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Get another ultrasound?


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Hi,

 

Before purchase, we had a survey done and he did an ultrasound. He told us the hull is in great condition. (I've attached a PDF of the ultrasound results.) However, the survey also says he couldn't examine the internal hull.

 

After buying the boat, I’ve taken up the ply flooring and scraped the rust off the floor. But some sections had huge sheets of rusty steel coming off (see photos) which has alarmed me.

 

I spoke to a boat builder who works at the boatyard we're moored at he suggested possibly getting an ultrasound.

 

My question is… is it worth getting one even though my pre purchase survey included an ultrasound and the surveyor said the hull was in great condition?

 

Am I overreacting to this rust? Or should I invest in the additional ultrasound to be on the safe side before I pour more money into this project?

We are going to treat all rust with rust converter, then two layers of red oxide paint. But before doing that, should we get the ultrasound?

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Rust is approximately 8 or 10 times the thickness of the metal (depending on which book you read) it has 'consumed'

 

So 2mm of rust = 0.2mm of metal thickness.

 

Don't worry, nothing there to be concerned about.

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9 minutes ago, Alan de Enfield said:

Rust is approximately 8 or 10 times the thickness of the metal (depending on which book you read) it has 'consumed'

 

So 2mm of rust = 0.2mm of metal thickness.

 

Don't worry, nothing there to be concerned about.

Wow, that's great to know. Thanks!

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1 hour ago, n3sh14 said:

Wow, that's great to know. Thanks!

I see the comments about not "nothing to be concerned about" but I would respectfully suggest that if possible  it might be wise, if practical,  to get another ultrasound check carried out including this time an internal inspection, if for nothing else but your own peace of mind. I do acknowledge that this will undoubtedly  incur additional cost but with great respect to the contributors to this forum, we are all, in the end,   just a collection of  anonymous boaters with varying degrees of knowledge and expertise.  It is regretful  that an internal inspection couldn't be carried out by the original surveyor but hopefully a second one will confirm what you have been told in a previous post.

Do you know the "as built hull thicknesses" as a comparison?

 

Howard 

Edited by howardang
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17 minutes ago, howardang said:

I see the comments about not "nothing to be concerned about" but I would respectfully suggest that if possible  it might be wise, if practical,  to get another ultrasound check carried out including this time an internal inspection, if for nothing else but your own peace of mind. I do acknowledge that this will undoubtedly  incur additional cost but with great respect to the contributors to this forum, we are all, in the end,   just a collection of  anonymous boaters with varying degrees of knowledge and expertise.  It is regretful  that an internal inspection couldn't be carried out by the original surveyor but hopefully a second one will confirm what you have been told in a previous post.

 

Howard 

Thanks for your comment, I'm definitely going to consider it. It's a nuisance and they will probably say what I was told in the previous post (as someone offline also said the same thing) but I'm going to inquire about getting one whilst the floor is up I think. 

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Thing is why is there that much rust. As said it's probably nothing to worry about although I am no expert. But if it's condensation that is causing it then that will have to be minimised and if it's a leak, either external or from the domestic water supply that will have to be detected and sorted. One of my boats had rust below the flooring but nothing like that and that boat had been built in the early eighties. The rust on my boat was caused by condensation that had got under the broken flag stones ballast.

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There aren't really enough thickness measurements in that table to confirm that all is well. But the baseplate looks to have been 12mm (or 1/2") originally and has corroded down to 9mm in the worst place measured.  So while the baseplate has lost some thickness there is loads left and nothing really to worry about.

My guess is that water which has found its way into the bilge has been trapped there - perhaps under paving slab ballast, or worse, ballast standing on roofing felt, so the baseplate never dries out. Better to sit paving slabs on lengths of rubber tubing or old electric cable so there is a definite air gap underneath, which will allow bilge water to flow back to the lowest point and the bilge space to dry out.

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4 hours ago, n3sh14 said:

Am I overreacting to this rust?

 

 

Just noticed a potential worrying number in the durveyors grid.

 

Stbd side / base plate / test No7 = .9mm

 

Is it really 0.9mm or is there a digit mising ?

