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Zinc spray & two-pack, what happens at hull survey time?


Joe the plumber
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This isn't something we've got to worry about as we've not had it done, but I was thinking the other day about what happens if someone has spent around

five grand on their hull having zinc spray and two pack work done with a ten year guarantee (for example at Debdale Wharf) and then has to sell the boat

a couple of years later.

 

The buyer's surveyor is presumably going to need to remove the blacking to check the hull thickness, which is going to damage the coatings.

If the buyer then doesn't go through with the sale, the owner is left with the hull coating compromised and it can't just be fixed with a quick tin of

bitumastic.

 

I suppose it may be that proof of having had the work done will show a buyer that the hull doesn't need surveying, but it's something that could

be a real issue for the seller.

 

Has anyone had this problem yet and how do you repair this coating system if it gets damaged?

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Our boat has epoxy blacking, when surveyed for pre purchase only the surface crud was removed with a wire brush in a battery drill prior to ultrasound. There was no damage to the hull coating.

I'd be sure to ascertain the method any potential surveyor proposed to adopt.

 

Roger

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You repair the coating one of two ways:Spot blast/

Angle grinder and sanding disc to clean up small damaged areas, renew zinc spray on damaged areas, overcoat with epoxy.

or Angle grinder and sanding disc , cold zinc spray in-a-can from Screwfix et al, overcoat with epoxy.

 

If it is really badly damaged then you need to start over with a full blast.

 

N

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I read the earlier thread on ultrasonic testing with interest and I guess it prompted this question.

 

The obvious answer is to have the hull surveyed before applying such a system. To remove a coating that remains soundly adhered and visibly doing its job is to a degree unnecessary because if the plate the coating was applied to was sound at the time of application it will remain sound as long as the protection remains sound. At least on the side that is coated. Ironically the only plate wastage you can logically be measuring will be on the inside of the hull where the plate has been covered over and isn't visible.

 

Do the folk who have mentioned the ultrasonic testing device that works without removal of the coating have any knowledge of its effectiveness on narrowboats? For instance does it work with the coatings that are used on boats and are the results sufficiently accurate to be usable?

 

JP

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Do the folk who have mentioned the ultrasonic testing device that works without removal of the coating have any knowledge of its effectiveness on narrowboats? For instance does it work with the coatings that are used on boats and are the results sufficiently accurate to be usable?

 

JP

No personal experience but the specs and description say yes and yes.

 

Have you checked out the 2 links?

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No personal experience but the specs and description say yes and yes.

Have you checked out the 2 links?

Yes, and at that price you would hope it works but I wondered if anyone saying there is no need to scrape off the hull coating categorically knows that is the case. I can think of reasons why such technology may have it's limitations but i don't know the state of the art.

 

JP

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This isn't something we've got to worry about as we've not had it done, but I was thinking the other day about what happens if someone has spent around

five grand on their hull having zinc spray and two pack work done with a ten year guarantee (for example at Debdale Wharf) and then has to sell the boat

a couple of years later.

 

The buyer's surveyor is presumably going to need to remove the blacking to check the hull thickness, which is going to damage the coatings.

If the buyer then doesn't go through with the sale, the owner is left with the hull coating compromised and it can't just be fixed with a quick tin of

bitumastic.

 

 

I believe that the potential purchaser has to agree to pay to have any damage done to the hull repaired, even if they do not go through with purchasing the vessel. That is why a deposit may be required before allowing a survey to be carried out.

If I remember correctly it says something like that in the BMF selling/purchase contract.

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Yes, and at that price you would hope it works but I wondered if anyone saying there is no need to scrape off the hull coating categorically knows that is the case. I can think of reasons why such technology may have it's limitations but i don't know the state of the art.

 

JP

I don't know where the ultrasonic NDT leading edge is today either but when I was first involved with it, in aircraft maintenance, it was a black art involving special people special kit, special gels and polished surfaces to probe against. Interpreting the results was an art form. After a few years it was all much simpler with no need to remove paint and an easy to read display.

A few years after that narrowboat surveyors were putting away their sledgehammers and crawling under baseplates with a battery grinder and an ultrasonic thickness machine (and some chalk).

 

Today it is nearly automated it appears!

 

Progress is a wonderful thing.

 

N

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Our current boat was epoxy coated not long before we purchased it and none had to be removed for the hull survey to be completed.

However I did mention it to the surveyor prior to the survey being carried out and his response was that it was unlikely that he would need to remove any coating but it was written in the conditions of his contract that he may need to do so.

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I have not had any involvement for decades but when I worked in ship research I recall when ultrasound NDT was first being developed.

 

At that time, a ship's hull was surveyed not just for thickness but also laminations - although most of the testing would be done immediately prior to construction. (see http://www.inspection-for-industry.com/plate-lamination-defect.html).In a sense this would be even more hit and miss then thickness measurements and might only pick up the largest of laminations - which are important because they can lead to a loss of strength and hence structural integrity.

 

The first ultrasound NDT devices were demonstrated to shipyards but they cause consternation as they led to a very high rate of detection, far more then the old drilling of holes method. The salespeople devised the following approach: they asked the yard to pick out a sheet that had been tested conventionally and passed as OK. They then carried out the ultrasound test and showed just how many laminations it had that had been undetected.

 

Of course, this led to a re-evaluation of what standard was appropriate as a total absence of laminations was not practical and, in any case, ships had been sailing the oceans in such a condition for some time!

 

What this shows is that any form of testing is not automatic or mechanical - it requires skill, knowledge and judgement to apply in a practical context - the device reference above has a number of tests and settings and I suspect the surveyor will have invested at least as much of his/her time in learning how to use and interpret as the device itself costs. Time is what costs he money!

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Don't forget magnetic ink (iron filings in black paraffin) and the coin tap test!

Neither are much use on a steel NB I grant you, but the coin tap might tell you something about a GRP sandwich construction.

 

N

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  • 3 years later...
On 21/02/2017 at 19:39, b0atman said:

Its called NDT none destructive testing .Problem I have is I could not get any paperwork from Debdale very poor service from the office, but a great job by the workers.

I wasn't very impressed with the phone communication when I called, but glad to read the guys doing the job were great. How long did it take?

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