Talpidae: problems,

[Moley]

Edited to say:Sorry

That's quite ok, I think we're about done here

[David Schweizer]

I have been following this thread with some interst, as I have similar localized pitting on the hull of my boat (also similar age to Talpidae)

 

Unlike Adrian we do not have a 230v shoreline, and there is no 230v on our moorings, the bank side is concrete not steel and we all moor woth ropes not chains. Hovever, Gibbo has made a comment which is still causing me some concern, I have trawled through his web site but am still unsure of the answer to my dilema.

 

My boat is negative grounded and the negative is soundly connected to the hull via one of the starter motor fixing bolts which is immediatly adjacent to the engine mounting which is hard mounted to the steel bearers whuich are welded to the baseplate (all these connections are unpainted direct metal to metal contacts.

 

My concern is that battery isolation is via a single heavy duty isolation switch on the joint megative side, which Gibbo is (I think) suggesting can cause problems. Is the problem that isolating the negative when the boat is left will cause the hull to be the return path for any stray electricity, or something else. I do not have any electrical equopment which is permanently connected (such as bilge pumps) when the boat is left, and always switch all electrical equipment off before isolating the negative. I thought that should be ok, but am no longer that sure.

 

Following some advice given on an earlier thread, I am considering changing to positive side isolation, but was intending to retain the negative isolation, because it is there, and rewiring will be a pain. should I :-

 

a. Leave everthing as it is,

b. Fit two positive side isolators and retain the negative isolator ,

c. Fit two positive side isolators and remove the negative isolator.

 

Edited to add:- I am fairly certain that the joint negative cable runs to the isolation switch and then splits with one cable connecting with the negative contact block, and a second (very heavy) cable connecting with the engine/hull, would this be correct?

Edited February 25, 2009 by David Schweizer

[chris w]

Negative side isolation can cause big problems and is not recommended. Any moisture or condensation can cause a bridge which will mean your hull is positively bonded when the negative isolation switch is OFF. Positively bonded hulls corrode fast due to the physics of electrolysis and is the reason why cars are almost universally negatively bonded these days.

[Gibbo]

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

 

My concern is that battery isolation is via a single heavy duty isolation switch on the joint megative side, which Gibbo is (I think) suggesting can cause problems. Is the problem that isolating the negative when the boat is left will cause the hull to be the return path for any stray electricity, or something else. I do not have any electrical equopment which is permanently connected (such as bilge pumps) when the boat is left, and always switch all electrical equipment off before isolating the negative. I thought that should be ok, but am no longer that sure.

 

Yes.

 

Boats are damp. I don't give a sh*t about people saying their boat is dry as a bone. I've repaired enough p*ss*d wet through and corroded electronic equipment that had been removed from "bone dry" boats to know that they are damp.

 

If it's damp then there is going to be some stray leakage. That's just a fact of life.

 

If the negative is grounded then the only stray currents can be from wiring to the hull. This will erode the wire. Not a major problem.

 

If the positive is grounded, or if the system is isolated then stray current can leak from wiring to the hull (the wiring erodes) and also from the hull to the wiring. The hull erodes.

 

Following some advice given on an earlier thread, I am considering changing to positive side isolation, but was intending to retain the negative isolation, because it is there, and rewiring will be a pain. should I :-

 

a. Leave everthing as it is,

b. Fit two positive side isolators and retain the negative isolator ,

c. Fit two positive side isolators and remove the negative isolator.

 

Fit two +ve side isolators. keep the -ve ones there but make sure they never get switched off. Alternatively just move the connections onto the sdame bolt at the back.

 

http://www.smartgauge.co.uk/whereiso.html

 

Gibbo

[David Schweizer]

Yes.

 

Boats are damp. I don't give a sh*t about people saying their boat is dry as a bone. I've repaired enough p*ss*d wet through and corroded electronic equipment that had been removed from "bone dry" boats to know that they are damp.

 

If it's damp then there is going to be some stray leakage. That's just a fact of life.

 

If the negative is grounded then the only stray currents can be from wiring to the hull. This will erode the wire. Not a major problem.

