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Advice on Pitting Please!


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Hey all,

 

We've lived on a narrowboat for 4 years now and just in the midst of purchasing a 57x10ft widebeam, constructed in 2008. We had a survey done, and was surprised to find some pitting of 2.8mm on the baseplate. The hull thickness measurements were:

 

Base plate: 9.7mm to 10.mm

Side plates: 5.6mm to 6mm with pitting of 1.0mm

Uxter Plates: 9.7mm to 9.9mm

 

The boat was previously shot blasted 3 years ago, and blacked earlier this year, so the surveyor made it clear that due to the shot blasting it wasn't a pretty sight, but was a very honest baseplate. He said if all boats were shot blasted prior to surveying they'd likely find the same kind of pitting. He also said as long as we look after her, take her out of the water every few years and check and touch-up any blacking, she should be fine for another 10 years with no further damage. 

 

Does anyone have any advice on whether or not this is sound? I'm concerned that when it comes to selling her on ourselves in say 5 years time, the next purchaser will be as worried as I am! Or am I just worrying too much? 

 

Photo below! 

IMG-20230523-WA0001.jpg

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My guess is that the baseplate has never been painted until now, so pitting can be expected. 

Has the bottom now been blacked, and if so what with? 2 pack epoxy over shot blasting should stop any further corrosion on the existing pits, and elsewhere (except where the coating gets damaged due to contact with the canal bottom or rubbish). But if it has just been bitumened, then you will have less protection.

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The surveyor is right.  Shot blast will remove all the pre existing rust then the epoxy will prevent it recurring, except where it gets mechanical damage from locks, bridges, shopping trolleys etc.

 

The pic looks a bit like our baseplate, but wider.  We were blasted in 2010 and it has been pressure washed, wire brushed, rust at damage sorted and over coated with epoxy every 4 or 5 years since.  There is no recurrence of old rust.

 

You don't say what the boat was blacked with.  It should have been an epoxy.  If it was anything else (bitumen, fancy vinyl,) the effect is most likely to be cosmetic and  I would expect the epoxy to eventually fail gradually  after about 10 years.

 

N

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Thank you all for your swift replies! 

 

I should've said yes the hull was shot blasted and both the hull and baseplate was blacked with 2pack epoxy 3 years ago. Then end of last year both hull and baseplate were blacked again with International intertuf.

 

There is no galvanic isolator so you could well be right on the corrosion. Should I get one fitted, even though we will be continuously cruising? 

 

 

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2.8mm sounds quite bad and we do hear reports of older boats with zero pitting, but I suspect they are fantasy. I also suspect that many if not most boats that are shotblasted would show that level of pitting. It is normal not to black the baseplate (thats why they are 10mm thick), most yards are not able to, but things are changing.

 

Do the baseplate in epoxy and that pitting will pretty much stop and the boat will last forever, just repair any scrapes every few years

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Bear in mind the logic of the move to 10mm baseplates was to give an additional 4mm of sacrificial steel because baseplates aren't (or weren't) routinely protected.

 

If all you've got on the baseplate is ordinary standard bituminous blacking then probably all you can sensibly do is to keep it in good order now. Personally I wouldn't shot blast a hull and then not put a more state of the art protective system on it.

 

You only need a galvanic isolator if you are connected to shore supply. You do need suitable anodes though but it's unlikely a yard will black a boat without at least consulting on that subject.

 

Corrosion is a product of environment and lack of protection. Moving about is good in respect of the former and you have the basics of the latter under control. 

 

Also note there is effectively no general corrosion of your plates. Those numbers are well within the manufacturing tolerance of the orginal plates, which I can guarantee were not actually 10.0mm thick.

 

Edited by Captain Pegg
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45 minutes ago, emma_naomi said:

I should've said yes the hull was shot blasted and both the hull and baseplate was blacked with 2pack epoxy 3 years ago. Then end of last year both hull and baseplate were blacked again with International intertuf.

That's a shame. Would be better if epoxy had been used again. You can put blacking over epoxy, but not the other way round. So you are now into a 3 yearly cycle of bitumen blacking, or you will have to blast the bitumen (and most of the epoxy) off in order to epoxy again.

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11 minutes ago, David Mack said:

That's a shame. Would be better if epoxy had been used again. You can put blacking over epoxy, but not the other way round. So you are now into a 3 yearly cycle of bitumen blacking, or you will have to blast the bitumen (and most of the epoxy) off in order to epoxy again.

 

I assume the epoxy underneath will still give it extra protection? 

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44 minutes ago, emma_naomi said:

 

I assume the epoxy underneath will still give it extra protection? 

 

Yes, it'll still be fully effective and the ordinary blacking on top should give it some protection against mechanical damage.

 

Sorry, I missed the detail on your earlier post that rendered my comments on the type of blacking redundant.

