It's the Pits

[Keeping Up]

I guess the only objection to that is that the alternator current flows down that wire, (assuming you don't have a marine alternator with isolated earth) so when your alternator is charging hard, the hull is positive WRT to the engine (and, possibly the prop and stern gear) by the alternator output times the resistance of the engine to battery wire and its connections. A big alternator working hard and that wire less than perfect, and there could be a significant flow of electrons out of the hull and back to the prop and shaft.

 

I guess the ideal would be a big cable from the engine to the hull, and a second big cable from the engine to battery negative. I'm preaching perfection here, this boat has a big cable from the engine to a bolt on the hull, and a second big cable from that bolt to battery negative, which has the same problem.

 

It's difficult to see how you'd measure to find out if this was a real problem: I suppose you'd have to uncouple the prop shaft and insert a multimeter. A more practical option might be to measure the current flowing from the engine to temporary electrode hung over the side when the alternator was working hard.

 

Cheers,

 

MP.

 

ETA I think I may have got my electron flow direction backwards, but I don;t think that affects the main point.

Hmmm that's a good point. Somehow it has always felt "right" to tie the battery negative, and hence all the black wires, to the hull as securely as possible. Maybe I need to rethink that, it probably wouldn't be difficult to swap the cables around. At least they are short and thick (about 18" of 70mm cable) with professionally made terminals so fairly low resistance, maybe when I get the boat back I can try measuring the voltage drop when charging.

 

I have my doubts that there's a big issue there, in particular given that there is little pitting in that area; it seems to be mainly confined to the parallel sides. But of course that might be because there are anodes nearby.

 

Incidentally I have spoken to very helpful technical people at both Safeshore and Duff. They both reckoned that the level of stray currents that I have measured though the hull are insignificant. The guy at Duff said that if such currents leave the hull, they usually do so via the anodes leaving them heavily depleted and if it is a high current then also with a noticeably smooth surface. I shall look carefully at them when I see the boat in dock again (and post some pictures here)

 

Edited to correct the autocorrect

Edited September 29, 2016 by Keeping Up

[magnetman]

This is where we need Chris W and Gibbo

[alan_fincher]

This is where we need Chris W and Gibbo

 

Noooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooo!!!!

[GUMPY]

This is where we need Chris W and Gibbo

Yes please but it will never happen

[sbillis]

A friend of mine had big problems with significant pitting. He claimed it was his radio which returned through the hull. He caught it just in time and had the hull grit blasted epoxy. Learning his lesson I turn off the power to my radio when I am off the boat just in case.

 

Simon

nb Bulrush

[Keeping Up]

I've been delving back through my diaries of work I've done (and my list of eBay purchases) and checking various dates because I had done a few bits of work around the radio within the last 12 months and I am now 100% certain that the radio aerial cable's capacitor issue could not possibly have existed before mid-June of this year.

 

I think this must almost certainly rule out the radio aerial as the source of the problem. Even if 5mA was flowing through the hull from one end to the other for a few hours every day while the radio was switched on, and 0.7 mA for the remaining hours of the day, I find it impossible to believe that it could cause this level of damage in just 3 months, especially as we have not been hooked up to a shore line during that time so only a tiny proportion of that current would have been passing from the hull into the water.

[magnetman]

Its definitely shocking if it has happened that quickly. I'm a bit sceptical and I suspect that the pitting may have been occuring already but not to an extent where anyone would be overly concerned.

 

Is it the insurance company insisting on replating?

 

Not sure if I have of right but j think the boat may be late 90s or early 00s. I may have got that totally wrong !!

If i'm right there will probably be plenty of boats that age which continue on happily with similar problems because the owners only take 3rs party cover.

A boat is a depreciating asset after all.