 

If the positive is grounded, or if the system is isolated then stray current can leak from wiring to the hull (the wiring erodes) and also from the hull to the wiring. The hull erodes.

 

 

 

Fit two +ve side isolators. keep the -ve ones there but make sure they never get switched off. Alternatively just move the connections onto the sdame bolt at the back.

 

http://www.smartgauge.co.uk/whereiso.html

 

Gibbo

Thanks for that, I had more or less concluded that would be the correct proceedure, but thought I would clarify it first.

 

Fortunately my boat is probably amongst the less damp ones on the system, all the bilges are dry, the boat is kept well ventilated, and there is never any condesation on anything (including the windows) but as you say, I know that there must be some moisture in the air, because all the timber sections swell slightly when the boat is left unattended over the winter. Fortunately this dampness does not seem to be causing any deterioration to the grounding wire, (yet) they were all taken off last year when the engine was re-built and seemed to be in excellent condition, nevertheless I shall be adding the instalation of two isolating switches to this years (growing) miantenance schedule.

 

Thanks for your continuing advice

 

P.S. Was that you playing the banjo on your music thread? very impressed, what Barney McKenna used to call "Cowboy Music", how about a slip jig or a reel.

Edited February 25, 2009 by David Schweizer

[Gibbo]

P.S. Was that you playing the banjo on your music thread? very impressed, what Barney McKenna used to call "Cowboy Music", how about a slip jig or a reel.

 

Yes it was me. Mrs Gibbo calls it "Inbreed music"

 

Gibbo

[Trev The Rev]

Although Metal chains used to secure a boat, will effectively short out its Galvanic Isolator (if fitted).

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?

 

The Alloy cleats are bolted through the metal sub frame of the moorings (I checked that one out a while ago) - wood on top but nut to metal on the underside!

The electrical bollards stand on the mooring and will be earth bonded to the metal sub frame of the mooring.

 

If there is a connection through the chain it would be the same as an earth bonded electrial shore line without a galvanic isolator. Easy to remedy, don't use chain, if you wish to use chain then use chain with a thick plastic coating, plus something to prevent your boat touching any steelwork on the mooring etc. (other boats as well to be safe). The moorings are wooden edged and, from memory, do not have any metal fixing on the their faces, so are quite good to start with. Plastic / rubber fender are also a good idea.

 

Basic principle: canal + shore + boat = a simpe battery (Around 1.0 Volt ish). Your boat being one of the batteries terminals, the shore the other terminal (the cut = the chemicals inside the battery). If you link boat and shore (both terminals) you get current flowing, Boat > Cut > shore > Boat = erosion.

 

Good to hear your hull problem is not as severe as you first thought, what a relief, certainly got me thinking though.

 

Trev

Edited February 25, 2009 by Trev The Rev

[Guest]

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?

 

 

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.

It had just been pressure washed for the survey, Pete. You must appreciate that it was a few years ago, but I remember the pits as being quite clean and free of rust, as I said it looked like somebody had gone berzerk with a sharp drill bit.

[chris w]

Basic principle: canal + shore + boat = a simpe battery (Around 1.0 Volt ish). Your boat being one of the batteries terminals, the shore the other terminal (the cut = the chemicals inside the battery). If you link boat and shore (both terminals) you get current flowing, Boat > Cut > shore > Boat = erosion.

 

Trev

Actually it's canal + shore + boat + another boat = a battery. If another boat is connected to the shore, either through its mains lead earth or through a chain, then the two of you form a battery with a maximum voltage of 0.8v. which will cause galvanic corrosion.

 

Chris

[Gibbo]

Actually it's canal + shore + boat + another boat = a battery. If another boat is connected to the shore, either through its mains lead earth or through a chain, then the two of you form a battery with a maximum voltage of 0.8v. which will cause galvanic corrosion.

 

Chris

 

Eh?

 

You don't need two boats. Just one boat (the cathode), the world (the anode) and the water (the electrolyte).

