Hull Thickness

[veruska poni]

Hello everyone. I'm about to buy my first narrowboat, 55ft cruiser stern built 1996. A 4 years ago survey shows its hull sides measured between 6.8mm to 7.2mm, with some widespread pitting on hull side with pittings up to 1.4mm. I am about to book a new survey BUT do you guys think I am likely to find the boat in need of overplating in the near future (3/4 years?)

 

My questions are:

1) how a boat which is believed to be originally 10/6/4 can measure 6.8 to 7.2mm on hull sides with ultrasonic testing? Shouldn't they be 6mm?

2) should I be concerned about the 1.4mm pit depths as shown in the survey 4 years ago, and should I expect to replate/overplate it in the near future? Am I right to believe that at the time of the survey the remaining hull thickness around the pitting was not less than 6.8mm-1.4mm= 5.4mm?

3) will I be able to sell a boat which in 3/4 years time might be almost in need for overplating without losing much of its value?

 

I find hull survey a bit confusing... but as I would spend quite a lot of money on this boat I would need to know where these money are going... :))

[matty40s]

In 24 years it has lost 1.4mm.....itll be a while before it sinks if it has been looked after.

More important will be the base plate....rarely blacked.

Fully comp insurance is difficult but not impossible to get under 4mm thickness.

Some boats had 8mm lower side plating, CTS amongst others.

[Sea Dog]

If it had the issues you refer to at a survey 4 years ago, the one thing you can be sure of is that it won't have healed up.  7.2mm of steel on a 6mm side sounds odd, doesn't it, and it raises enough concern that I'd not be relying on that survey, especially after 4 more years of potential degradation.  I'd say you need your own survey before risking your capital.

[Bee]

I have seen a few boats with heavy scale on the inside, was hauled out next to a lovely old dutch barge 2 years ago and the owner was shovelling sacks full of scale out of it, I,ve seen the same with an old joey boat as well so it is quite common for the inside to be trying to meet the outside. More modern boats that usually have a nice dry bilge are usually much better but if you can only see the outside it only seems to be half the inspection. I think a hull survey would be a good idea and that includes the bottom. If its reasonable then you have a choice of paint for the hull - and that is a whole new can of worms.

[veruska poni]

47 minutes ago, matty40s said:

In 24 years it has lost 1.4mm.....itll be a while before it sinks if it has been looked after.

More important will be the base plate....rarely blacked.

Fully comp insurance is difficult but not impossible to get under 4mm thickness.

Some boats had 8mm lower side plating, CTS amongst others.

Thanks Matty40, that reassures me quite a bit... sinking is not exactly what you hope to get from your first boat innit

 

Yes, it seems it has been well looked after, including base plate that was blacked just after that survey. But corrosion progress is a bit of a gamble right? Not really predictable how quickly it can let you down

 

I don't understand why boat builders don't all make 8mm side plating as it's the part that rots the most...

[veruska poni]

46 minutes ago, Sea Dog said:

If it had the issues you refer to at a survey 4 years ago, the one thing you can be sure of is that it won't have healed up.  7.2mm of steel on a 6mm side sounds odd, doesn't it, and it raises enough concern that I'd not be relying on that survey, especially after 4 more years of potential degradation.  I'd say you need your own survey before risking your capital.

Yes quite strange...  I know the ultrasonic testing was made through paint but I don't think that matters...

 

Anyway yes, will get my survey done and the mystery might be unveiled. Hopefully the pittings haven't got much worse by now!

[veruska poni]

12 minutes ago, Bee said:

I have seen a few boats with heavy scale on the inside, was hauled out next to a lovely old dutch barge 2 years ago and the owner was shovelling sacks full of scale out of it, I,ve seen the same with an old joey boat as well so it is quite common for the inside to be trying to meet the outside. More modern boats that usually have a nice dry bilge are usually much better but if you can only see the outside it only seems to be half the inspection. I think a hull survey would be a good idea and that includes the bottom. If its reasonable then you have a choice of paint for the hull - and that is a whole new can of worms.

Yes, good point, the inside needs to be checked as well... not always possible for what I understand, right?

[Sea Dog]

1 hour ago, veruska poni said:

I know the ultrasonic testing was made through paint but I don't think that matters...