[Keeping Up]

Two years ago, when the boat was 23 years old (it was launched in December 1991), the steelwork had just some minor pitting along the waterline (nothing more than 1mm) and the steel from there down to the baseplate was perfectly sound. I didn't take many photos of it when it was in dock in 2014 - after all, why take photos of uncorroded steel? These two are not perfectly clear but give an idea; first from the bows including the side, prior to cleaning and re-blacking:

 

 

And then the stern, with new blacking but with the skin-tank area left un-blacked:

 

 

 

Yes the insurance company are insisting on re-plating, because at 2mm the steel behind the pits is too thin; I tend to agree with them, especially if it's corroding away at a rate of 2mm per year. I know that a boat is a depreciating asset but ours had held its value extremely well because it was well built by a highly respected builder (Stoke on Trent Boat Building), and had been meticulously maintained so it was in exceptionally good condition both inside and out. But at the rate of corrosion in the last 2 years it was rapidly turning into a sinking fund!

[magnetman]

Was was a bit surprised that a 20+ year old narrow boat would be 'valued' at £55k which I think was what you said earlier. SOT boatbuilding did build good boats but I don't think they were actually one of the top 5. Maybe they were.

 

Anyway I do hope you can track down the source of the pitting problem otherwise a 6mm overplaying is only good for 5 or 6 years if it carries on

 

Sorry you probably know that. Good luck with it all anyway

[bigcol]

Ho Allan

 

Anything new happening re this subject. Am interested as although I have 2 forms of galvanic isolator

Plugged into mains here non stop for years, I'm wondering what damage is happening below.

I have had the boat out 2 years ago, there was plenty of pitting,mouth nothing major.

 

 

So any news be great?

[Keeping Up]

No news yet. Still waiting to get into the dry dock as the boat that's in there at the moment is taking longer than expected to repair, I'll try to get some photos when I can.

 

Otherwise, there are no further clues; I've checked all my wiring (12v and 240v) yet again, also confirmed that my GI is still working but I've replaced it with one that has LED indicators because it gives better protection. I've tested the mains supply at the mooring as far as I can and everything looks ok, but I've installed an isolation transformer just in case. Of the boats either side of me, one has had a GI fitted while it's been there and the other has an on-board IT, there hasn't been a way of examining their bottoms.

[bigcol]

Thanks for letting us know

I have the galvanic isolator with the 2 leds lights

Also the one that gos inline on the shore line.

 

Makes you think though

 

Very interested in what you find,and how you progress through.

 

Col

[Cheshire cat]

The rate at which the corrosion has suddenly taken place is almost an argument for an insurance payout. It's obviously accidental damage rather than normal wear and tear.

[Keeping Up]

The rate at which the corrosion has suddenly taken place is almost an argument for an insurance payout. It's obviously accidental damage rather than normal wear and tear.

I agree, but sadly my insurance company don't. They said that they aren't interested unless I could identify the specific incident that caused the damage.

[Boat&Bikes]

Mystifying isn't it. When we brought our first boat, steel 45 footer built by a budget, though decent builder we knew nothing about boats. The boat was 5 years old so we assumed it would be fine. We thought it a plus point that the boat had hardly been cruised as what damage could happen whilst moored up...it had spent 5 years continously plugged to shore power, no GI or isolation transformer. We had no survey as we believed the boat was a fair price. The boat had been recently blacked, first time since being launched 5 years previously, what could have gone wrong? We spent the next 2 years learning about boats, including galvanic issue's. 2 years later come the first time we had the boat in a dry dock for blacking we had a few sleepless nights prior to judgement day. Thankfully all was well, hardly a spot of rust or a pit to be seen. No logic is there?

 

Ian.

[Keeping Up]

I just heard that the boat won't be in the dock for at least another couple of weeks, so there'll be no pictures before that. The repairs will apparently take at least 3 weeks so we'll be boatless until late November - which also means we'll be stuck on the wrong side of the winter closures.