 

Look up "anode" and "cathode" before you argue that they are the wrong way round

 

Gibbo

[Neil TNC]

Yes.

 

Boats are damp. I don't give a sh*t about people saying their boat is dry as a bone. I've repaired enough p*ss*d wet through and corroded electronic equipment that had been removed from "bone dry" boats to know that they are damp.

 

If it's damp then there is going to be some stray leakage. That's just a fact of life.

 

If the negative is grounded then the only stray currents can be from wiring to the hull. This will erode the wire. Not a major problem.

 

If the positive is grounded, or if the system is isolated then stray current can leak from wiring to the hull (the wiring erodes) and also from the hull to the wiring. The hull erodes.

 

 

 

Fit two +ve side isolators. keep the -ve ones there but make sure they never get switched off. Alternatively just move the connections onto the sdame bolt at the back.

 

http://www.smartgauge.co.uk/whereiso.html

 

Gibbo

 

Earnest wasn't damp..............until I went to Ireland!

[FORTUNATA]

This sort of thing happens. Suddenly you get hit by loads of problems all at once.

Galvanic isolation concerns isolation of the DC negative and AC ground so DC currents don't pass to the AC ground. The problem of galvanic isolation can be a complicated issue which is probably why people tend to argue different pounts of view.

Normally AC and DC electrics should function totally apart and the AC should most definitely be grounded as should the shore supply.

Replating: Yes, this is expensive. I would have it done at a reputable boat builders if you decide you're going to push ahead and grit your teeth. You would probably find a buyer regardless of any pitting but, as you know, a survey would detract from the sale price. If you're happy with your boat and can afford it replating once done would add many years of extra life.

 

NB Talpidae went into dry dock yesterday morning for re-blacking and BSS. She has failed BSS on fuel lines not carrying the required numbers, there's a minor gas leak somewhere which we thought we had found and I had nipped up but on the manometer test when the level is supposed to remain constant for 5 minutes it dropped, installation condemned. Dry-dock man is able to rectify both. There were also a few minor niggles which I am able to rectify.

 

However, once the dock was drained and pressure washing commenced, my problems took a quantum leap.

 

I must confess that I have never really bothered with the threads on galvanic isolation and earth bonding because, although they frequently turn into quite amusing arguments (viewed from the outside), that rapidly detracts from any benefit those threads might be. Also, as I have a very minimal 240v installation, I didn't think it applied to me. Well it seems to be having a bloody good go at applying to me!

 

Let me describe my mains installation: Cable from shoreline bollard to plug on boat, from mains inlet to 2-way distribution point, 2 trip switches, 6-amp line to single socket outlet, 10-amp line to 2-gang socket outlet. System is entirely self-contained, nothing is earthed to the steelwork, I may as well bring in an extension lead from the bollard. Besides this, it is almost never connected to shoreline, there's no credit on my meter and no card which BW sell anywhere on the system works in our bollards, a couple of neighbouring boat owners have complained and had the meters taken out of their bollards and are on a constant supply. Very occasionally I might disconnect one of those and plug in for a short while to run a vacuum cleaner around and earlier this week I pinched a connection overnight to plug in a battery charger. That's it, we have no other mains appliances.

 

However, boats on either side of ours (one 3 feet away across a pontoon, the other a couple of fenders away) are permanently connected to their supplies. One of those is a forum member so please be tactful in any replies.

 

On pressure washing there is some long-term pitting and corrosion which would be expected with any boat 25+ years old. However, there is also quite a bit of shiny pitting due to electrolysis, some of it rather deep, and the propeller looks silver, not bronze.