It depends.  Some surveyors splash out on a meter which can discriminate between the steel itself and any coatings, but they're a fair bit more money so many (most?) do not.  Instead, they may remove "tickets" of protective coating as a way of measuring just the steel, thus leaving you with a problem to solve... or they may not.  Since you already have a mystery to solve, best find yourself one who has the necessary kit. I hope it works out for you.

 

[Mike Adams]

The problem you have is that for a proper inspection you need to access both sides at least in some places. You could have anything up to about 6mm of scale in my experience on an internal surface and it can show up as metal on some meters. Internally it depends how well it was protected when built and how much water has got into the bilge. I don't think any coating is likely to make a big difference to the measurement. You really need a really big hammer but this is frowned upon now and with that you can soon tell if the scale comes off on the inside and by the noise of the hammer on the plating. If I was buying a boat I would like to have a look into the bilge at a few places especially where water would collect to look at the condition. Some surveyors have been known do the survey on the hull without even looking inside. If the hull is shot walk away and find one that isn't. I chinese meter available on ebay for about £50 is worth the investment if you are looking at buying a boat. Measuring the thickness is not rocket science.

[veruska poni]

23 hours ago, Sea Dog said:

It depends.  Some surveyors splash out on a meter which can discriminate between the steel itself and any coatings, but they're a fair bit more money so many (most?) do not.  Instead, they may remove "tickets" of protective coating as a way of measuring just the steel, thus leaving you with a problem to solve... or they may not.  Since you already have a mystery to solve, best find yourself one who has the necessary kit. I hope it works out for you.

 

I see... Best thing is to ask the surveyor which kit he got and figure out how accurate it can be. Thanks!!! And keep your fingers crossed for me ?

[David Mack]

On 08/06/2020 at 14:48, veruska poni said:

A 4 years ago survey shows its hull sides measured between 6.8mm to 7.2mm, with some widespread pitting on hull side with pittings up to 1.4mm. 

 

Is the 6.8 - 7.2 mm the thickness in unpitted areas, or is that the remaining thickness at the bottom of the 1.4 mm pits? A good hull survey will identify the remaining thickness at pitted areas since that is the number that really matters.

[veruska poni]

23 hours ago, Mike Adams said:

The problem you have is that for a proper inspection you need to access both sides at least in some places. You could have anything up to about 6mm of scale in my experience on an internal surface and it can show up as metal on some meters. Internally it depends how well it was protected when built and how much water has got into the bilge. I don't think any coating is likely to make a big difference to the measurement. You really need a really big hammer but this is frowned upon now and with that you can soon tell if the scale comes off on the inside and by the noise of the hammer on the plating. If I was buying a boat I would like to have a look into the bilge at a few places especially where water would collect to look at the condition. Some surveyors have been known do the survey on the hull without even looking inside. If the hull is shot walk away and find one that isn't. I chinese meter available on ebay for about £50 is worth the investment if you are looking at buying a boat. Measuring the thickness is not rocket science.

 

Right... At the time of the survey, 4 years ago, the cabin bilge was not accessed. During the viewing I saw it was painted and treated so everything was looking good but I'll ask the surveyor to have a look. I'm checking the ultrasonic meters on ebay.... if they are reliable they are well worth the cost!

[veruska poni]

6 minutes ago, David Mack said:

 

Is the 6.8 - 7.2 mm the thickness in unpitted areas, or is that the remaining thickness at the bottom of the 1.4 mm pits? A good hull survey will identify the remaining thickness at pitted areas since that is the number that really matters.

Exactly, that's what I was trying to understand! I think that's the thickness of the unpitted areas of the side hull, so where there is the 1.4mm pitting the remaining thickness should be (hopefully) at least 5.4mm. It sounds too good to be true tho. I'll need to ask the next surveyor how the measurements need to be read cos even a 0.3 mm difference matters when it comes to hull sides! 

[Mike Adams]

1 hour ago, veruska poni said:

I'm checking the ultrasonic meters on ebay.... if they are reliable they are well worth the cost!

I bought this one.  https://www.ebay.co.uk/itm/LCD-Digital-Ultrasonic-Thickness-Gauge-Meter-Depth-Measurement-Tester-1-2-220mm. Couple of years ago now so far worked without problems. works when boat is in the  water too!