 

Insurance companies are refusing to give comprehensive cover on 4mm and below.

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7 minutes ago, Alan de Enfield said:

Stbd side / base plate / test No7 = .9mm

 

Is it really 0.9mm or is there a digit mising ?

No. It says ".9." So obviously a typo of some sort. I assumed it is supposed to be 9.x which is OK. If it really was 0.9mm the surveyor would have been duty bound to point it out.

 

Worth checking back with the surveyor anyway though.

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Looks like it is  12mm base plate originally and that the ultrasound shows areas down to 9 mm which in itself is no cause for concern if the boat is  a good number of years  old as most boats from the '90 on have a 10mm base plate as new.

 

However there is no mention of any pitting under the base plate. If the base plate has never been blacked or better epoxy painted, there will be pitting biting into your remaining 9mm. Did the surveyor inspect the base plate for localised pitting as well as taking broad ultrasound readings?

 

The internal rust scale would not frighten me on its own but add a few 5mm deep pits and it could mean that the base plate is below the insurance 4mm limit.

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If its on 1/2 " plate,it should be OK......chipping with a air hammer might be the budget way to go.....with plenty of dust extraction..........What may be of concern is the area where the side (6mm?) joins the baseplate.....be wise to chip out the corner ,and see if there isnt a line wasted metal behind the weld.

Edited by john.k
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3 hours ago, john.k said:

If its on 1/2 " plate,it should be OK......chipping with a air hammer might be the budget way to go.....with plenty of dust extraction..........What may be of concern is the area where the side (6mm?) joins the baseplate.....be wise to chip out the corner ,and see if there isnt a line wasted metal behind the weld.

This thread exactly demonstrates the thought behind my earlier post, with views and differing advice on what course of action to take and no way of knowing which is the best course  of action to follow, unless you know the background and expertise of each contributor. 

Good luck with what ever course of action you take. In my mind it highlights the problems of sites such as these for the unwary. 

 

Howard

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9 hours ago, Tracy D'arth said:

Looks like it is  12mm base plate originally and that the ultrasound shows areas down to 9 mm which in itself is no cause for concern if the boat is  a good number of years  old as most boats from the '90 on have a 10mm base plate as new.

 

However there is no mention of any pitting under the base plate. If the base plate has never been blacked or better epoxy painted, there will be pitting biting into your remaining 9mm. Did the surveyor inspect the base plate for localised pitting as well as taking broad ultrasound readings?

 

The internal rust scale would not frighten me on its own but add a few 5mm deep pits and it could mean that the base plate is below the insurance 4mm limit.

He walked us around the boat for an hour after the survey and explained that there is barely any pitting. The boat has also been blacked every two years since new (it was built in '89.)

This particularly rusty area is where the bathroom used to be, and I was told that the previous owner used to have washes in a tub and the water would go everywhere... so I'm assuming that may be one reason why this area is notably worse than the other areas so far on the boat. 

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5 minutes ago, howardang said:

This thread exactly demonstrates the thought behind my earlier post, with views and differing advice on what course of action to take and no way of knowing which is the best course  of action to follow, unless you know the background and expertise of each contributor. 

Good luck with what ever course of action you take. In my mind it highlights the problems of sites such as these for the unwary. 

 

Howard

I understand, and don't worry, I'm wary of what advice I take. I mainly asked this question to get a general consensus on whether this was alarming and an ultrasound would be worthwhile. However, yesterday I asked the boat builder I hire for the odd job if the ultrasound was worthwhile and he said if the surveyor did one then it should be ok, and that an ultrasound from the inside while the boat is on water could give false readings, but he's advised me to speak to his boss to be sure. I'm also going to contact the surveyor today to show him photos, ask for his opinion, and also ask him to rectify the typo in the survey.

Thanks to everyone for pointing out the survey typo!

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3 minutes ago, n3sh14 said:

I understand, and don't worry, I'm wary of what advice I take. I mainly asked this question to get a general consensus on whether this was alarming and an ultrasound would be worthwhile. However, yesterday I asked the boat builder I hire for the odd job if the ultrasound was worthwhile and he said if the surveyor did one then it should be ok, and that an ultrasound from the inside while the boat is on water could give false readings, but he's advised me to speak to his boss to be sure. I'm also going to contact the surveyor today to show him photos, ask for his opinion, and also ask him to rectify the typo in the survey.