 

If a picture's worth a thousand words, this lot should save me pages:

http://i20.photobucket.com/albums/b204/Mol...ck/09022001.jpg

http://i20.photobucket.com/albums/b204/Mol...ck/09022002.jpg

http://i20.photobucket.com/albums/b204/Mol...ck/09022005.jpg

http://i20.photobucket.com/albums/b204/Mol...ck/09022101.jpg

http://i20.photobucket.com/albums/b204/Mol...ck/09022102.jpg

http://i20.photobucket.com/albums/b204/Mol...ck/09022103.jpg

http://i20.photobucket.com/albums/b204/Mol...ck/09022104.jpg

http://i20.photobucket.com/albums/b204/Mol...ck/09022105.jpg

http://i20.photobucket.com/albums/b204/Mol...ck/09022106.jpg

http://i20.photobucket.com/albums/b204/Mol...ck/09022107.jpg

http://i20.photobucket.com/albums/b204/Mol...ck/09022108.jpg

http://i20.photobucket.com/albums/b204/Mol...ck/09022109.jpg

http://i20.photobucket.com/albums/b204/Mol...ck/09022110.jpg

 

My options would appear to be:

 

Have dry-dock man blow some weld into the deeper pits, black over and try to get rid of ASAP (and caveat emptor).

 

New baseplate and half way up the sides.

 

If we re-plate there is still a question mark over whether or not or for how long we keep the boat but I am certain that, in the present climate, any prospective buyer is going to shy away from any older boat which could potentially require plating.

 

If we re-plate, who (within sensible proximity) could I ask to do it at a reasonable price, what might that price be, and what's to stop it happening again?

 

Alternately, I might be open to offers, as seen.

 

 

Differing metals and fittings in proximity sure do cause galvanic cells. If you wanna be really clever there's something I heard of called impressed current cathodic protection. You will have heard of this and it's often used on boats that are long periods moored up at marinas. The device gives off a protective counter current and converts anodes into cathodes. The downside is they cost power to run but are said to be effective.

 

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

[chris w]

Galvanic isolation concerns isolation of the DC negative and AC ground so DC currents don't pass to the AC ground.

Not quite correct.

 

Galvanic isolation is the prevention of DC galvanic currents from completing a circuit through your AC ground lead. ie: nothing to do with the normal DC battery negative.

 

A DC circuit will otherwise be formed from your boat's hull, then through your AC earth lead to shore, then down another boat's AC shore earth lead to his/her hull, then through the water back to your hull. You have effectively created a battery.

 

The galvanic isolator puts a DC break in your earth shore lead but still allows AC earth currents to flow in the case of an AC fault, thus ensuring that earth safety trips do indeed still trip.

 

Chris

[FORTUNATA]

The whole issue can get complicated. Maybe I'm confusing with isolation transformers. These latter galvanically isolate the vessel when plugged into shore power.

The issues I've encountered that cause concern are:

(1) AC and DC electrical systems on a vessel. Some vessels have both of these. I'm told the best system is to isolate AC from DC and make sure the AC is always properly grounded.

(2) Galvanic corrosion: Interaction of differing alloys, metals and fittings in an electrolyte.

(3) Electrolytic corrosion: This seems more complex and can cover stuff such as stray current leaks to the hull via damp, dirt, bad connections. Or indeed, current faults from a marina going back to the vessel.

There is a test I found using a basic multimeter where you just connect between, say, the positive battery terminal to the hull using DC volts. Or vice versa.

 

Not quite correct.

 

Galvanic isolation is the prevention of DC galvanic currents from completing a circuit through your AC ground lead. ie: nothing to do with the normal DC battery negative.

 

A DC circuit will otherwise be formed from your boat's hull, then through your AC earth lead to shore, then down another boat's AC shore earth lead to his/her hull, then through the water back to your hull. You have effectively created a battery.

 

The galvanic isolator puts a DC break in your earth shore lead but still allows AC earth currents to flow in the case of an AC fault, thus ensuring that earth safety trips do indeed still trip.

 

Chris

[Radiomariner]

We had a full side skin up to the waterline done on a 28 y.o. 60 foot boat a few years back for about £8K. This was the last job this yard did before the owner retired so I can't make any recommendations now. This re-plating was forced by the BSS exam where the "examiner" hit the plates with a sledge hammer and he then made his judgment on the basis of the "clang" and the size of dent. Not very scientific IMHO.

.

What on earth was the BSS Examiner doing bashing hell out of your hull? BSS Examiners do not do this. If he was also a "surveyor" perhaps you should say so. BSS examiners have a bad reputation as it is!!!