 

[___]

On 08/06/2020 at 14:48, veruska poni said:

Hello everyone. I'm about to buy my first narrowboat, 55ft cruiser stern built 1996. A 4 years ago survey shows its hull sides measured between 6.8mm to 7.2mm, with some widespread pitting on hull side with pittings up to 1.4mm. I am about to book a new survey BUT do you guys think I am likely to find the boat in need of overplating in the near future (3/4 years?)

 

My questions are:

1) how a boat which is believed to be originally 10/6/4 can measure 6.8 to 7.2mm on hull sides with ultrasonic testing? Shouldn't they be 6mm?

2) should I be concerned about the 1.4mm pit depths as shown in the survey 4 years ago, and should I expect to replate/overplate it in the near future? Am I right to believe that at the time of the survey the remaining hull thickness around the pitting was not less than 6.8mm-1.4mm= 5.4mm?

3) will I be able to sell a boat which in 3/4 years time might be almost in need for overplating without losing much of its value?

 

I find hull survey a bit confusing... but as I would spend quite a lot of money on this boat I would need to know where these money are going... :))

1). The obvious answer is that the belief the plate was originally 6mm think is erroneous. Do you have anything other than someone’s word for it? Specified plate thicknesses are nominal. In extreme a 6mm plate could be anything from 5.6mm to 7.5mm thick and still be compliant to specification although it’s unlikely that the manufacturer was in the habit of giving away a free 1mm with all their plate. Nor will a plate record an entirely uniform thickness over its entire surface even when new.

 

2 & 3). You should get it surveyed to confirm it wasn’t something aggressive 4 years ago that’s now worse. If it’s still just pitting of 1.4mm or thereabouts I would think good maintenance would see you through that period of time with no problem.
 

This quoting of steel thickness and pit depth to tenths of millimetres is a bit optimistic. An ultrasonic test meter actually measures time and converts what can be some pretty variable signal responses into a seemingly precise measurement using algorithms. I wonder what the true capability of some of the meters actually is, particularly those that can ‘see’ through coatings as it strikes me they probably work mostly by having more complex algorithms to cope with the weaker and more variable signals they receive back. Does anyone know for certain?
 

I think key is to confirm that the increase in thickness isn’t due to swelling by corrosion of the inner face but I very much doubt an ultrasonic test would be be able to identify de-laminated steel.

 

JP

[WotEver]

13 hours ago, Mike Adams said:

I bought this one.  https://www.ebay.co.uk/itm/LCD-Digital-Ultrasonic-Thickness-Gauge-Meter-Depth-Measurement-Tester-1-2-220mm. Couple of years ago now so far worked without problems. works when boat is in the  water too!

 

Working link:

https://www.ebay.co.uk/itm/LCD-Digital-Ultrasonic-Thickness-Gauge-Meter-Depth-Measurement-Tester-1-2-220mm/352987071399

[veruska poni]

19 hours ago, Mike Adams said:

I bought this one.  https://www.ebay.co.uk/itm/LCD-Digital-Ultrasonic-Thickness-Gauge-Meter-Depth-Measurement-Tester-1-2-220mm. Couple of years ago now so far worked without problems. works when boat is in the  water too!

 

Thanks Mike!

5 hours ago, WotEver said:

Working link:

https://www.ebay.co.uk/itm/LCD-Digital-Ultrasonic-Thickness-Gauge-Meter-Depth-Measurement-Tester-1-2-220mm/352987071399

Yes, here he is. Thanks a lot

[veruska poni]

13 hours ago, Captain Pegg said:

1). The obvious answer is that the belief the plate was originally 6mm think is erroneous. Do you have anything other than someone’s word for it? Specified plate thicknesses are nominal. In extreme a 6mm plate could be anything from 5.6mm to 7.5mm thick and still be compliant to specification although it’s unlikely that the manufacturer was in the habit of giving away a free 1mm with all their plate. Nor will a plate record an entirely uniform thickness over its entire surface even when new.

 

2 & 3). You should get it surveyed to confirm it wasn’t something aggressive 4 years ago that’s now worse. If it’s still just pitting of 1.4mm or thereabouts I would think good maintenance would see you through that period of time with no problem.
 