Thanks to everyone for pointing out the survey typo!

 

Presumably this area corresponds to the locations where the survey records 2mm of plate loss and hence gives you a logical set of evidence?

 

I don't see why an ultrasonic test wouldn't work adequately done from inside but equally I don't see any reason why it would give you a different answer from the one you already have. The ultrasonic reading you have should reflect the boundary of the sound steel and not include the thickness of rust.

 

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15 hours ago, David Mack said:

There aren't really enough thickness measurements in that table to confirm that all is well. But the baseplate looks to have been 12mm (or 1/2") originally and has corroded down to 9mm in the worst place measured.  So while the baseplate has lost some thickness there is loads left and nothing really to worry about.

My guess is that water which has found its way into the bilge has been trapped there - perhaps under paving slab ballast, or worse, ballast standing on roofing felt, so the baseplate never dries out. Better to sit paving slabs on lengths of rubber tubing or old electric cable so there is a definite air gap underneath, which will allow bilge water to flow back to the lowest point and the bilge space to dry out.

Thanks for sharing your thoughts. They seem to have used blocks of cement as ballast and it isn't sitting on top of anything like roofing felt. I plan to replace all the ballast as I read that cement is not a good material to use as ballast in a boat. The area where I found this particularly rusty spot is where the bathroom was, so I assume water was somehow leaking under the flooring.

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5 minutes ago, Captain Pegg said:

 

Presumably this area corresponds to the locations where the survey records 2mm of plate loss and hence gives you a logical set of evidence?

 

I don't see why an ultrasonic test wouldn't work adequately done from inside but equally I don't see any reason why it would give you a different answer from the one you already have. The ultrasonic reading you have should reflect the boundary of the sound steel and not include the thickness of rust.

 

Yes, to confirm, it corresponds with the area of 2mm plate loss. 

OK, thanks for your input on the ultrasound reflecting the sound steel and not the rust.

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1 hour ago, n3sh14 said:

Yes, to confirm, it corresponds with the area of 2mm plate loss. 

OK, thanks for your input on the ultrasound reflecting the sound steel and not the rust.

 

I appreciate why @howardang said what he did but my counter to that is that there is consistent evidence here.

 

I was tempted to say that the ultrasonic test reading WILL be of the sound steel and if it had been done by a skilled ultrasonic test operator interpreting the primary signal output I'd be confident of that. Exactly how a modern ultrasonic test meter that can 'see' through paint layers and the like calculates thickness is less obvious, it doubtless involves some fancy algorithms in the programming. Remember that ultrasound is primarily used to find very fine internal cracks, the meter isn't going to miss the interface between steel and rust, it's just a question of how accurately it can position that interface but you have lots of room for error here. Quoting thicknesses to 0.1mm is a bit misleading anyway.

 

I took the .9. to be someone hitting the keys the wrong way round and it should be 9.9, presumably you've checked that out.

 

So you have a 1989 boat - that appears to have been built with a 12mm baseplate - and has some localised loss of material that you are dealing with properly and it retains 10mm minimum baseplate and 5mm minimum side thickness. Even more importantly you have little to no pitting because that's what will get you rather than general hull loss.

 

I'd say that's pretty good, I've seen worse in the bottom of boats and I think many folk are oblivious to what is happening to the top surface of their baseplate.

 

   

Edited by Captain Pegg
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1 hour ago, Captain Pegg said:

 

Presumably this area corresponds to the locations where the survey records 2mm of plate loss and hence gives you a logical set of evidence?

 

I don't see why an ultrasonic test wouldn't work adequately done from inside but equally I don't see any reason why it would give you a different answer from the one you already have. The ultrasonic reading you have should reflect the boundary of the sound steel and not include the thickness of rust.

 

 

 

I bet two adjacent ultrasound thickness readings on the baseplate from one of my boats would ALWAYS give different results...

 

 

 

 

image.jpeg.c4f8a7406521ff7cb2aac025fca9b0dd.jpeg

 

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