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Residual Plate Thickness


Ambull

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Good evening,

 

I would be grateful if someone could help me with my query regarding baseplate
Nominal plate thickness 10 mm

Average pit depth  0.8 mm

Maximum pit depth 1.3 mm

Coverage of pitting 20 percent 

Average Plate Thickness 9.5 mm

Residual Plate thickness 8.3mm

 

is this anything to be concerned about on a 6 year old narrowboat?

 

Thank you in advance 

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4 minutes ago, Ambull said:

is this anything to be concerned about on a 6 year old narrowboat?

 

 

It does seem to be a high rate of corrosion for a 6 year old boat.

Are you moored in a marina ?

 

 

The good news is that it is unlikley to sink in the next 30 years if the corrosion rate stays the same.

Edited by Alan de Enfield
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4 minutes ago, Alan de Enfield said:

 

 

It does seem to be a high rate of corrosion for a 6 year old boat.

Are you moored in a marina ?

 

 

The good news is that it is unlikley to sink in the next 30 years if the corrosion rate stays the same.

Hi Alan,

 

Thanks for your reply. I am looking at purchasing this boat, the surveyor said it’s nothing to worry about ?

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Just now, Ambull said:

Hello Sea Dog 

 

I think it was the 8.3 mm residual thickness figure that alarmed me.  Do you think it is concerning?

 

 

Not so long ago NB hulls were built with an 8mm base plate, before that some were even built with 4mm.

Many boats built in Europe (Dutch Barges) will have 6mm- 8mm. Its a UK narrowboat 'thing' that it has to be 'thick'.

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4 minutes ago, Alan de Enfield said:

 

 

Not so long ago NB hulls were built with an 8mm base plate, before that some were even built with 4mm.

Many boats built in Europe (Dutch Barges) will have 6mm- 8mm. Its a UK narrowboat 'thing' that it has to be 'thick'.

I owned a colecraft from 1981 some years ago with a 6 mil baseplate. It is still alive and well.

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Just now, Ambull said:

Hello Sea Dog 

 

I think it was the 8.3 mm residual thickness figure that alarmed me.  Do you think it is concerning?

It sounds like it's actually 8.7 if @TheBiscuits maths is as good as he thinks it is! It wouldn't put me off - as @Alan de Enfield said,it's gonna be a long time at that rate. The question about whether it's been in a marina is about whether it's been left plugged in which can cause putting if there are earthing issues, particularly in boats without a galvanic isolator or isolation transformer fitted to protect it. Check if it has and if it hasn't, most would advise that you fit one.

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Thanks TheBiscuits and Mrsmelly for your replies

2 minutes ago, Sea Dog said:

It sounds like it's actually 8.7 if @TheBiscuits maths is as good as he thinks it is! It wouldn't put me off - as @Alan de Enfield said,it's gonna be a long time at that rate. The question about whether it's been in a marina is about whether it's been left plugged in which can cause putting if there are earthing issues, particularly in boats without a galvanic isolator or isolation transformer fitted to protect it. Check if it has and if it hasn't, most would advise that you fit one.

It does have a galvanic isolator fitted ??

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I think the residual thickness is taken from the average plate thickness 9.5mm minus the maximum pit depth 1.3mm, which gives 8.2mm, so the maths isnt quite as bad, but still incorrect.

Having lost this much metal in 6 years  IS a concern to me, either the GI isnt working properly, or the boats electrics arent as good as they should be, or it is moored in a marina with problems. Or several other reasons.

 

The surveyor shouldnt have said it's nothing to worry about unless he is being paid by the buyer or broker, what he should be doing is advising you on how to slow/stop the corrosion occurring.

 

My boat is 30 years old this year, its last survey 2 years ago showed maximum pits of 1.3mm in a 10mm baseplate.

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26 minutes ago, Sea Dog said:
3 minutes ago, matty40s said:

I think the residual thickness is taken from the average plate thickness 9.5mm minus the maximum pit depth 1.3mm, which gives 8.2mm, so the maths isnt quite as bad, but still incorrect.

Having lost this much metal in 6 years  IS a concern to me, either the GI isnt working properly, or the boats electrics arent as good as they should be, or it is moored in a marina with problems. Or several other reasons.

 

The surveyor shouldnt have said it's nothing to worry about unless he is being paid by the buyer or broker, what he should be doing is advising you on how to slow/stop the corrosion occurring.

 

My boat is 30 years old this year, its last survey 2 years ago showed maximum pits of 1.3mm in a 10mm baseplate.