This quoting of steel thickness and pit depth to tenths of millimetres is a bit optimistic. An ultrasonic test meter actually measures time and converts what can be some pretty variable signal responses into a seemingly precise measurement using algorithms. I wonder what the true capability of some of the meters actually is, particularly those that can ‘see’ through coatings as it strikes me they probably work mostly by having more complex algorithms to cope with the weaker and more variable signals they receive back. Does anyone know for certain?
 

I think key is to confirm that the increase in thickness isn’t due to swelling by corrosion of the inner face but I very much doubt an ultrasonic test would be be able to identify de-laminated steel.

 

JP

That sounds very interesting. The only doc I have specifying the original plate thickness (as it was believed to be) is the past survey, so no, I have no certainty. It's a Liverpool boat if that's of any use. I agree the manufacturer is unlikely to give away a free 1mm steel so I tend to assume it's more about steel swelling due to corrosion/rust scale/coating rather than the manufacturer's generosity (how nice would that be tho). What it's strange is that I saw another few surveys in which the hull seemed to be thicker (as for ultrasonic testing) than the supposed original 10/6/4... I wonder how a surveyor can be sure of the accuracy of their measurement with so many variables

[Mike Adams]

In my limited experience scale which normally forms inside the hull and depending on the circumstances is a very solid material and forms a strong bond with the parent metal. Any ultrasonic testing uses the reflection from the surface boundaries within the sample. In the case of internal scale the strong refection would come from the scale/air interface rather than the metal/scale interface. Remember that rust, as a uniform thickness of scale without much pitting might be ten times as thick as the metal displaced so losing 1mm of metal slowly might give you anything up to 10mm rust. Depending on how they are set up they can measure things like paint thickness on the outside of the material. If neither surface is flat because of pitting or scale then it becomes quite difficult to get a stable reading. An experienced surveyor will be able to interpret these measurements. Probably on the hull sides internally the worst corrosion is at the base of the sides and very little further up so if you take a reading say 50mm from the base of the side and then another one 200 mm up the side and the lower is higher I would suspect a problem with internal scale. I am sure there are a lot of boats floating around where there  is very little metal left and it is only scale stopping the water coming in. You only find this out when you have them shot blasted. I remember a very nice Dutch steel boat being lifted out and the paintwork looked great but when they lowered it on to the blocks the hull colasped but it had been sailing around quite happily no external corrosion but all from the inside. With a survey you are buying some sort of indemnity for your insurance.

[___]

Ultrasound encountering any boundary between material of differing densities will get at least a partial reflection. In reality it’s actually quite difficult to get it from the transmitter into the test piece which is why historically it required clean steel to make the coupling. You also need to consider the speed of the ultrasound waves through any surface coating or rust is different than it is through steel. Therefore if the meter really is capable of reading through two or three coatings of blacking on the hull sides and a well adhered layer of rust on the inside then the return signal is likely to be quite complex and weak. A digital meter will simply display what it’s algorithms tell it to be the thickness of the sound metal without the need for an intelligent operator’s interpretation and the uninitiated will simply believe it because it’s displayed in a precise manner to the nearest tenth of a millmetre.

 

1mm plus of well adhered rust on the inside of the side plates doesn’t sound a very realistic prospect to me. Sheets of rust on the top of the baseplate I’m all too familiar with and those can be removed by hand and I doubt are detectable by ultrasound.

 

This discussion is mostly guesswork. The OP has said they are getting a survey done. The best advice is to ensure the inside of the hull is inspected and a fresh steel thickness survey is undertaken. Any decisions should be based upon those findings. I wouldn’t worry too much about the outputs or someone else’s survey 4 years ago other than it forms a useful baseline for the findings of a new survey.

 

JP

[David Mack]

2 hours ago, Mike Adams said:

I am sure there are a lot of boats floating around where there  is very little metal left and it is only scale stopping the water coming in.

Some years ago I went to a Thames Water presentation on replacing water mains. They had photographs of a piece of iron water pipe which had been removed from below a London street. It looked rusty but intact. And then they handed round the same piece of pipe after it had been shotblasted. It was completely full of holes - more hole than pipe!

 

Might well be a few boats out there in similar condition.

[Alan de Enfield]

13 minutes ago, David Mack said:

Might well be a few boats out there in similar condition.