 

Glad you are all sorted out Moley. I thought that your original pic's looked ok. Hulls nearly always look worse to the owner than they actually are.

 

Regards

[alan_fincher]

What on earth was the BSS Examiner doing bashing hell out of your hull? BSS Examiners do not do this. If he was also a "surveyor" perhaps you should say so. BSS examiners have a bad reputation as it is!!!

Yes, I thought this.

 

Surely you can present a boat for BSS that has serious wastage to the steel, and maybe even a few small leaks. It doesn't even need to have a bilge pump, despite it's condition.

 

If all the BSS items are in order, it should pass with flying colours, even if it sinks the next day.

 

Has the BSS ever considered hull condition ?

[chris w]

The whole issue can get complicated. Maybe I'm confusing with isolation transformers.

Isoaltion transformers do the same job as galvanic isolators, ie: they allow AC through but block DC galvanic currents. An isolation transformer is inherently safer than a galvanic isolator because, if one or more of a galvanic isolator's diodes go "pop" and are left open circuit, you lose your AC earth continuity as well, a potentially dangerous situation.

 

The downside of an isolation transformer is cost and size as compared to a galvanic isolator.

[Rob@BSSOffice]

Yes, I thought this.

 

Surely you can present a boat for BSS that has serious wastage to the steel, and maybe even a few small leaks. It doesn't even need to have a bilge pump, despite it's condition.

 

If all the BSS items are in order, it should pass with flying colours, even if it sinks the next day.

 

Has the BSS ever considered hull condition ?

 

No, not in the way of a formal examination. It has never been set that task by the owners.

 

Examiners have a form for general notes. If there is an obvious safety related problem found by coincidence ie not related to the BSS requirements or checking procedures, examiners may use report form e to make a note of the observation and draw it to the owners attention through that route, but it is not expected for the examiners to intentionally identify anything such as hull condition as a part of the BSS examination.

 

NB: examiners who are also commissioned to undertake a survey may well consider hull condition and report back on that as a part of their survey work/report which is a separate piece of work to any BSS examination undertaken at the same time.

 

HTH

Rob

[NB Alnwick]

Eh?

 

You don't need two boats. Just one boat (the cathode), the world (the anode) and the water (the electrolyte).

 

Look up "anode" and "cathode" before you argue that they are the wrong way round

 

Gibbo

 

So will a galvanic isolator be useful in a boat that never connects to AC shore power? If so where in the circuit should it be installed?

 

And by bonding the DC negative and AC earth to the hull, have I exposed the steelwork to an errosion risk that did not previously exist?

[Gibbo]

So will a galvanic isolator be useful in a boat that never connects to AC shore power? If so where in the circuit should it be installed?

 

No. There's nowhere to connect it and it won't do anything anyway.

 

And by bonding the DC negative and AC earth to the hull, have I exposed the steelwork to an errosion risk that did not previously exist?

 

Only if you don't have a GI or isolation transformer.

 

Gibbo

[FORTUNATA]

This needs to be taken step at a time. I agree the BSS has nothing to do with the condition of the hull so first of all the O.P. simply needs to show he's addressing any issues raised, mainly the gas leak. Fixing a has leak shouldn't be expensive to do.

As for the hull, there is probably time to play with before any water starts to seep in. There's a harsh economic crisis at the moment so maybe a boatyard can be knocked down in price at a later date to do whatever welding is required. It is a very expensive business but once done will add many years life to the hull of the vessel. I'm not clear if the O.P. is living on this boat or is it just recreational?

I know too well a BSS can knock the stuffing out of an owner who suddenly finds a whole load of problems to deal with and mounting expenses. Bear in mind if you contact BW by letter with the proposal you intend to address all issues raised, I'm sure they'll allow plenty of time to sort it out. It could be staggered out over time and the main thing is really the slight gas leak.

 

No, not in the way of a formal examination. It has never been set that task by the owners.