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2 hours ago, dmr said:

Do the baseplate in epoxy and that pitting will pretty much stop and the boat will last forever,

 

 

Whereas if you don't, it will probably only last another 40 years at the current rate of corrosion.

 

And you might be dead by then, so will you care?! 

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9 minutes ago, MtB said:

 

 

Whereas if you don't, it will probably only last another 40 years at the current rate of corrosion.

 

And you might be dead by then, so will you care?! 

 

Thats a very depressing way of looking at things.

I've got a really big grease gun for doing the propshaft UJ's and pillowblock. Last filled it ten years ago. Did it again last week. Did cross my mind that this might be the last time 😀

That's what old men say "That'll see me out".😀

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12 minutes ago, dmr said:

 

Thats a very depressing way of looking at things.

I've got a really big grease gun for doing the propshaft UJ's and pillowblock. Last filled it ten years ago. Did it again last week. Did cross my mind that this might be the last time 😀

That's what old men say "That'll see me out".😀

 

I blame Simon Wain. 

 

When he replaced the baseplate on Reg I was barfing slightly at the cost of the steel to do it in 10mm. He pointed out 8mm would be 20% cheaper and would easily last me out.

 

 

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9 hours ago, emma_naomi said:

We had a survey done, and was surprised to find some pitting of 2.8mm on the baseplate. The hull thickness measurements were:

 

Base plate: 9.7mm to 10.mm

Side plates: 5.6mm to 6mm with pitting of 1.0mm

Uxter Plates: 9.7mm to 9.9mm

 

This doesn't make sense to me. If it started out with a 10mm baseplate and it's now got pitting 2.8mm deep then surely the baseplate thickness measurements are now 7.2mm - 10mm, not 9.7mm - 10mm?

 

Same with the sides. With 1mm pitting it's now 5mm - 6mm thick.

 

You have to include the depth of the pitting in minimum hull thickness measurements.

Edited by blackrose
  • Greenie 1
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1 hour ago, blackrose said:

 

This doesn't make sense to me. If it started out with a 10mm baseplate and it's now got pitting 2.8mm deep then surely the baseplate thickness measurements are now 7.2mm - 10mm, not 9.7mm - 10mm?

 

Same with the sides. With 1mm pitting it's now 5mm - 6mm thick.

 

You have to include the depth of the pitting in minimum hull thickness measurements.


What is expressed is two different sets of measurements.

 

One is the overall plate thickness measured by use of an ultrasonic gauge at a series of pre-determined points on the side and hull, and the other is the directly measured depth of the visible localised pitting.

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3 minutes ago, ditchcrawler said:

I thought base plates didnt corrode because there is no oxygen down there, was I told wrong?

 

Not really, there is much less available oxygen in water than in air so it rusts much more slowly.


Key word is rusts. Other forms of corrosion, or aggressive rusting due to catalysts, are more of a risk to boats.

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13 minutes ago, ditchcrawler said:

I thought base plates didnt corrode because there is no oxygen down there, was I told wrong?

 

A convenient myth from the yards that cant get access to the baseplate?

I offer two bits of evidence..

1 Fish stay alive

2 have a look at the steel stuff that the magnet fishers pull out.

(a magnet fisher on the Marple flight gave me 4 windlasses that he had fished out.......repair way beyond my welding skills 😀 )

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Water is comprised of oxygen (H2O) and some people will tell you that's why steel rusts in water, but there's a difference between the oxygen atoms bound up in water molecules and dissolved oxygen that fish breathe. It's the dissolved oxygen that's available to react with submerged steel causing corrosion. The deeper you go the less dissolved oxygen there is but it's a gradient and at canal and river depths it doesn't disappear entirely so as Cpt. Pegg says, bare steel will still corrode, just more slowly than it does at the surface.

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1 hour ago, emma_naomi said:

Thank you all for your help and comments. It seems as though as long as we look after it there should be no further corrosion so I think we will be going through with the purchase. 

 

 

 

That's essentially what your surveyor is saying which is more important than what we think. There is no risk free way to purchase a boat but that baseplate is far better protected than at least 95% of boats on the water, and as was also said quite likely no more pitted.

 

I'm not sure "no damage" is a wise statement, I'd be more inclined to say "tolerable wear and tear" as you may get some mechnical damage and surface rust but that will get sorted every time you bring it out the water for a re-coat. You'll probably find striations along the blacking next time it comes out of the water from where the boat has run across items on the bed of the canal. They may or may not go through to the steel. If you're river cruising it may be less prone to this.

 

I'm sure it should last many times longer than 10 years so I wonder if that's the lifespan the surveyor is putting on the current 2 pack epoxy.

 

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8 hours ago, dmr said:

1 Fish stay alive

 

Point of Order...

 

This is a self-selecting population. 

 

Only the fish with successful strategies for surviving the oxygen-depleted environment 600mm down, are alive for you to notice!

 

Hope that helps. ;) 

 

 

(Nice wine, this.) 

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