marina for at least a year.

There was just one 'earth' wire, joining a keel bolt to the neg side of a transducer..

( .. actually 'marina' is too grand a term. It was more a sort of floating junk yard. And not everything was floating.. )

 

[Mirelle]

Re: Yes, definitely.

The wire will form one half of a circuit; the other half is through the water.
If you've got a zinc- free boat, with negligible corrosion, it ain't broke, and does not require fixing!

 

[trouville]

Re: Yes, definitely.

Some time ago i read books and books about this and asked anyone that seemed to know somthing abour this.I also had no zink and the family that owned the boat before had no zink

Apart from the keel bolts and engin nothing has changed ,(the old huge 38HP was changed for a Volvo,) and nothing looks furry green or pitted,ive even fitted 2 earth plates for the VHF and SSB ???And no greeness nor increased voltage.

From time to time i go around with the volt meter but nothing more than 0.2v or sometimes 0.4v

I do let her dry out twice a year for a week or sometimes more.

Indeed if its working leave it, I do look at my keel bolts though and wonder??!!

I just rememberd,in one port the marina power was leeking into the harbour, in another a steel boat had his hull positivly charged????In both cases i left!

Again in both cases i only found out becouse i was told!

 

[Ships_Cat]

Re: Yes, definitely.

But surely if you wire everything together then there won't be any need for any one item to act as an anode, another a a cathode; any current will surely take the easiest route - ie, through the wire?

If you wire everything together then you WILL make all those things cathodes and anodes and current will flow through the wire AND also through the sea to complete the circuit.

If they are not wired together, NO current whatsoever will flow through the sea - what you are saying is if one puts a lump of metal in the sea in one place and another somewhere else then current will magically flow between them. Despite the common belief of some marina myth makers, it won't, there is no circuit.

John

 

[tim3057]

Re: Yes, definitely.

Thanks John.