 

Examiners have a form for general notes. If there is an obvious safety related problem found by coincidence ie not related to the BSS requirements or checking procedures, examiners may use report form e to make a note of the observation and draw it to the owners attention through that route, but it is not expected for the examiners to intentionally identify anything such as hull condition as a part of the BSS examination.

 

NB: examiners who are also commissioned to undertake a survey may well consider hull condition and report back on that as a part of their survey work/report which is a separate piece of work to any BSS examination undertaken at the same time.

 

HTH

Rob

[Moley]

This needs to be taken step at a time. I agree the BSS has nothing to do with the condition of the hull so first of all the O.P. simply needs to show he's addressing any issues raised, mainly the gas leak. Fixing a has leak shouldn't be expensive to do.

 

I'm not clear if the O.P. is living on this boat or is it just recreational?

 

I think you might be confusing this thread with the one about what the examiner didn't like.

 

My BSS examiner never went anywhere near my hull with a hammer, the hull surveyor did, and decreed that I didn't need any plating this time around.

 

O.P. is already addressing the issues raised by the BSS, fully compliant fuel lines are already installed and properly secured, the gas leak is being dealt with today by a Corgi registered gas fitter. I am pretty sure I had already dealt with the minor leak near the new cooker which was only installed a couple of weeks ago (old one didn't have flame failure on hob) and the other area for concern I think I may have located is outdoors, above deck. She will be gas-tight before she leaves the dock and re-test is booked for next Tuesday.

 

O.P. has also made it crystal clear from the outset to anyone who has followed this thread that the boat is recreational.

 

 

Actually, I'll retract that last bit, now I'm confusing it with the “what the examiner didn't like” thread, I didn't mention that until half-way through this thread but it wasn't relevant as we were discussing corrosion here, not gas.

Edited February 26, 2009 by Moley

[Keeping Up]

Found it at last!

 

For some time now I've suspected that I had an occasional spurious contact between negative and hull, somewhere that here shouldn't be one. Whenever I've looked for it, there has been no such connection, but I've still had the feeling taht it was there sometimes. Today I found it, and I'm posting this in case anyone else has the same issue.

 

On our foredeck is a metal 3-pin socket like this one (picture of plug and socket screwed together) which the horn and spotlamp plug into.

 

 

The socket sits on a rubber waterproofing washer, which the mounting screws pass through. The plug, which carries the cable to the horn and spotlamp, is held together after assembly by a small screw which passes through the body of the plug and - you've possibly guessed it by now - into the brasswork of the 'earth' pin which carries the negative connection. So when the whole lot is pushed together, the negative wire is earthed through from the body of the plug to the body of the socket and through the mounting bolts to the hull.

 

So when everything is in place there is a spurious hull connection which disappears when the spotlamp is removed for the winter, which was when I would be looking for the fault, and which also disappears if you dismantle the plug to check for problems.

 

The socket had a similar screw connecting the negative pin to the metalwork, but this and the original spotlamp plug had had the screw replaced by a nylon version when the boat was built. Not having noticed this, when I made up a spare horn/spotlamp assembly I had used a metal screw, hence my occasional fault depending which lamp I was using!

 

So if you have a similar plug or socket connection on your boat, it may be worth checking that it isn't accidentally earthing your negative supply and causing a possible source of corrosion.

[Derek R.]

If the negative is grounded then the only stray currents can be from wiring to the hull. This will erode the wire. Not a major problem.

 

If the positive is grounded, or if the system is isolated then stray current can leak from wiring to the hull (the wiring erodes) and also from the hull to the wiring. The hull erodes.

 

Fit two +ve side isolators. keep the -ve ones there but make sure they never get switched off. Alternatively just move the connections onto the sdame bolt at the back.

 

http://www.smartgauge.co.uk/whereiso.html

 

Gibbo

 

Many thanks to Gibbo and Chris W for putting me straight on that one. I've no AC anywhere, but will now check on which side I put the DC isolator. You can be sure It'll be changed pronto if it's on the neg side!

 

Cheers - Derek

Edited February 28, 2009 by Derek R.