Hi Matty40s

 

Thank you for your reply. I was concerned by the 8.3 mm figure given the boats age but with my limited knowledge, I wanted to ask the members of this forum what they thought.  

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Just now, Phoenix_V said:

it is a lot of pitting and its happening fast, the current amount is not a problem but you would want to slow it down if you bought the boat

Hi Phoenix V

 

Yes, as much as this seemed to be the almost perfect boat for me, I am starting to think that I will walk away from this one.

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depends how much you want it at .2mm / year its not that fast although there seem to be a lot of them, things you might want to do in no particular order; move mooring, check out electrics, get plate 2 pack epoxied

strange its just the base and not the sides

  • Greenie 1
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Residual plate thickness of sides 5.9mm

Just now, Tony Brooks said:

Could it have been sitting on mud? Many marinas are not very deep. It may be the microbial corrosion that we are now hearing about and nothing to do the electrics. I would have liked the surveyor to advise you on the likely cause.

Hi Tony Brooks,

 

The surveyor didn’t advise on the likely cause, this boat has also had three owners during its relatively short life.  I don’t know if that is significant or not, possibly not

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7 minutes ago, Ambull said:

Residual plate thickness of sides 5.9mm

Hi Tony Brooks,

 

The surveyor didn’t advise on the likely cause, this boat has also had three owners during its relatively short life.  I don’t know if that is significant or not, possibly not

 

As none of us here have had a chance to inspect the corrosion and also don't know where the boat has been kept we can't really comment apart from to say that according to the figures it is likely to last a long time before it becomes an issue. The reason the boat's location is important is because some locations are potentially worse for some forms of corrosion than others.

Edited by Tony Brooks
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From what I have found out, this boat has spent a fair bit of time in a small marina.  I am thinking along the lines that if purchased, the baseplate corrosion will always be in the back of my mind, so therefore not worth it.  It is a very nice boat, but also a considerable amount of money.  

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Yes 0.2mm is above average but even if it were maintained then you would only lose 4mm in the next 20 years and would then still have a perfectly good (usable, saleable  and insurable) boat. And if you are lucky, a change of location would halt the corrosion anyway. So I would say ignore it, if you like the boat just buy it enjoy it and stop worrying about it. And I speak as someone whose boat had 23 years without corrosion followed by a year of incredible but unexplained pitting and now several years without any corrosion at all.

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32 minutes ago, Ambull said:

From what I have found out, this boat has spent a fair bit of time in a small marina.  I am thinking along the lines that if purchased, the baseplate corrosion will always be in the back of my mind, so therefore not worth it.  It is a very nice boat, but also a considerable amount of money.  

Thing is, most folk with boats that age have no idea of what their baseplate is really like. It's usually only checked with this level of accuracy if the boat is sold and the new buyer commissions a surveyor. Modern boats use 10mm baseplates, but as has been said, there are many older boats around with 6mm and 8mm bottoms, so you already have a margin to allow for corrosion. My first question was "What made you not believe him?" after your surveyor said it was fine. We can all speculate having not seen the boat and having our own opinions about what is and what isn't acceptable, but you've paid a professional to give you a professional opinion based on his own inspection and  measurements. Surveyors are often maligned here, rightly or wrongly, but if yours is a good one... "What made you not believe him?" 

Edited by Sea Dog
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1 minute ago, Ambull said:

Hi Keeping Up,

 

Yes maybe a change of location combined with good maintenance schedule would halt the corrosion. 

 

Gritblasting, (optionally) pits welding up and 2-pack epoxy should be doable for a few grand depending on where you get it done.  If you really like the boat and are worried about the pits this should sort it for the long term.

 

Does the seller have a previous survey you could compare yours to?

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2 minutes ago, Sea Dog said:

Thing is, most folk with boats that age have no idea of what their baseplate is really like. It's usually only checked with this level of accuracy if the boat is sold and the new buyer commissions a surveyor. Modern boats use 10mm baseplates, but as has been said, there are many older boats around with 6mm and 8mm bottoms, so you already have a margin to allow for corrosion. My first question was "What made you not believe him?" after your surveyor said it was fine. We can all speculate having not seen the boat and having our own opinions about what is and what isn't acceptable, but you've paid a professional to give you a professional opinion based on his own inspection and  measurements. Surveyors are often maligned here, rightly or wrongly, but if yours is a good one... "What made you not believe him?" 

Hi Sea Dog,

 

What made me doubt him was that 20 percent of the baseplate just seems a lot for a relatively newish boat that has been 2 pack epoxied, has sacrificial anodes, galvanic isolator and seems immaculate in every other way

  • Greenie 1
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