[Stevec]

Some years ago I came across a boat with pitting that suggested it was 10 years old rather than only about 6 months. A surveyor found this and suggested an electrical fault. I was asked to take a look and noticed a very faint glow on circuit breaker indicators which increased when cabin light switches were turned on (battery master switch was off)

The boat only had 12 volt circuits no shore mains and I traced the problem to a headlight with its metal frame connected to the lamp & bolted onto the hull and an incorrectly fitted earth strap from battery negative to engine frame. The circuit seemed to be from batt positive through cabin lights left switched on via the negative commoning busbar and the headlight casing onto the hull. Thence onto the engine frame oil pressure warning light to the ignition switch (on position) and thence ?? - The earth strap was wired onto one of the master switch terminals so that the battery was always connected to engine frame. Moving the strap to the other side of the master switch properley isolated the battery.. When the boat was vacated, there were only small currents but with engine running they could have been much bigger - hence the significant pitting.

[Detling]

I noticed today that a boat in our marina has coiled his mains lead round his steel taff rail, this will basically cause an induced current/voltage in his boat as he has created a transformer. Is is possible the OP did something similar and could this lead to his problems, my knowledge of electrics tells me that coiling a Mains lead is bad and then providing a steel core to concentrate the magnetic field can only make things worse.

[Keeping Up]

A couple of interesting points in the last two points, thanks, however:

 

In the first case the alternative route via the grounded spotlight bypassed the isolator switch, hence the high current through the hull. My own spotlight is fully isolated and uses a wire for the negative return. But it was one of the things I checked, and there is no stray connection between negative and hull (apart from the stray radio aerial connection that I mentioned already, which existed for a couple of months and is common on many boats without causing any problems; and at no time during that time was the isolator switched off)

 

Not that I have my mains lead coiled around any steelwork, but thinking about the latter post I don't think it would generate significant electromagnetic coupling because the live and neutral would be carrying equal and opposite currents so there should be no induction effect. Having said that, it would be intriguing to know if there could be something measurable.

[NigelD]

Just read through this last night, and really sorry to hear this has happened! Just one question you say "Anodes OK - slightly used" why were these not completely dissolved? surely this is their job? Equally, when were the last changed and do you know the "purity" of the material used?

 

​Hope the repairs all go well.

[Naughty Cal]

Just read through this last night, and really sorry to hear this has happened! Just one question you say "Anodes OK - slightly used" why were these not completely dissolved? surely this is their job? Equally, when were the last changed and do you know the "purity" of the material used?

 

​Hope the repairs all go well.

They rarely, if ever, get "completely dissolved"

[NigelD]

It seems that for whatever reason, the whole hull became active and ended up being more "sacrificial" than the anodes. Surely these should always go first, not the hull?

[Tony Brooks]

 

It seems that for whatever reason, the whole hull became active and ended up being more "sacrificial" than the anodes. Surely these should always go first, not the hull?

 

But anodes have a relatively limited area of influence so most narowboats have the whole centre section of their hull unprotected unless several "mid point" anodes are fitted.

[Alan de Enfield]

The last few posts set me off 'wondering'

 

Anodes not eroded away :-

 

Maybe the wrong anode material was used / specified (Zinc or aluminium in lieu of magnesium)

Maybe the yard fitting took the 'wrong' anodes out of stores

Maybe the manufacturer labelled the anodes incorrectly

Maybe the manufacturer was supplied with the 'wrong' material to manufacture the anodes.

 

If, in error, a Zinc anode(s) was used then that may be the reason - in freshwater, Zinc tends to forms an insulating oxide film over itself stopping it 'working'.

 

Could it be worth having the anodes checked to see what they are ?

[Keeping Up]

The anodes are asbout 75% eroded, and the pitting is rather less in their vicinity but they are only at the bow and stern so the parallel-sided part of the boat is the worst. I would have though that the wrong type would be either totally gone or else not gone at all?

 

Once again I checked everything electrical; the only feature that seems relevant, as suggested by Moominpapa, is the way the negative is bonded to the hull at the "battery" end of the cable that connects the engine rather than at the block itself, so that some of the very high alternator current (175 Amps) may be "leaking" around that cable via the hull & exhaust. I'm not convinced that's the problem but I will change that anyway.

 

I tried to take some photos yesterday but the light in the dock was very poor and it wasn't possible to get anything that showed the depth of the pits, but the attached does show the extent of the pitting. Most pits are the size of a fingernail and 3 or 4 mm deep.