There was the example a couple of years ago where a boat was shotblasted, painted and put back into the water where it sank overnight.

Owner tried legal recourse but it seemed to sort of go quiet.

 

There was a very long thread on the subject.

 

This was part of the story

 

I am the owner who is mentioned here whose boat sank during routine maintenance carried out by a marina. My blog, also mentioned here was to inform other boat owners of the potential to loose everything without really trying, so that they might be more prudent in their own dealings with marinas, boatyards and insurance companies. I know my posts are long and difficult to read, but so was the incident that lasted most of 2014, and still didn't have a satisfactory resolution, where I felt I had no alternative in the end other than to dispose of my boat as salvage due to all the circumstances I have already described. It was never my intention to name and shame, just to make others aware of what can happen, and in full detail, although I am still unable to publish the name of the marina concerned due to ongoing attempts to recoup some of the unfair costs they levied before I could remove my boat from their premises. (The boat sank while they were working on it and in their care, and then they chose to send invoices for storage charges throughout the year while I attempted to sort things out, without any help from them and even before my insurance claim was settled).

• My fully comprehensive insurance was with Craftinsure, underwritten by Navigators & General. I had cover with them without claim from 2006. My claim was passed to the underwriters due to the possible size of the claim.
• They made it clear from the start that I had no recourse to the 3rd party insurer to claim from them if I continued with my claim to my own insurer. It was of no concern to me if N&G chose to reclaim from a 3rd party insurer their payout to me or not. Any claim to Craftinsure would have to be within the terms of my policy with them, which came nowhere near to what was required by my own surveyor. Nothing that had been damaged could be replaced with new. In the end N&G did reclaim their costs from the marina's insurer, but all I got out of it was the refund of my £300 excess payment.
• In the first instance, using the advice of N&G's representative, I wrote to the marina owner requesting details of their insurer so a direct claim could be made to them and perhaps I might have access to a more detailed repair that way. I never received a reply, other than an acknowledgemnt of my request from the marina manager.
• As such, again on the advice of N&G, I arranged to use legal expenses insurance I had with another policy to pursue uninsured losses. My solicitor, after gathering all related information to my case and making an assessment, also wrote to the marina owner on the 10th June requesting the same information. I have a later letter from my solicitor stating that "the marina owner had not yet given him the decency of a reply" and he would write again.
• The long process of obtaining estimates for repair that would satisfy my own insurer went on from there, with many dificulties, as I found other yards did not want to become involved. Eventually, after many months I felt I had no alternative but to accept a settlement of £5000 in lieu of repairs. I had argued black and blue that their repairs were insufficient to provide a reliable future for our boat. This is why we decided to sell it unrepaired. My own surveyor's estimates for satisfactory detailed repairs were around £20,000.
• During this process N&G were informed of the marina's insurer, but by the end of July I had settled with them in full and final payment. The legal process went on, and by now my solicitor also knew the identity of the marina's insurer, and put a claim together for reasonable uninsured losses. This was rejected out of hand by the other insurer, as I had already accepted my claim from N&G. They did however accept full liability, but would only repay N&G's costs, their payout to me.
• Due to the nature of legal expenses insurance (which I now consider to be a waste of time), with no 3rd party insurer to claim from, in their eyes my action now had a less than 51% chance of success. As a consequence legal funding was withdrawn, although I was advised that I still had a viable claim direct with the marina owners. However, this could not be funded by this action, although I was free to make a further claim to use LEI to pursue that. This meant starting from scratch, and with potential costs escalating, (storage charges), I decided I was on a route to nowhere there.

I made enquiries with two specialist marine law companies, who wanted £10,000 to undertake preliminary case assessments, with an estimated £30,000 final cost to gain success at court. Should a court case fail, then costs could double due to having to fund all court costs. This of course was not viable, and that is where we ended up - selling the boat for salvage and adding the £5000 insurance payout to that. That equated to a loss of around £20,000 on what I might have expected to sell our boat for in an unsunken condition. Yes, I threw the towel in, but I was getting nowhere with anybody, and after all those months of worry, I had had enough.

 

My reply has again been long, but if I made every detail known I could probably fill a book. I hope this answers some questions that have been posted on this forum about insurance as well as trusting marinas and boatyards to undertake work on your behalf.